184 results on '"Vöhringer, Peter"'
Search Results
152. Vertical Photoionization of Liquid-to-SupercriticalAmmonia: Thermal Effects on the Valence-to-Conduction Band Gap.
- Author
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Urbanek, Janus and Vöhringer, Peter
- Subjects
- *
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
- Full Text
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153. Zweidimensionale IR-Spektroskopie und H-Brücken.
- Author
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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
- Published
- 2012
- Full Text
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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.
- Author
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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]
- Published
- 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.
- Author
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Vennekate, Hendrik, Schwarzer, Dirk, Torres-Alacan, Joel, and Vöhringer, Peter
- Subjects
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.
- 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., Cringus, Dan, Pshenichnikov, Maxim S., Wiersma, Douwe A., Mostovoy, Maxim, Lindner, Jörg, and Vöhringer, Peter
- Abstract
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.
- 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, 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).
- Published
- 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
- Full Text
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163. Elementary solute-solvent interactions and the photophysical properties of photoacids
- Author
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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
164. 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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165. 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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166. 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.)
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- 2023
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167. Ultrafast Dynamics of Photochemical Nitrile Imine Formation.
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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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168. Photo-Initiated Cobalt-Catalyzed Radical Olefin Hydrogenation.
- Author
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Sang S, Unruh T, Demeshko S, Domenianni LI, van Leest NP, Marquetand P, Schneck F, Würtele C, de Zwart FJ, de Bruin B, González L, Vöhringer P, and Schneider S
- 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 H
2 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., (© 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)- Published
- 2021
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169. 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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170. Photochemical Kinetics of a Phosphine Oxide Free Radical Initiator from Femtosecond UV-Pump/Mid-IR-Probe Spectroscopy.
- Author
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Straub S, Lindner J, and Vöhringer P
- Abstract
Femtosecond UV-pump/mid-infrared-probe spectroscopy was used to explore in detail the primary photochemical events of the free radical initiator, (2,4,6-trimethylbenzoyl)diphenylphosphine oxide, in liquid dichloromethane solution at room temperature. Following electronic excitation of its lowest excited singlet state, S
1 , the radical initiator undergoes an intersystem crossing to the triplet ground state, T1 , with a time constant of 135 ps. A subsequent α-cleavage occurs from the triplet state with a time constant of 15 ps and yields a trimethylbenzoyl radical together with a diphenylphosphinoyl radical. Transient absorptions from the S1 and T1 states are observed that can be assigned to the P═O stretching mode and the symmetric in-plane deformation mode of the trimethylphenyl moiety of the radical initiator.- Published
- 2017
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171. The Femtochemistry of a Ferracyclobutadiene.
- Author
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Wezisla B, Lindner J, Das U, Filippou AC, and Vöhringer P
- Abstract
The eminent role of metallacyclobutadienes as catalytic intermediates in organic synthesis and polymer chemistry is widely acknowledged. In contrast, their photochemistry is as yet entirely unexplored. Herein, the photo-induced primary processes of a ferracyclobutadiene tricarbonyl complex in solution are revealed by femtosecond mid-infrared spectroscopy. The time-resolved vibrational spectra expose an ultrafast substitution of a basal CO ligand by a solvent molecule in a consecutive dissociation-association mechanism. Following optical excitation, the system relaxes non-radiatively to the triplet ground state from which a CO is expelled. Since the triplet state is bound with respect to Fe-CO cleavage, the dissociation can only occur from vibrationally excited states. The excitation energy, vibrational relaxation, and intersystem crossing to the singlet ground state control the primary quantum yield for formation of the ferracyclic dicarbonyl-solvent product complex., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)
- Published
- 2017
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172. Photolysis of a High-Spin Azidoiron(III) Complex Studied by Time-Resolved Fourier-Transform Infrared Spectroscopy.
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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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173. Photochemistry of a Puckered Ferracyclobutadiene in Liquid Solution Studied by Time-Resolved Fourier-Transform Infrared Spectroscopy.
- Author
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Torres-Alacan J, Das U, Wezisla B, Straßmann M, Filippou AC, and Vöhringer P
- Abstract
Flash photolysis combined with step-scan and rapid-scan Fourier-transform infrared spectroscopy was carried out to explore the photochemistry of a puckered, quasi-square pyramidal ferracyclobutadiene, [Fe{κ(2) -C3 (NEt2 )3 }(CO)3 ]BF4 ([1]BF4 ), that features three additional carbonyl ligands in the metal coordination sphere. In liquid solution at room temperature, an excitation with λ=355 nm light resulted in the loss of one CO ligand, which is cleaved from a basal metal-coordination site. Within the time resolution of the experiment, a solvent molecule promptly refills the resultant vacancy at the coordinatively unsaturated metal center. In the weakly interacting liquid, dichloromethane, the counterion of the complex is subsequently able to substitute the solvent in the coordination sphere of the iron center, thereby forming as a stable product a neutral dicarbonyl tetrafluoroborato iron(0) species containing a four-membered ferracycle., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)
- Published
- 2015
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174. 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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175. 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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176. Observing the formation and the reactivity of an octahedral iron(V) nitrido complex in real time.
- Author
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Torres-Alacan J, Das U, Filippou AC, and Vöhringer P
- Subjects
- Magnetic Resonance Spectroscopy, Photochemical Processes, Spectrophotometry, Ultraviolet, Spectroscopy, Fourier Transform Infrared, Ferric Compounds chemistry
- Abstract
Give me five: Time-resolved Fourier-transform IR spectroscopy is used to time-resolve the formation and the reaction dynamics of a fourfold symmetrical nitrido iron(V) complex (light blue C, red Fe, blue N) in liquid solution under physiological and technologically relevant conditions., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)
- Published
- 2013
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177. From single hydrogen bonds to extended hydrogen-bond wires: low-dimensional model systems for vibrational spectroscopy of associated liquids.
- Author
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Olschewski M, Knop S, Lindner J, and Vöhringer P
- Abstract
It is fair to say that if we ever wish to understand the anomalous properties of water, we need to study hydrogen bonds. Such a statement is based on statistical mechanics, which tells us how to calculate the structure and the thermodynamic properties of fluids and dense liquids from the forces between the particles. However, in the case of complex associated liquids, such calculations present a formidable--if not even insurmountable--challenge, which largely reflects our still-limited understanding of the hydrogen-bonding phenomenon itself. More experimental research on hydrogen-bonded systems is required to develop a comprehensive, satisfactory theory for associated liquids. This Review gives an introduction to the latest experimental technique currently being used to study the ultrafast structural dynamics of hydrogen bonds, namely two-dimensional infrared spectroscopy, and its applications to hydrogen-bonded systems of systematically increasing complexity, starting from the single hydrogen bond of a diol to low-dimensional extended networks of stereoselectively synthesized polyalcohols., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)
- Published
- 2013
- Full Text
- View/download PDF
178. A hydrogen-bond flip-flop through a Bjerrum-type defect.
- Author
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Olschewski M, Lindner J, and Vöhringer P
- Subjects
- Kinetics, Models, Molecular, Protons, Hydrogen Bonding, Spectrophotometry, Infrared methods
- Abstract
Back and forth: Femtosecond two-dimensional infrared exchange spectroscopy was used to study the dynamics of the reversal of an intramolecular hydrogen bond. The H-bond reversal resembles a flip-flop motion that is facilitated by two concerted disrotatory torsional isomerizations and that occurs on a time scale of about 2 ps., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)
- Published
- 2013
- Full Text
- View/download PDF
179. 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
- Full Text
- View/download PDF
180. Vibrational relaxation of azide ions in liquid-to-supercritical water.
- Author
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Olschewski M, Knop S, Lindner J, and Vöhringer P
- 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 ν(3) 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., (© 2011 American Institute of Physics)
- Published
- 2011
- Full Text
- View/download PDF
181. 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
- Full Text
- View/download PDF
182. 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
183. OH-stretch vibrational relaxation of HOD in liquid to supercritical D2O.
- Author
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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
184. The coming of age.
- Author
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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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