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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. [ABSTRACT FROM AUTHOR]

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 (ν3) 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 H2O. 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 H2O. 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. [ABSTRACT FROM AUTHOR]

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 C3-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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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]

The population relaxation of the OD stretching vibration of HOD diluted in H2O is studied by time-resolved infrared pump-probe spectroscopy for temperatures between 278 and 663 K in the density range 0.28≤ρ≤1.01 g/cm3. Transient spectra recorded after exciting the v=0→1 OD stretching transition at low temperatures show a delay between excited state decay and formation of the thermalized spectrum pointing to an intermediately populated state. Above 400 K, the rates of excited state decay and ground state recovery become equivalent and the intermediate state is not detectable anymore. Over the entire thermodynamic range, the derived OD stretch relaxation rate constant kr depends linearly on the static dielectric constant [variant_greek_epsilon] of water, indicating a correlation of kr with the average hydrogen bond connectivity of HOD within the H2O network. However, in contrast to the OH stretch relaxation rate constant of the complementary system of HOD in D2O, the low density data of kr([variant_greek_epsilon]) extrapolate to a nonzero intercept for [variant_greek_epsilon]→1. Our analysis suggests that at ambient conditions the OD excited state is mainly depopulated by a direct v=1→0 transition, avoiding the excited v=1 HOD bending state. Therefore, at room temperature the detected intermediate is assigned to a nonthermalized state with respect to nuclear degrees of freedom of the solvent molecules, and subsequent formation of the final product spectrum is related to a rearrangement of the hydrogen bond network. Passing over to the gas phase the excited OD stretch state shifts into close resonance with the HOD bend overtone, thereby opening up an additional relaxation channel. [ABSTRACT FROM AUTHOR]

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-NH3 solutions. [ABSTRACT FROM AUTHOR]

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]

The dynamics of product vibrational deactivation and subsequent geminate recombination of diiodide ions with atomic iodine following 400-nm photolysis of triiodide in ethanol solution has been studied using femtosecond transient absorption spectroscopy. The excess vibrational energy of the diatomic product was found to decay on two distinct time scales. An ultrafast subpicosecond component, which accounts for the dissipation of most of the energy that is initially deposited into fragment vibrations, is followed by thermalization near the bottom of the I[SUP-,SUB2]potential on a time scale of several picoseconds. The former process is associated with recoil of the fragments in the exit channel of the potential energy surface relevant to bond breakage whereas the latter process represents relaxation in the asymptotic limit where interaction between the atom-diatom fragments becomes negligible. Transient product vibrational distributions are determined for delay times larger than the dephasing time of nuclear coherences in the diiodide product ions, thereby providing new information about the mechanism for bond fission. These product distributions are translated into energy-time profiles which are analyzed by a master-equation approach using various model functions for the power spectrum of solvent forces acting on the I[SUP-,SUB2]vibrational coordinate. The dynamics of geminate recombination are found to exhibit a strongly nonexponential character and are interpreted with a simple diffusion model that takes the initial stages of bond breakage and recoil of the fragments into account. [ABSTRACT FROM AUTHOR]

A rigorous theoretical connection between the polarizability spectral density obtained from optical Kerr effect (OKE) measurement with the correlation function describing solvent-induced optical dephasing detected in photon echo measurements is given. The experimentally obtained spectral density has a more reasonable physical basis than model correlation function descriptions of solvent fluctuations. The experimental OKE spectrum is demonstrated to provide a natural description of solvent motions that modulate the chromophore electronic states in the case of weak induced-dipolar interactions. The chromophore optically active vibrational modes are obtained from pump-probe spectra and are employed in the calculation of echo signals. It is found that the fast decays of the two-pulse echo signals result from both solvent and solute intramolecular motions while the echo peak shifts are dominated by the solvent intermolecular modes. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The population relaxation of the OD stretching vibration of HOD diluted in H(2)O is studied by time-resolved infrared pump-probe spectroscopy for temperatures between 278 and 663 K in the density range 0.281 OD stretching transition at low temperatures show a delay between excited state decay and formation of the thermalized spectrum pointing to an intermediately populated state. Above 400 K, the rates of excited state decay and ground state recovery become equivalent and the intermediate state is not detectable anymore. Over the entire thermodynamic range, the derived OD stretch relaxation rate constant k(r) depends linearly on the static dielectric constant epsilon of water, indicating a correlation of k(r) with the average hydrogen bond connectivity of HOD within the H(2)O network. However, in contrast to the OH stretch relaxation rate constant of the complementary system of HOD in D(2)O, the low density data of k(r)(epsilon) extrapolate to a nonzero intercept for epsilon-->1. Our analysis suggests that at ambient conditions the OD excited state is mainly depopulated by a direct v=1-->0 transition, avoiding the excited v=1 HOD bending state. Therefore, at room temperature the detected intermediate is assigned to a nonthermalized state with respect to nuclear degrees of freedom of the solvent molecules, and subsequent formation of the final product spectrum is related to a rearrangement of the hydrogen bond network. Passing over to the gas phase the excited OD stretch state shifts into close resonance with the HOD bend overtone, thereby opening up an additional relaxation channel.

The pressure and temperature-dependent linear absorption spectrum of partially deuterated water HOD dissolved in heavy water D(2)O was measured in the OH-stretching spectral region. The temperature was varied in the interval of 298 K

Femtosecond midinfrared pump-probe spectroscopy was carried out to explore the dynamics of vibrational energy relaxation of NH(2)D in fluid ammonia NH(3). The ND-stretching fundamental of the partially deuterated solute NH(2)D 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 (NH(3)) onto the vibrationally excited solute (NH(2)D).

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 by femtosecond mid-infrared spectroscopy. Using the dielectric constant of the fluid both, the OH-stretching absorption frequency and 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.