200 results on '"Vöhringer, Peter"'
Search Results
152. Vertical Photoionization of Liquid-to-SupercriticalAmmonia: Thermal Effects on the Valence-to-Conduction Band Gap.
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Urbanek, Janus and Vöhringer, Peter
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AMMONIA , *PHOTOIONIZATION of gases , *THERMAL analysis , *VALENCE (Chemistry) , *BAND gaps , *ELECTRONS - Abstract
We recently reported first femtosecondpump–probe experimentson the geminate recombination dynamics of solvated electrons in fluidammonia (Urbanek et al., J. Phys. Chem. B2012, 116, 2223–2233). The electrons were generatedthrough a vertical two-photon ionization at a total energy of 9.3eV. Here, we present a full Monte Carlo analysis of the time-resolveddata to determine the solvated electron’s thermalization distancefrom the ionization hole, NH3+. The simulations are compared with the experimentover wide thermodynamic conditions to obtain insight into the dependenceof the vertical ionization mechanism on the electronic propertiesof the solvent network. The simulations reveal that the average thermalizationdistance, ⟨r0⟩, decreasesstrongly with both increasing temperature, T, anddecreasing density, ρ, from 3.2 nm in the cryogenic fluid downto roughly 0.5 nm in the dilute supercritical phase with almost gas-likedensities. We combine our results with the current understanding ofthe T,ρ-dependence of the electronic structureof the liquid phase and discuss in detail the role of thermally inducedenergy level shifts for the valence-to-conduction band gap. The observedchanges of the thermalization distance can be well attributed to agradual decrease of the excess energy initially imparted on the ejectedelectron as gas-like conditions are progressively approached. [ABSTRACT FROM AUTHOR]
- Published
- 2013
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153. Zweidimensionale IR-Spektroskopie und H-Brücken.
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Lindner, Jörg and Vöhringer, Peter
- Published
- 2013
154. The Photochemistry of [FeIIIN3(cyclam-ac)]PF6 at 266 nm.
- Author
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Torres-Alacan, Joel, Krahe, Oliver, Filippou, Alexander C., Neese, Frank, Schwarzer, Dirk, and Vöhringer, Peter
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- 2012
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155. OH-Stretch Vibrational Relaxation of HOD in Liquid to Supercritical D2O †.
- Author
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Schwarzer, Dirk, Lindner, Jörg, and Vöhringer, Peter
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- 2006
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156. Femtosecond two-photon ionization of fluid NH3 at 9.3 eV.
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Urbanek, Janus, Dahmen, Annika, Torres-Alacan, Joel, and Vöhringer, Peter
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FEMTOSECOND pulses ,TWO-photon-spectroscopy ,IONIZATION (Atomic physics) ,PHOTONS ,PHOTOIONIZATION ,FLUID dynamics ,AMMONIA - Abstract
Liquid and supercritical ammonia (NH3) is photo-ionized at an energy of 9.3 eV with 100-fs duration pulses at a wavelength of 266 nm. The ionization involves two photons and generates fully solvated electrons via the conduction band of the solvent within the time resolution of the experiment. The dynamics of their ensuing geminate recombination is followed in real time with femtosecond near-infrared (IR) probe pulses. The recombination mechanism can be understood as an ion-pair mediated reaction. The electron survival probability is found to be in quantitative agreement with the classical Onsager theory for the initial recombination of ions [ABSTRACT FROM AUTHOR]
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- 2013
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157. Femtosecond UV-pump mid-IR probe spectroscopy of the ultrafast photodissociation of azide radicals from an azidoiron( III) complex.
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Vennekate, Hendrik, Schwarzer, Dirk, Torres-Alacan, Joel, and Vöhringer, Peter
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FEMTOSECOND pulses ,PICOSECOND pulses ,PHOTODISSOCIATION ,PHOTOLYSIS (Chemistry) ,FLASH photolysis ,IRON oxidation ,ELECTRONIC spectra - Abstract
The ultrafast photolysis of the cation complex [(cyclam-ac)FeN
3 ]+ is studied by femtosecond spectroscopy with ultraviolet excitation and mid-infrared probing. Immediately after UV absorption, the excited complex undergoes internal conversion and azide dissociation within 2 ps. The subsequent vibrational relaxation in the electronic ground state and geminate recombination of the fragments take place on time scales of 13 and 20 ps, respectively [ABSTRACT FROM AUTHOR]- Published
- 2013
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158. Anharmonic Bend-Stretch Coupling in Water.
- Author
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Castleman, A. W., Toennies, J. P., Zinth, W., Yamanouchi, K., Corkum, Paul, Jonas, David M., Miller, R. J. Dwayne., Weiner, Andrew M., Lindner, Jörg, Vöhringer, Peter, Pshenichnikov, Maxim S., Cringus, Dan, and Wiersma, Douwe A.
- Abstract
Following excitation of the H-O-H bending mode of pure liquid water, the stretching mode is monitored over its entire spectral width. The anharmonic coupling between the two modes results in a substantial change of the transient stretch absorption that decays with the bend depopulation time. Unlike in the gas phase, the stretch transition shifts to the blue, which is a direct consequence of the weakened hydrogen-bond network. [ABSTRACT FROM AUTHOR]
- Published
- 2007
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159. Multicolor IR Spectroscopy of Pure Liquid Water.
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Castleman, A. W., Toennies, J. P., Zinth, W., Yamanouchi, K., Corkum, Paul, Jonas, David M., Miller, R. J. Dwayne., Weiner, Andrew M., Cringus, Dan, Pshenichnikov, Maxim S., Wiersma, Douwe A., Mostovoy, Maxim, Lindner, Jörg, and Vöhringer, Peter
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Multicolor infrared ultrafast spectroscopy is applied to investigate the vibrational relaxation dynamics in liquid water at room temperature with both the stretching and the bending mode being photoexcited and probed. A unified model, capable of the reproduction of as much as 150 transients, yielded cross-sections and relaxation times for the stretching and bending modes. It is demonstrated, that the energy from the initially excited stretching vibration is partitioned to the bending modes of approximately two water molecules. [ABSTRACT FROM AUTHOR]
- Published
- 2007
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160. Femtosecond Infrared Spectroscopy of HOD in Liquid to Supercritical Heavy Water.
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Castleman, A. W., Toennies, J. P., Zinth, W., Yamanouchi, K., Corkum, Paul, Jonas, David M., Miller, R. J. Dwayne., Weiner, Andrew M., Lindner, Jörg, Vöhringer, Peter, and Schwarzer, Dirk
- Abstract
The dynamics of vibrational energy relaxation (VER) of the OH-stretching vibration of HOD in liquid-to-supercritical heavy water is studied as a function of temperature and solvent density. The VER rate can be correlated phenomenologically with the average hydrogen-bond connectivity within the random D2O-network. This correlation enables the identification of thermodynamic conditions under which spectral diffusion due to hydrogen-bond breakage/formation is much faster than VER. [ABSTRACT FROM AUTHOR]
- Published
- 2007
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161. The Photochemistry of [FeIIIN3(cyclam‐ac)]PF6at 266 nm
- Author
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Torres‐Alacan, Joel, Krahe, Oliver, Filippou, Alexander C., Neese, Frank, Schwarzer, Dirk, and Vöhringer, Peter
- Abstract
The photochemistry of iron azido complexes is quite challenging and poorly understood. For example, the photochemical decomposition of [FeIIIN3(cyclam‐ac)]PF6([1]PF6), where cyclam‐ac represents the 1,4,8,11‐tetraazacyclotetradecane‐1‐acetate ligand, has been shown to be wavelength‐dependent, leading either to the rare high‐valent iron(V) nitrido complex [FeVN(cyclam‐ac)]PF6([3]PF6) after cleavage of the azide NαNβbond, or to a photoreduced FeIIspecies after FeNazidebond homolysis. The mechanistic details of this intriguing reactivity have never been studied in detail. Here, the photochemistry of 1in acetonitrile solution at room temperature has been investigated using step‐scan and rapid‐scan time‐resolved Fourier transform infrared (FTIR) spectroscopy following a 266 nm, 10 ns pulsed laser excitation. Using carbon monoxide as a quencher for the primary iron‐containing photochemical product, it is shown that 266 nm excitation of 1results exclusively in the cleavage of the FeNazidebond, as was suspected from earlier steady‐state irradiation studies. In argon‐purged solutions of [1]PF6, the solvent‐stabilized complex cation [FeII(CH3CN)(cyclam‐ac)]+(2 red) together with the azide radical (N3.) is formed with a relative yield of 80 %, as evidenced by the appearance of their characteristic vibrational resonances. Strikingly, step‐scan experiments with a higher time resolution reveal the formation of azide anions (N3−) during the first 500 ns after photolysis, with a yield of 20 %. These azide ions can subsequently react thermally with 2 redto form [FeIIN3(cyclam‐ac)] (1 red) as a secondary product of the photochemical decomposition of 1. Molecular oxygen was further used to quench 1 redand 2 redto form what seems to be the elusive complex [Fe(O2)(cyclam‐ac)]+(6).
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- 2012
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162. OH-Stretch Vibrational Relaxation of HOD in Liquid to Supercritical D2O †.
- Author
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Schwarzer, Dirk, Lindner, Jörg, and Vöhringer, Peter
- Abstract
The population relaxation of the OH-stretching vibration of HOD diluted in D2O is studied by time-resolved infrared (IR) pump−probe spectroscopy for temperatures of up to 700 K in the density range 12 ≤ ρ ≤ 58 mol/L. For selected state points of the fluid solution, transient IR spectra were recorded following resonant excitation of the v = 0 → 1 OH stretching transition with a 200 fs laser pulse centered at ∼3500 cm-1. Above 400 K these spectra show no indication of spectral diffusion after pump−probe delays of 0.3 ps. Over nearly the entire density range and for sufficiently high temperatures (T > 360 K), the vibrational relaxation rate constant, kr, is strictly proportional to the dielectric constant, ε, of water. Together with existing molecular dynamics simulations, this result suggests a simple linear dependence of kr on the number of hydrogen-bonded D2O molecules. It is shown that, for a given temperature, an isolated binary collision model is able to adequately describe the density dependence of vibrational energy relaxation even in hydrogen-bonded fluids. However, dynamic hydrogen bond breakage and formation is a source of spectral diffusion and affects the nature of the measured kr. For sufficiently high temperatures when spectral diffusion is much faster than energy transfer, the experimentally observed decays correspond to ensemble averaged population relaxation rates. In contrast, when spectral diffusion and vibrational relaxation occur on similar time scales, as is the case for ambient conditions, deviations from the linear kr(ε) relation occur because the long time decay of the v = 1 population is biased to slower relaxing HOD molecules that are only weakly connected to the hydrogen bond network. [ABSTRACT FROM AUTHOR]
- Published
- 2006
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163. Photo‐Initiated Cobalt‐Catalyzed Radical Olefin Hydrogenation.
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Sang, Sier, Unruh, Tobias, Demeshko, Serhiy, Domenianni, Luis I., van Leest, Nicolaas P., Marquetand, Philipp, Schneck, Felix, Würtele, Christian, de Zwart, Felix J., de Bruin, Bas, González, Leticia, Vöhringer, Peter, and Schneider, Sven
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ABSTRACTION reactions , *COBALT , *TRANSITION metal hydrides , *HYDROGENATION , *ALKENES , *HYDROGEN atom , *CATALYSTS - Abstract
Outer‐sphere radical hydrogenation of olefins proceeds via stepwise hydrogen atom transfer (HAT) from transition metal hydride species to the substrate. Typical catalysts exhibit M−H bonds that are either too weak to efficiently activate H2 or too strong to reduce unactivated olefins. This contribution evaluates an alternative approach, that starts from a square‐planar cobalt(II) hydride complex. Photoactivation results in Co−H bond homolysis. The three‐coordinate cobalt(I) photoproduct binds H2 to give a dihydrogen complex, which is a strong hydrogen atom donor, enabling the stepwise hydrogenation of both styrenes and unactivated aliphatic olefins with H2 via HAT. [ABSTRACT FROM AUTHOR]
- Published
- 2021
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164. Preface to the special issue: The beauty of chemical reaction kinetics, a Festschrift in honour of Jürgen Troe.
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Quack, Martin, Abel, Bernd, Merkt, Frédéric, Hippler, Horst, Vöhringer, Peter, Wodtke, Alec, and Willitsch, Stefan
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CHEMICAL kinetics , *PHYSICAL & theoretical chemistry , *FLASH photolysis , *MOLECULAR physics , *CHEMICAL reactions , *DIATOMIC molecules - Published
- 2021
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165. Vibrational relaxation of azide ions in liquid-to-supercritical water.
- Author
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Olschewski, Martin, Knop, Stephan, Lindner, Jörg, and Vöhringer, Peter
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RELAXATION phenomena , *AZIDES , *IONS , *SUPERCRITICAL fluids , *WATER , *VIBRATION (Mechanics) , *MOLECULAR dynamics , *HYDROGEN bonding , *ASYMMETRY (Chemistry) - Abstract
The dynamics of vibrational energy relaxation (VER) of the aqueous azide anion was studied over a wide temperature (300 K ≤ T ≤ 663 K) and density (0.6 g cm-3 ≤ ρ ≤ 1.0 g cm-3) range thereby covering the liquid and the supercritical phase of the water solvent. Femtosecond mid-infrared spectroscopy on the ν3 band associated with the asymmetric stretching vibration of the azide anion was used to monitor the relaxation dynamics in a time-resolved fashion. The variation of the vibrational relaxation rate constant with temperature and density was found to be rather small. Surprisingly, the simple isolated binary collision model is able to fully reproduce the experimentally observed temperature and density dependence of the relaxation rate provided a local density correction around the vibrationally excited solute based on classical molecular dynamics simulations is used. The simulations further suggest that head-on collisions of the solvent with the terminal nitrogen atoms rather than side-on collisions with the central nitrogen atom of the azide govern the vibrational energy relaxation of this system. Finally, the importance of hydrogen bonding for the VER dynamics in this system is briefly discussed. [ABSTRACT FROM AUTHOR]
- Published
- 2011
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166. ND-stretching vibrational energy relaxation of NH2D in liquid-to-supercritical ammonia studied by femtosecond midinfrared spectroscopy.
- Author
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Schäfer, Tim, Schwarzer, Dirk, Lindner, Jörg, and Vöhringer, Peter
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AMMONIA , *INFRARED spectroscopy , *FLUIDS , *MOLECULAR dynamics , *ENERGY transfer , *FORCE & energy , *DENSITY - Abstract
Femtosecond midinfrared pump-probe spectroscopy was carried out to explore the dynamics of vibrational energy relaxation of NH2D in fluid ammonia NH3. The ND-stretching fundamental of the partially deuterated solute NH2D was excited by femtosecond pulses centered at 2450 cm-1, and both the ground-state bleach and the anharmonically shifted transient absorption of the same vibration was probed. The temperature of the sample was varied between 230 and 450 K, while the pressure was tuned from 10 to 1500 bar, thereby entering both the liquid and the supercritical phase of the fluid solution. The density and temperature dependence of the ND-stretching lifetime suggests that hydrogen bonding is of negligible importance for vibrational energy relaxation. Rather, the energy transfer dynamics can be understood qualitatively in terms of a simple Landau-Teller description for vibrational energy relaxation using molecular dynamics simulations to estimate the spectral density of the fluctuating forces exerted by a weakly interacting Lennard-Jones solvent (NH3) onto the vibrationally excited solute (NH2D). [ABSTRACT FROM AUTHOR]
- Published
- 2008
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167. Frontispiece: Observing the Entry Events of a Titanium‐Based Photoredox Catalytic Cycle in Real Time.
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Schmidt, Jonas, Domenianni, Luis I., Leuschner, Marcel, Gansäuer, Andreas, and Vöhringer, Peter
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DELAYED fluorescence , *MID-infrared spectroscopy - Abstract
In their Communication (e202307178), Andreas Gansäuer, Peter Vöhringer et al. report time-resolved fluorescence and ultrafast mid-infrared spectroscopies to study the primary events of entry into a sustainable photoredox-catalytic cycle utilizing a titanium(IV) photocatalyst. Photo-Redox Catalysis, Thermally Activated Delayed Fluorescence, Time-Resolved Fluorescence, Transition-Metal Photochemistry, Ultrafast Infrared Spectroscopy Keywords: Photo-Redox Catalysis; Thermally Activated Delayed Fluorescence; Time-Resolved Fluorescence; Transition-Metal Photochemistry; Ultrafast Infrared Spectroscopy EN Photo-Redox Catalysis Thermally Activated Delayed Fluorescence Time-Resolved Fluorescence Transition-Metal Photochemistry Ultrafast Infrared Spectroscopy 1 1 1 08/25/23 20230828 NES 230828 B Time-Resolved Spectroscopy b . [Extracted from the article]
- Published
- 2023
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168. Frontispiz: Beobachtung der ersten Schlüsselschritte eines titanbasierten Photoredox‐Katalysezyklus in Echtzeit.
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Schmidt, Jonas, Domenianni, Luis I., Leuschner, Marcel, Gansäuer, Andreas, and Vöhringer, Peter
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MID-infrared spectroscopy , *INFRARED spectroscopy , *DELAYED fluorescence , *FLUORESCENCE , *PHOTOCHEMISTRY , *CATALYSIS - Abstract
In their Communication (e202307178), Andreas Gansäuer, Peter Vöhringer et al. report time-resolved fluorescence and ultrafast mid-infrared spectroscopies to study the primary events of entry into a sustainable photoredox-catalytic cycle utilizing a titanium(IV) photocatalyst. Keywords: Thermally Activated Delayed Fluorescence; Time-Resolved Fluorescence; Transition-Metal Photochemistry; Ultrafast Infrared Spectroscopy; Photo-Redox Catalysis DE Thermally Activated Delayed Fluorescence Time-Resolved Fluorescence Transition-Metal Photochemistry Ultrafast Infrared Spectroscopy Photo-Redox Catalysis 1 1 1 08/23/23 20230828 NES 230828 B Time-Resolved Spectroscopy b . [Extracted from the article]
- Published
- 2023
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169. The femtochemistry of the ferrioxalate actinometer.
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Straub, Steffen, Brünker, Paul, Lindner, Jörg, Vöhringer, Peter, Cerullo, G., Ogilvie, J., Kärtner, F., Khalil, M., and Li, R.
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FEMTOCHEMISTRY , *ACTINOMETERS , *LIGHT sources , *PHOTOCHEMISTRY , *LIGHT absorption - Abstract
The ferrioxalate actinometer is widely used as an analytical standard to determine the photon flux of light sources in photochemical reactors. Yet, the underlying mechanistic functioning of the actinometer at the molecular level is entirely unknown. Here, we present results from femtosecond UV-pump/mid-infrared-probe spectroscopy to reveal the elementary events including the primary C–C and Fe–O-bond breakages that follow an initial photon absorption by the ferrioxalate complex. [ABSTRACT FROM AUTHOR]
- Published
- 2019
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170. Anharmonic bend–stretch coupling in neat liquid water
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Lindner, Jörg, Cringus, Dan, Pshenichnikov, Maxim S., and Vöhringer, Peter
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FEMTOCHEMISTRY , *INFRARED spectroscopy , *RELAXATION (Nuclear physics) , *WATER - Abstract
Abstract: Femtosecond mid-IR spectroscopy is used to study the vibrational relaxation dynamics in neat liquid water. By exciting the bending vibration and probing the stretching mode, it is possible to reliably determine the bending and librational lifetimes of water. The anharmonic coupling between the bending and the stretching degrees of freedom is quantified in terms of a differential absorption cross-section for the fundamental stretching transition carrying one spectating bending quantum. A positive off-diagonal anharmonicity may be caused by the initial excitation of the anharmonic bending mode and is mediated by the hydrogen bonded network. [Copyright &y& Elsevier]
- Published
- 2007
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171. Femtosecond water dynamics in reverse-micellar nanodroplets
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Cringus, Dan, Lindner, Jörg, Milder, Maaike T.W., Pshenichnikov, Maxim S., Vöhringer, Peter, and Wiersma, Douwe A.
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COLLOIDS , *SPECTRUM analysis , *SURFACE active agents , *HYDROGEN - Abstract
Abstract: Vibrational energy relaxation and ultrafast thermalization following impulsive excitation of the OH-stretching band of water nanodroplets confined to reverse micelles is studied by infrared pump–probe spectroscopy with sub-100 fs time resolution. The self-consistent analysis of experimental data for micelles diameters ranging from 1 to 10nm as well as for bulk water reveals distinctly different vibrational lifetimes for the water molecules in the bulk-like core (270 fs) and in the surfactant vicinity (800 fs), which is a direct proof of a strongly disturbed hydrogen-bond network. [Copyright &y& Elsevier]
- Published
- 2005
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172. Elementary solute-solvent interactions and the photophysical properties of photoacids
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Premont-Schwarz, Mirabelle, Elsässer, Thomas, Vöhringer, Peter, and Röde, Beate
- Subjects
Femtosekunden-UV-Anrege/IR-Abtast-Spektroskopie ,OH stretch vibration ,photoacids ,Photosäure ,ddc:530 ,UH 5710 ,Zustandskreuzung ,530 Physik ,level crossing ,Femtosecond UV pump-IR probe spectroscopy ,OH-Streckschwingung ,29 Physik, Astronomie ,VE 8307 - Abstract
Photosäuren sind aromatische Alkohole, sogenannt aufgrund der Erhöhung ihrer Azidität, die aus der elektronischen Anregung folgt. Allerdings muss ein plausibles Verständnis der Prozesse, die zu der Erhöhung der Azidität führen noch etabliert werden. Zu diesem Zweck wird die Photophysik zweier Photosäuren, 1-Naphthol (1N) und 2-Naphthol (2N), untersucht. Mit Hilfe der Femtosekunden-UV-Anrege/IR-Abtast-Spektroskopie wird die OH-Streckschwingung sowohl im Grundzustand als auch im angeregten Zustand gemessen. Die intrinsische elektronische Ladungsverteilung, die aufgrund der Anregung in der Säure auftritt, in apolaren Lösungsmitteln untersucht. Der Vergleich mit Resultaten eines theoretischen Modells stellte einwandfrei fest, dass eine geringe Ladungsverteilung in der Photosäure auftritt. Die OH-Streckschwingung von wasserstoffverbrückten Komplexen zwischen 2N und Acetonitril wird gemessen. Obwohl experimentell gefunden wurde, dass der angeregte Zustand verglichen mit dem Grundzustand eine nahezu doppelt so große solvatochromatische Verschiebung zeigt, scheitern theoretische Modelle daran, diese Ergebnisse wiederzugeben. Die Ladungstransferreaktion im angeregten Zustand von 1N und 2N zu halogenierten Lösungsmitteln wird zur Abfrage der elektronischen Dichte am aromatischen Ring nach Anregung verwendet. Wurde ermittelt, dass verglichen mit dem unkomplexierten Molekül die Elektrontransferrate in einem wasserstoffverbrückten Komplex mit Acetonitril 10 mal höher ist. Auf diese Weise hat sich der Einfluss der Wasserstoffbrückenbindung auf das Ausmaß der Ladungsverteilung gezeigt. Mittels zeitaufgelöster Anisotropie, Fluoreszenz und IR-Messungen war es möglich festzustellen, dass die ultraschnelle ( Photoacids are aromatic alcohols, characterized by a dramatic increase in acidity upon electronic excitation. A coherent view of the processes giving rise to this increase in acidity has yet to be established. To this effect, the photophysics of photoacids 1-naphthol (1N) and 2-naphthol (2N) are investigated. Using femtosecond UV pump-IR probe spectroscopy, the OH stretch vibration in both the ground and excited-state is measured. The intrinsic electronic charge redistribution in the acid upon excitation is investigated in non-polar solvents where specific interactions are absent. Comparison with results from a theoretical model based on the Pullin-van der Zwan-Hynes perturbative approach established that little charge redistribution occurs in the photoacid. The OH stretch vibration of hydrogen-bonded complexes of 2N with acetonitrile is measured. While it was found experimentally that the excited-state is characterized by a solvatochromic response that is almost twice as large as in the ground-state, the theoretical model failed to reproduce these results. Instead, the calculations predict no significant differences between the behaviour of the two states. The excited-state charge transfer reaction of 1N and 2N to halogenated solvents is used as a probe for the electronic density on the aromatic ring upon excitation. The charge transfer rate for the hydrogen-bonded complex with acetonitrile is found to be ten times higher than for the uncomplexed molecule. In this way, the influence of a hydrogen-bond on the extent of charge redistribution was evinced. Using time resolved anisotropy, fluorescence and IR measurements, it was determined that ultrafast (
- Published
- 2013
173. Primary processes of the archetypal model complex azido(porphinato)iron(III) from ultrafast vibrational-electronic spectroscopy.
- Author
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Flesch S, Domenianni LI, and Vöhringer P
- Abstract
Azidoiron complexes serve as valuable photochemical precursors for catalytically active species containing high-valent iron. In bioinorganic chemistry, azido(tetraphenylporphinato)iron(III), i.e., [FeIII(tpp)(N3)] with tpp = 5, 10, 15, 20-tetraphenylporphyrin-21, 23-diido, constitutes the archetypal model system that was used to access for the first time the terminal nitridoiron core, FeV ≡ N, in the biomimetic redox-non-innocent ligand environment. So far, the light-induced dynamics leading to the oxidation of the metal and the release of dinitrogen from the N3-ligand have only been studied for precursors featuring redox-innocent auxiliary ligands that simplify the electronic structure change accompanying the photo-transformation. Here, we monitored the primary events of the above paradigmatic complex, following its optical excitation in the ultraviolet-to-visible spectral range using femtosecond spectroscopy with probing in both the UV-vis and mid-infrared regions. Following ultrafast Soret-excitation at 400 nm, the complex relaxes to the lowest excited sextet state by a first internal conversion in less than 200 fs. The excited state then undergoes vibrational relaxation on a time scale of roughly 2 ps before internally converting yet again to recover the sextet electronic ground state within 19.5 ps. Spectroscopic evidence is obtained neither for a transient occupation of the energetically lowest metal-centered state, 41A1, nor for vibrational relaxation in the ground-state. The primary processes seen here are thus in contrast to those previously derived from ultrafast UV-pump/vis-probe and UV-pump/XANES-probe spectroscopies for the halide congener [FeIII(tpp)(Cl)]. Any photochemical transformation of the complex arises from two-photon-induced dynamics., (© 2024 Author(s). Published under an exclusive license by AIP Publishing.)
- Published
- 2024
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174. Ultrafast photogeneration of a metal-organic nitrene from 1,1'-diazidoferrocene.
- Author
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Scherz F, Bauer M, Domenianni LI, Hoyer C, Schmidt J, Sarkar B, Vöhringer P, and Krewald V
- Abstract
Ferrocene and its derivatives have fascinated chemists for more than 70 years, not least due to the analogies with the properties of benzene. Despite these similarities, the obvious difference between benzene and ferrocene is the presence of an iron ion and hence the availability of d-orbitals for properties and reactivity. Phenylnitrene with its rich photochemistry can be considered an analogue of nitrenoferrocene. As with most organic and inorganic nitrenes, nitrenoferrocene can be obtained by irradiating the azide precursor. We study the photophysical and photochemical processes of dinitrogen release from 1,1'-diazidoferrocene to form 1-azido-1'-nitrenoferrocene with UV-pump-mid-IR-probe transient absorption spectroscopy and time-dependent density functional theory calculations including spin-orbit coupling. An intermediate with a bent azide moiety is identified that is pre-organised for dinitrogen release via a low-lying transition state. The photochemical decay paths on the singlet and triplet surfaces including the importance of spin-orbit coupling are discussed. We compare our findings with the processes discussed for photochemical dinitrogen activation and highlight implications for the photochemistry of azides more generally., Competing Interests: There are no conflicts of interest to declare., (This journal is © The Royal Society of Chemistry.)
- Published
- 2024
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175. A Chiral Titanocene Complex as Regiodivergent Photoredox Catalyst: Synthetic Scope and Mechanism of Catalyst Generation.
- Author
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Zhang Z, Slak D, Krebs T, Leuschner M, Schmickler N, Kuchuk E, Schmidt J, Domenianni LI, Kleine Büning JB, Grimme S, Vöhringer P, and Gansäuer A
- Abstract
We describe a combined synthetic, spectroscopic, and computational study of a chiral titanocene complex as a regiodivergent photoredox catalyst (PRC). With Kagan's complex catCl
2 either monoprotected 1,3-diols or 1,4-diols can be obtained in high selectivity from a common epoxide substrate in a regiodivergent epoxide opening depending on which enantiomer of the catalyst is employed. Due to the catalyst-controlled regioselectivity of ring opening and the broader substrate scope, the PRC with catCl2 is also a highly attractive branching point for diversity-oriented synthesis. The photochemical processes of cat(NCS)2 , a suitable model for catCl2 , were probed by time-correlated single-photon counting. The photoexcited complex displays a thermally activated delayed fluorescence as a result of a singlet-triplet equilibration, S1 ⇄ T1 , via intersystem crossing and recrossing. Its triplet state is quenched by electron transfer to the T1 state. Computational and cyclic voltammetry studies highlight the importance of our sulfonamide additive. By bonding to sulfonamide additives, chloride abstraction from [catCl2 ]- is facilitated, and catalyst deactivation by coordination of the sulfonamide group is circumvented.- Published
- 2023
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176. Photoinduced Metallonitrene Formation by N 2 Elimination from Azide Diradical Ligands.
- Author
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Domenianni LI, Bauer M, Schmidt-Räntsch T, Lindner J, Schneider S, and Vöhringer P
- Abstract
Transition-metal nitrides/nitrenes are highly promising reagents for catalytic nitrogen-atom-transfer reactivity. They are typically prepared in situ upon optically induced N
2 elimination from azido precursors. A full exploitation of their catalytic potential, however, requires in-depth knowledge of the primary photo-induced processes and the structural/electronic factors mediating the N2 loss with birth of the terminal metal-nitrogen core. Using femtosecond infrared spectroscopy, we elucidate here the primary molecular-level mechanisms responsible for the formation of a unique platinum(II) nitrene with a triplet ground state from a closed-shell platinum(II) azide precursor. The spectroscopic data in combination with quantum-chemical calculations provide compelling evidence that product formation requires the initial occupation of a singlet excited state with an anionic azide diradical ligand that is bound to a low-spin d8 -configured PtII ion. Subsequent intersystem crossing generates the Pt-bound triplet azide diradical, which smoothly evolves into the triplet nitrene via N2 loss in a near barrierless adiabatic dissociation. Our data highlight the importance of the productive, N2 -releasing state possessing azide ππ* character as a design principle for accessing efficient N-atom-transfer catalysts., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)- Published
- 2023
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177. Photolysis of an Archetypal Model Complex. Photooxidation Versus Photoreduction of Azido(porphinato)iron(III).
- Author
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Flesch S and Vöhringer P
- Abstract
Azidoporphinatoiron(III) ([1]) is an archetypal model complex for the photochemical generation of nitridoiron(V) complexes via cleavage of dinitrogen. So far, this process has only been studied with continuous irradiation in thin films under cryogenic conditions or in frozen solutions. In addition, the photooxidation from iron(III) to iron(V) competes with photoreduction to iron(II) via cleavage of an azidyl radical. The quantum yields of both pathways remained hitherto undisclosed. Here, we investigated the photolysis of this model complex in room temperature liquid solution using stationary and time-resolved infrared spectroscopy. The two reaction pathways are unambiguously identified in quenching studies and their quantum yields are accurately determined. Nitridoporphinatoiron(V) ([2]) exhibits N-atom-2-electron-transfer reactivity toward tert-butyl isonitrile and forms a carbodiimido species. In the presence of tert-butyl isonitrile, the two products of the photoreduction pathway react to cationic diisonitriloporphinatoiron(III) and azide anions, which in turn combine to reform [1] and the quencher., (© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)
- Published
- 2023
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178. Observing the Entry Events of a Titanium-Based Photoredox Catalytic Cycle in Real Time.
- Author
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Schmidt J, Domenianni LI, Leuschner M, Gansäuer A, and Vöhringer P
- Abstract
Titanium-based catalysis in single electron transfer (SET) steps has evolved into a versatile approach for the synthesis of fine chemicals and first attempts have recently been made to enhance its sustainability by merging it with photo-redox (PR) catalysis. Here, we explore the photochemical principles of all-Ti-based SET-PR-catalysis, i.e. in the absence of a precious metal PR-co-catalyst. By combining time-resolved emission with ultraviolet-pump/mid-infrared-probe (UV/MIR) spectroscopy on femtosecond-to-microsecond time scales we quantify the dynamics of the critical events of entry into the catalytic cycle; namely, the singlet-triplet interconversion of the do-it-all titanocene(IV) PR-catalyst and its one-electron reduction by a sacrificial amine electron donor. The results highlight the importance of the PR-catalyst's singlet-triplet gap as a design guide for future improvements., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)
- Published
- 2023
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179. Ultrafast Dynamics of Photochemical Nitrile Imine Formation.
- Author
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Flesch S and Vöhringer P
- Abstract
The chemical reactivity of nitrile imines is of great utility in organic synthesis with applications rapidly expanding into the materials and life sciences. Yet, our understanding of the electronic and molecular structures of nitrile imines remains incomplete and the elementary mechanism of their photoinduced generation is entirely unknown. Here, femtosecond infrared spectroscopy after 266 nm-excitation of 2,5-diphenyltetrazole has been carried out to temporally resolve the formation and structural relaxation dynamics of the nascent diphenylnitrile imine in liquid solution under ambient conditions. The infrared-spectroscopic evolution is interpreted by an initial sequence of intersystem crossings within 250 fs followed by the cleavage of N
2 with formation of a structurally relaxed nitrile imine on the adiabatic ground-state singlet surface within a few tens of picoseconds. The infrared spectrum supports the notion of a "floppy" nitrile imine molecule whose equilibrium character ranges from fully propargylic to fully allenic in the room temperature liquid solution., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)- Published
- 2022
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180. Vibrational relaxation of carbon dioxide in water.
- Author
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Gleim J, Lindner J, and Vöhringer P
- Abstract
The dynamics of vibrational relaxation of carbon dioxide in water has been studied using femtosecond mid-infrared pump-probe spectroscopy with excitation of the anti-symmetric stretching (ν
3 ) fundamental state of the solute. The relaxation dynamics were recorded at a constant pressure of 500 bars and in the temperature range between 300 and 600 K, thereby covering the liquid-to-near-critical region of the solvent. The excited state of the ν3 -mode is deactivated in two competing pathways: (i) direct relaxation to the ground state with resonant transfer of the excess vibrational energy into the bending-librational continuum of the water solvent and (ii) relaxation to the bending fundamental state with transfer into the intramolecular bending mode of H2 O. The rate of pathway (i) decreases with increasing temperature, from ∼1/(9 ps) at 300 K to ∼(1/16 ps) at 600 K and obeys Fermi's golden rule strictly, provided that the spectral density of energy-accepting solvent states is derived from the stationary infrared absorption profile of H2 O. The rate of pathway (ii) is 1/(23 ps) and assumed to be temperature-independent within our data analysis. Finally, the bending fundamental of CO2 can also relax to the ground state by resonantly transferring the remaining excess energy to the librational fundamentals of the solvent.- Published
- 2022
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181. Ultrafast "end-on"-to-"side-on" binding-mode isomerization of an iron-carbon dioxide complex.
- Author
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Straub S and Vöhringer P
- Abstract
Carbon dioxide (CO
2 ) binding by transition metals is a captivating phenomenon with a tremendous impact in environmental science and technology, most notably, for establishing circular economies based on greenhouse gas emissions. The molecular and electronic structures of coordination compounds containing CO2 can be studied in great detail using photochemical precursors bearing the photolabile oxalato-ligand. Here, we study the photoinduced elementary dynamics of the ferric complex, [FeIII (cyclam)(C2 O4 )]+ , in dimethyl sulfoxide solution using femtosecond mid-infrared spectroscopy following oxalate-to-iron charge transfer excitation with 266 nm pulses. The pump-probe response in the ν3 -region of carbon dioxide gives unequivocal evidence that a CO2 -molecule is detached from the metal within only 500 fs and with a primary quantum yield of 38%. Simultaneously, a primary ferrous product is formed that carries a carbon dioxide radical anion ligand absorbing at 1649 cm-1 , which is linked to the metal in a bent-O-"end-on" fashion. This primary ηO,bent 1 -product is formed with substantial excess vibrational energy, which relaxes on a time scale of several picoseconds. Prior to full thermalization, however, a fraction of the ferrous primary product can structurally isomerize at a rate of 1/(3.5 ps) to a secondary ηCO 2 -product absorbing at 1727 cm-1 , which features a bent carbon dioxide ligand that is linked to the metal in a "side-on" fashion. The ηO,bent 1 -to-ηCO 2 isomerization requires an intersystem crossing from the sextet to the quartet state, which rationalizes a partial trapping of the system in the metastable bent-O-"end-on" geometry. Finally, a fraction (62%) of the initially photoexcited complexes can return without structural changes to the parent's electronic ground state, but dressed with excess kinetic energy, which relaxes again on a time scale of several picoseconds.- Published
- 2021
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182. Intramolecular O-H⋯S hydrogen bonding in threefold symmetry: Line broadening dynamics from ultrafast 2DIR-spectroscopy and ab initio calculations.
- Author
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Brünker P, Domenianni LI, Fleck N, Lindner J, Schiemann O, and Vöhringer P
- Abstract
The dynamics of intramolecular hydrogen-bonding involving sulfur atoms as acceptors is studied using two-dimensional infrared (2DIR) spectroscopy. The molecular system is a tertiary alcohol whose donating hydroxy group is embedded in a hydrogen-bond potential with torsional C
3 -symmetry about the carbon-oxygen bond. The linear and 2DIR-spectra recorded in the OH-stretching region of the alcohol can be simulated very well using Kubo's line shape theory based on the cumulant expansion for evaluating the linear and nonlinear optical response functions. The correlation function for OH-stretching frequency fluctuations reveals an ultrafast component decaying with a time constant of 700 fs, which is in line with the apparent decay of the center line slopes averaged over absorption and bleach/emission signals. In addition, a quasi-static inhomogeneity is detected, which prevents the 2DIR line shape to fully homogenize within the observation window of 4 ps. The experimental data were then analyzed in more detail using a full ab initio approach that merges time-dependent structural information from classical molecular dynamics (MD) simulations with an OH-stretching frequency map derived from density functional theory (DFT). The latter method was also used to obtain a complementary transition dipole map to account for non-Condon effects. The 2DIR-spectra obtained from the MD/DFT method are in good agreement with the experimental data at early waiting delays, thereby corroborating an assignment of the fast decay of the correlation function to the dynamics of hydrogen-bond breakage and formation.- Published
- 2021
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183. Spin-Controlled Binding of Carbon Dioxide by an Iron Center: Insights from Ultrafast Mid-Infrared Spectroscopy.
- Author
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Straub S and Vöhringer P
- Abstract
The influence of the spin on the mode of binding between carbon dioxide (CO
2 ) and a transition-metal (TM) center is an entirely open question. Herein, we use an iron(III) oxalato complex with nearly vanishing doublet-sextet gap, and its ultrafast photolysis, to generate TM-CO2 bonding patterns and determine their structure in situ by femtosecond mid-infrared spectroscopy. The formation of the nascent TM-CO2 species according to [L4 FeIII (C2 O4 )]+ + hν → [L4 Fe(CO2 )]+ + CO2 , with L4 =cyclam, is evidenced by the coincident appearance of the characteristic asymmetric stretching absorption of the CO2 -ligand between 1600 cm-1 and 1800 cm-1 and that of the free CO2 -co-fragment near 2337 cm-1 . On the high-spin surface (S=5/2), the product complex features a bent carbon dioxide radical anion ligand that is O-"end-on"-bound to the metal. In contrast, on the intermediate-spin and low-spin surfaces, the product exhibits a "side-on"-bound, bent carbon dioxide ligand that has either a partial open-shell (for S=3/2) or fully closed-shell character (for S=1/2)., (© 2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)- Published
- 2021
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184. Probing the primary processes of a triazido-cobalt(III) complex with femtosecond vibrational and electronic spectroscopies. Photochemical selectivity and multi-state reactivity.
- Author
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Flesch S, Domenianni LI, and Vöhringer P
- Abstract
The elementary dynamics following 355 nm-excitation of the complex, mer-[Co(dien)(N3)3], were studied in liquid dimethyl sulfoxide (DMSO) solution using femtosecond-ultraviolet-pump/mid-infrared-to-near-ultraviolet probe spectroscopy in conjunction with electronic structure calculations based on density functional theory. Following the initial N3--to-Co charge transfer excitation, the parent complex undergoes an ultrafast metal-to-ligand back electron transfer (BET) within 2 ps thereby populating a metal-centered singlet excited state, 1MC, which can either repopulate the electronic ground state or cleave an azido ligand from the ligand sphere surrounding the metal center. From the asymptotic ground-state bleaching signal after 1 ns, a primary quantum yield for ligand loss of ca. 13% is estimated. The IR-spectrum of the product demonstrates that the photodissociation occurs selectively from the equatorial binding site thereby leading exclusively to the solvolysis product, mer-trans-[Co(dien)(N3)2(DMSO)]+, which features the solvent ligand in the equatorial coordination plane and the azides in the two axial positions. The remarkable photochemical selectivity is traced back to the initial BET and the nature of the intermediate state, 1MC, whose electronic structure entails occupancy of the σ-antibonding d(x2-y2)-orbital. A stereochemical scrambling at the stage of the primary penta-coordinated diazido product is kinetically inhibited on the singlet surface by an energy barrier of roughly 27 kJ mol-1. Primary penta-coordinated products that may be born on the triplet surface are funneled to their singlet ground-state preferentially from geometries with trans-oriented azido ligands thereby also preventing a stereochemical isomerization that could possibly arise from an intersystem crossing.
- Published
- 2020
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185. Vibrational Relaxation Dynamics of an Azido-Cobalt(II) Complex from Femtosecond UV-Pump/MIR-Probe Spectroscopy and Model Simulations with Ab Initio Anharmonic Couplings.
- Author
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Straub S, Stubbe J, Lindner J, Sarkar B, and Vöhringer P
- Abstract
Vibrational energy relaxation is of critical importance for the light-controlled reactivity of transition-metal complexes. In time-resolved optical spectroscopies, it gives rise to pronounced spectral redistributions with complex band shifts and thus to nonexponential kinetics, all of which are very difficult to quantify. Here we study the vibrational relaxation dynamics of a pentacoordinated azido-cobalt(II) complex in liquid solution following its ultrafast charge-transfer excitation in the near-ultraviolet (UV). The complex is photochemically remarkably stable and returns within the experimental time resolution back to its quartet electronic ground state via internal conversion. The nonadiabatic transition effectively instantaneously converts the entire photon energy into kinetic energy of the vibrational degrees of freedom. The ensuing relaxation dynamics of the vibrationally highly excited complex are monitored as a function of time using femtosecond mid-infrared (MIR) spectroscopy in the antisymmetric stretching region of the azido ligand and occur on a time scale of a few tens of picoseconds. The dynamic evolution of the MIR spectrum due to vibrational cooling of the complex can be understood quantitatively within the framework of an anharmonic coupling model, which relies on an ab initio intramolecular cubic/quartic force field from density functional theory combined with second-order vibrational perturbation theory. The simulations suggest that the primary internal conversion preferentially dumps the excess energy into the low-frequency bending modes of the azido ligand, whereas its high-frequency stretching modes are barely affected by the initial nonadiabatic transition. Surprisingly, the two bending vibrations appear to relax independently of one another, each with its own characteristic cooling time.
- Published
- 2020
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186. Vibrations tell the tale. A time-resolved mid-infrared perspective of the photochemistry of iron complexes.
- Author
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Vöhringer P
- Abstract
Owing to its superb structure-specificity, time-resolved infrared spectroscopy (TR-IR) has emerged in recent years as one of the most powerful tools for the exploration of photochemical dynamics in solution. It can be carried out in two distinct modes of operation: real-time Fourier-transform spectroscopy following flash photolysis or purely laser-based pump-probe spectroscopy. These two approaches combine to access nearly seamlessly a time window ranging from around hundred femtoseconds all the way up to hundreds of seconds. In this article, we highlight the most recent applications of these techniques in temporally resolving the multiscalar photochemical dynamics of iron complexes. The processes of interest here entail ligand dissociations or fragmentations, which lead to the formation of unique Fe-containing intermediates with remarkable electronic structure, exceptional molecular geometry, or unprecedented chemical reactivity. Thus, TR-IR can provide a novel unexpected view into the realm of iron chemistry including such fascinating topics like high-valent iron, ferracyclic compounds, or carbon dioxide binding to iron centers.
- Published
- 2020
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187. Time-resolved Fourier-transform infrared spectroscopy reveals the hidden bimolecular process of the ferrioxalate actinometer.
- Author
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Pilz FH, Lindner J, and Vöhringer P
- Abstract
Step-scan Fourier-transform infrared spectroscopy was used to monitor the photochemical reactions following the 266 nm-photolysis of aqueous ferrioxalate solutions on microsecond-to-millisecond time scales. Together with most recent observations from ultrafast infrared spectroscopy the reported results finally disclose the full molecular-level mechanism of a photochemical system that is widely known as the Hatchard-Parker actinometer.
- Published
- 2019
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188. Photoinduced Dynamics of a Diazidocobalt(III) Complex Studied by Femtosecond UV-Pump/IR-to-Vis-Probe Spectroscopy.
- Author
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Straub S, Domenianni LI, Lindner J, and Vöhringer P
- Abstract
The photochemistry of the cationic diazidocobalt(III) complex, trans -[Co(cyclam)(N
3 )2 ]+ , following its ligand-to-metal charge transfer (LMCT) excitation is studied in liquid dimethyl sulfoxide (DMSO) solution using femtosecond spectroscopy with detection in a very broad spectral region covering the near-ultraviolet (near-UV) all the way to the mid-infrared (MIR), thereby enabling a combined probing of electronic and vibrational degrees of freedom of the dynamically evolving system. The initially prepared singlet LMCT-state decays, via the metal-centered singlet excited state,1 MC(1 Eg ), into the triplet ground state,3 MC (3 Eg /3 A1g ), on a time scale shorter than 25 ps. During this time period, the vibrational spectrum demonstrates uniquely that the nature of the complex changes from a monoazidocobalt(II) species bearing a neutral azide radical ligand immediately after photon absorption to a metal-centered open-shell diazidocobalt(III) species. At the same time, the3 MC state is characterized by a very strong absorption band centered at 710 nm, which can be assigned to a transition to the triplet LMCT state. The1 LMCT lifetime is about 2 ps, whereas that of the excited state,1 MC, is defined by the primary intersystem crossing time of 6 ps. The ensuing intersystem recrossing from3 MC to the parent's singlet ground state,1 A1g , occurs with a rate of 1/(110 ps). The mid-infrared pump-probe spectrum after 1 ns, gives evidence for a heterolytic Co-N bond fission with a quantum yield of ∼5%, leading to free azide anions and the monoazido species, trans -[Co(cyclam)(N3 )(OSMe2 )]+ , featuring an oxygen-bound DMSO ligand in its coordination sphere.- Published
- 2019
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189. Molecular and electronic structure of an azidocobalt(iii) complex derived from X-ray crystallography, linear spectroscopy and quantum chemical calculations.
- Author
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Domenianni LI, Fligg R, Schäfermeier A, Straub S, Beerhues J, Sarkar B, and Vöhringer P
- Abstract
The photochemistry of transition-metal azides is remarkably complex and can involve multiple competing pathways leading to different fragmentation patterns. Therefore, an in-depth study of such rich photochemistry requires a thorough prior understanding of the molecular and electronic structures of these complexes. To this end, stationary (i.e. linear) spectroscopies in the ultraviolet-to-visible (UV/vis) and the mid-infrared (MIR) spectral regions are most often employed. Here, we investigate the electronic and vibrational spectroscopies of the cationic diazidocobalt(iii) complex, trans-[Co(cyclam)(N3)2]+, in liquid dimethyl sulfoxide (DMSO) solution and interpret the experimental data in terms of detailed quantum chemical calculations. The X-ray crystallography reveals a Ci-symmetric molecular structure of the complex whereas in liquid solution, evidence for symmetry breakage with loss of the inversion center of the ligand sphere is found from both, the UV/vis and MIR-data. This interpretation is fully corroborated by a stereochemical and conformational analysis of the complex using ab initio calculations involving nuclear degrees of freedom of both, the equatorial cyclam ancillary ligand and the two axial azido ligands.
- Published
- 2019
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190. Probing the band gap of liquid ammonia with femtosecond multiphoton ionization spectroscopy.
- Author
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Vogler T and Vöhringer P
- Abstract
The electronic band gap, i.e. the energy difference between the top of the valence band and the bottom of the conduction band, is widely recognized as the key property characterizing the electronic structure of bulk liquids and liquid solvents like water or ammonia. Here, the band gap of liquid ammonia at 270 K and 300 bar was studied with 2-photon ionization spectroscopy using the solvated electron primary yield as a near-infrared action-spectroscopic probe. The experimentally determined escape probability, which is the fraction of solvated electrons that is able to avoid geminate recombination within the first nanosecond after ionization, was used to extract a value of -(1.27 ± 0.03) eV for the vertical electron affinity of the liquid.
- Published
- 2018
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191. Femtosecond infrared spectroscopy reveals the primary events of the ferrioxalate actinometer.
- Author
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Straub S, Brünker P, Lindner J, and Vöhringer P
- Abstract
Chemical actinometry is an indispensable analytical tool in preparative photochemistry that allows for a precise measurement of radiant fluxes inside photoreactors. An actinometer thus enables an absolute determination of the quantum yield of a photochemical reaction of interest. The "gold standard" of chemical actinometry in liquid systems is the Hatchard-Parker actinometer, i.e. an aqueous solution of potassium trisoxalatoferrate(iii), which is based on the light-induced net transformation of ferric into ferrous oxalate complexes. Although the absolute photochemical quantum yield for this fundamental standard system has been accurately known for many years, the underlying molecular-level mechanisms and time scales associated with a photoreduction of the ferrioxalate actinometer remained so far largely obscured. Here, we use femtosecond mid-infrared spectroscopy combined with ultrafast laser photolysis to obtain unique structural-dynamical information associated with the primary light-triggered processes thereby finally providing the missing quantitative molecular-level foundations that ultimately justify a utilization of aqueous ferrioxalate as a true photochemical standard. Following photon absorption by the octahedral parent complex, an ultrafast decarboxylation occurs within 500 fs, which generates a penta-coordinated ferrous dioxalate that carries a bent carbon dioxide radical anion ligand in an "end-on" O-coordinated fashion. This unique intermediate structurally isomerizes on a tens of picoseconds time scale and subsequently loses a CO2˙--ligand to form a square-planar bisoxalatoferrate(ii) on a hundreds of picoseconds time scale.
- Published
- 2018
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192. An Iron Complex with a Bent, O-Coordinated CO 2 Ligand Discovered by Femtosecond Mid-Infrared Spectroscopy.
- Author
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Straub S, Brünker P, Lindner J, and Vöhringer P
- Abstract
The activation of carbon dioxide by transition metals is widely recognized as a key step for utilizing this greenhouse gas as a renewable feedstock for the sustainable production of fine chemicals. However, the dynamics of CO
2 binding and unbinding to and from the ligand sphere of a metal have never been observed in the time domain. The ferrioxalate anion is used in aqueous solution as a unique model system for these dynamics and femtosecond UV-pump mid-infrared-probe spectroscopy is applied to explore its photoinduced primary processes in a time-resolved fashion. Following optical excitation, a neutral CO2 molecule is expelled from the complex within about 500 fs to generate a highly intriguing pentacoordinate ferrous dioxalate that carries a bent carbon dioxide radical anion ligand, that is, a reductively activated form of CO2 , which is end-on-coordinated to the metal center by one of its two oxygen atoms., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)- Published
- 2018
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193. Photolysis of a High-Spin Azidoiron(III) Complex Studied by Time-Resolved Fourier-Transform Infrared Spectroscopy.
- Author
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Torres-Alacan J and Vöhringer P
- Abstract
The laser-flash photolysis of the high-spin azidoiron(III) complex [Fe
III (Me3 Cyclam-ac)(N3 )]PF6 ([1]PF6 ) in liquid acetonitrile solution at room temperature was explored by time-resolved Fourier-transform infrared spectroscopy. Excitation of [1] at 480 and 266 nm induced a photoreduction of the metal center and generated [FeII (Me3 Cyclam-ac)(NCCH3 )]+ ([2]) and azidyl radicals. Both photoproducts were detected independently through scavenging experiments. The metal-containing fragment was quenched with carbon monoxide to generate an iron(II) carbonyl complex, whereas the nitrogen-containing fragment was quenched with iodide to form azide anions. In the presence of N3 - , the photoreduction created the elusive hexanitrogen radical anion N6 .- as a transient byproduct., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)- Published
- 2017
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194. Ultrafast 2DIR spectroscopy of ferric azide precursors for high-valent iron. Vibrational relaxation, spectral diffusion, and dynamic symmetry breaking.
- Author
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Czurlok D, Torres-Alacan J, and Vöhringer P
- Subjects
- Diffusion, Spectrophotometry, Infrared, Time Factors, Vibration, Azides chemistry, Ferric Compounds chemistry, Molecular Dynamics Simulation
- Abstract
Femtosecond mid-infrared pump-probe and two-dimensional mid-infrared spectroscopy have been used to investigate the dynamics of vibrational relaxation and vibrational spectral diffusion of the asymmetric N3-stretching vibration of pseudo-octahedral azidoiron(III) complexes, [L6-nFe(N3)n](+) with n = 1 or 2 and L being an auxiliary ligand of denticity 6-n, in acetonitrile at room temperature. Compared to the free azide anion in acetonitrile solution, the vibrational relaxation dynamics are considerably accelerated. Vibrational energy transfer to the solvent is accelerated by virtue of a resonance with an overtone transition of the solvent. Intramolecular vibrational redistribution is found to be accelerated by virtue of a coupling between the initial azide stretching vibration and the torsional modes involving the axial ligands. Vibrational spectral diffusion within the asymmetric N3-stretching resonance was found to be insensitive to solvent fluctuations because the axial azide ligands are only partially accessible to the solvent. The particular role of intramolecular structural relaxations of the complex for shaping the linear and nonlinear two-dimensional infrared spectra is discussed in terms of ultrafast symmetry-breaking torsional fluctuations and on the basis of density functional theory calculations.
- Published
- 2015
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195. Below-band-gap ionization of liquid-to-supercritical ammonia: geminate recombination via proton-coupled back electron transfer.
- Author
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Urbanek J and Vöhringer P
- Abstract
Femtosecond multiphoton ionization experiments have been conducted on ammonia over a wide range of temperature (225 K ≤ T ≤ 490 K) and density (0.18 g/cm(3) ≤ ρ ≤ 0.7 g/cm(3)), thereby covering the liquid and supercritical phases. The experiments were carried out with excitation pulses having a wavelength of 400 nm, and the ionization was found to involve two photons. Therefore, the total ionization energy in this study corresponds to 6.2 eV, which is roughly 2 eV below the valence-to-conduction band gap of the fluid. The ionization generates solvated electrons, which have been detected through their characteristic near-infrared resonance, and must be facilitated through a coupling to nuclear degrees of freedom of the liquid. The recombination of the solvated electron with the geminate fragments was found to obey predominantly single-exponential kinetics with time constants between 500 fs and 1 ps. Only a very minor fraction of the photogenerated electrons is able to escape from the geminate recombination. The results indicate that the majority of electrons are injected into suitable trapping sites located between the first and second solvation shells of the initially ionized ammonia molecules. Such configurations can be considered as instantly reactive and facilitate an ultrafast barrierless electron annihilation. This process is found to exhibit a pronounced kinetic isotope effect, which indicates that the electronic decay is accompanied by the transfer of a proton. The sequence of ionization and recombination events can therefore be described appropriately as a proton-coupled electron transfer (PCET) followed by a proton-coupled back electron transfer (PCBET).
- Published
- 2014
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196. The photochemistry of [Fe(III)N3(cyclam-ac)]PF6 at 266 nm.
- Author
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Torres-Alacan J, Krahe O, Filippou AC, Neese F, Schwarzer D, and Vöhringer P
- Subjects
- Crystallography, X-Ray, Models, Molecular, Molecular Conformation, Molecular Structure, Spectroscopy, Fourier Transform Infrared, Ferric Compounds chemistry, Heterocyclic Compounds chemistry, Photochemistry methods
- Abstract
The photochemistry of iron azido complexes is quite challenging and poorly understood. For example, the photochemical decomposition of [Fe(III)N(3)(cyclam-ac)]PF(6) ([1]PF(6)), where cyclam-ac represents the 1,4,8,11-tetraazacyclotetradecane-1-acetate ligand, has been shown to be wavelength-dependent, leading either to the rare high-valent iron(V) nitrido complex [Fe(V)N(cyclam-ac)]PF(6) ([3]PF(6)) after cleavage of the azide N(α)-N(β) bond, or to a photoreduced Fe(II) species after Fe-N(azide) bond homolysis. The mechanistic details of this intriguing reactivity have never been studied in detail. Here, the photochemistry of 1 in acetonitrile solution at room temperature has been investigated using step-scan and rapid-scan time-resolved Fourier transform infrared (FTIR) spectroscopy following a 266 nm, 10 ns pulsed laser excitation. Using carbon monoxide as a quencher for the primary iron-containing photochemical product, it is shown that 266 nm excitation of 1 results exclusively in the cleavage of the Fe-N(azide) bond, as was suspected from earlier steady-state irradiation studies. In argon-purged solutions of [1]PF(6), the solvent-stabilized complex cation [Fe(II)(CH(3)CN)(cyclam-ac)](+) (2red) together with the azide radical (N(3)(.)) is formed with a relative yield of 80%, as evidenced by the appearance of their characteristic vibrational resonances. Strikingly, step-scan experiments with a higher time resolution reveal the formation of azide anions (N(3)(-)) during the first 500 ns after photolysis, with a yield of 20%. These azide ions can subsequently react thermally with 2red to form [Fe(II)N(3)(cyclam-ac)] (1red) as a secondary product of the photochemical decomposition of 1. Molecular oxygen was further used to quench 1red and 2red to form what seems to be the elusive complex [Fe(O(2))(cyclam-ac)](+) (6)., (Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)
- Published
- 2012
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197. Femtosecond spectroscopy of solvated electrons from sodium-ammonia-d3 solutions: temperature jump versus local density jump.
- Author
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Lindner J, Unterreiner AN, and Vöhringer P
- Abstract
The relaxation dynamics of solvated electrons from sodium-ammonia-d3 solutions was studied by femtosecond time-resolved near-infrared spectroscopy. The experimental pump-probe data reveal a pulse-width limited pump-induced redshift of the absorption spectrum of the ammoniated electron and a subsequent slower blueshift on a time scale of roughly 200 fs. The spectrotemporal response is interpreted using the nonadiabatic relaxation mechanism for cavity-bound solvated electrons in condensed phases. In particular, we develop a local density-jump model, which traces the dynamic spectrum back to a sequence of a pump-induced cavity expansion due to Pauli repulsion and a succeeding cavity contraction upon nonadiabatic return of the electron back to its ground state. Using the existing thermodynamic data of the solvent and experimental temperature and density-dependent absorption spectra of metal-ammonia solutions, an overall increase in the interparticle distance within the solvent cavity of 25% is crudely estimated. The density-jump model is compared to the temperature-jump model we proposed previously for the femtosecond relaxation dynamics of metal-NH(3) solutions.
- Published
- 2008
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198. Ultrafast energy transfer in water-AOT reverse micelles.
- Author
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Cringus D, Bakulin A, Lindner J, Vöhringer P, Pshenichnikov MS, and Wiersma DA
- Subjects
- Emulsions, Hydrogen Bonding, Kinetics, Solubility, Solutions chemistry, Spectrum Analysis, Dioctyl Sulfosuccinic Acid chemistry, Energy Transfer, Micelles, Nanostructures chemistry, Surface-Active Agents chemistry, Water chemistry
- Abstract
A spectroscopic investigation of the vibrational dynamics of water in a geometrically confined environment is presented. Reverse micelles of the ternary microemulsion H2O/AOT/n-octane (AOT = bis-2-ethylhexyl sulfosuccinate or aerosol-OT) with diameters ranging from 1 to 10 nm are used as a model system for nanoscopic water droplets surrounded by a soft-matter boundary. Femtosecond nonlinear infrared spectroscopy in the OH-stretching region of H2O fully confirms the core/shell model, in which the entrapped water molecules partition onto two molecular subensembles: a bulk-like water core and a hydration layer near the ionic surfactant headgroups. These two distinct water species display different relaxation kinetics, as they do not exchange vibrational energy. The observed spectrotemporal ultrafast response exhibits a local character, indicating that the spatial confinement influences approximately one molecular layer located near the water-amphiphile boundary. The core of the encapsulated water droplet is similar in its spectroscopic properties to the bulk phase of liquid water, i.e., it does not display any true confinement effects such as droplet-size-dependent vibrational lifetimes or rotational correlation times. Unlike in bulk water, no intermolecular transfer of OH-stretching quanta occurs among the interfacial water molecules or from the hydration shell to the bulk-like core, indicating that the hydrogen bond network near the H2O/AOT interface is strongly disrupted.
- Published
- 2007
- Full Text
- View/download PDF
199. OH-stretch vibrational relaxation of HOD in liquid to supercritical D2O.
- Author
-
Schwarzer D, Lindner J, and Vöhringer P
- Subjects
- Deuterium, Half-Life, Hydrogen Bonding, Kinetics, Models, Chemical, Temperature, Deuterium Oxide chemistry, Spectrophotometry, Infrared methods, Vibration, Water chemistry
- Abstract
The population relaxation of the OH-stretching vibration of HOD diluted in D2O is studied by time-resolved infrared (IR) pump-probe spectroscopy for temperatures of up to 700 K in the density range 12
1 OH stretching transition with a 200 fs laser pulse centered at approximately 3500 cm(-1). Above 400 K these spectra show no indication of spectral diffusion after pump-probe delays of 0.3 ps. Over nearly the entire density range and for sufficiently high temperatures (T > 360 K), the vibrational relaxation rate constant, kr, is strictly proportional to the dielectric constant, epsilon, of water. Together with existing molecular dynamics simulations, this result suggests a simple linear dependence of kr on the number of hydrogen-bonded D2O molecules. It is shown that, for a given temperature, an isolated binary collision model is able to adequately describe the density dependence of vibrational energy relaxation even in hydrogen-bonded fluids. However, dynamic hydrogen bond breakage and formation is a source of spectral diffusion and affects the nature of the measured kr. For sufficiently high temperatures when spectral diffusion is much faster than energy transfer, the experimentally observed decays correspond to ensemble averaged population relaxation rates. In contrast, when spectral diffusion and vibrational relaxation occur on similar time scales, as is the case for ambient conditions, deviations from the linear kr(epsilon) relation occur because the long time decay of the v = 1 population is biased to slower relaxing HOD molecules that are only weakly connected to the hydrogen bond network. - Published
- 2006
- Full Text
- View/download PDF
200. The coming of age.
- Author
-
Haran G and Vöhringer P
- Subjects
- Green Fluorescent Proteins chemistry, Humans, Magnetic Resonance Spectroscopy, Models, Molecular, Protein Conformation, Radiation, Spectrophotometry methods, Spectrum Analysis, Raman, Biophysics methods, Chemistry, Physical methods
- Published
- 2005
- Full Text
- View/download PDF
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