Tobias Steinel

Spectral diffusion in a fluctuating charge model of water.

We apply the combined electronic structure/molecular dynamics approach of Corcelli, Lawrence, and Skinner [J. Chem. Phys. 120, 8107 (2004)] to the fluctuating charge (SPC-FQ) model of liquid water developed by Rick, Stuart, and Berne [J. Chem. Phys. 101, 6141 (1994)]. For HOD in H(2)O the time scale for the long-time decay of the OD stretch frequency time-correlation function, which corresponds to the time scale for hydrogen-bond rearrangement in the liquid, is about 1.5 ps. This result is significantly longer than the 0.9 ps decay previously calculated for the nonpolarizable SPC/E water model. Our results for the SPC-FQ model are in better agreement with recent vibrational echo experiments.

Hydrogen bond breaking probed with multidimensional stimulated vibrational echo correlation spectroscopy

H bond population dynamics are extricated with exceptional detail using ultrafast (<50 fs) IR multidimensional stimulated vibrational echo correlation spectroscopy with full phase information and frequency resolved IR pump-probe expts. performed on the hydroxyl stretch of MeOH-OD oligomers in CCl4. H bond breaking makes it possible to acquire data for times much greater than the hydroxyl stretch vibrational lifetime. The correlation spectra and detailed calcns. demonstrate that vibrational relaxation leads to H bond breaking for oligomers that have hydroxyl stretch frequencies on the low energy (red) side of the hydroxyl stretch spectrum, the spectral region that is assocd. with the strongest H bonds. Frequency resolved pump-probe data support the conclusions drawn from the correlation spectra. Using a global fit to the pump-probe spectra, in conjunction with assignments made possible through the correlation spectra, the residual ground state and photoproduct of H bond breaking were prepd. near their thermal equil. distribution. The spectrum of the H bond breaking photoproduct and the residual ground state approach the steady-state temp. difference spectrum on the tens of picoseconds time scale, indicating the system thermalizes on this time scale. [on SciFinder(R)]

Ultrafast heterodyne detected infrared multidimensional vibrational stimulated echo studies of hydrogen bond dynamics

Multidimensional vibrational stimulated echo correlation spectra with full phase information are presented for the broad hydroxyl stretch band of methanol-OD oligomers in CCl4 using ultrashort ( 1 ps) shows that there is frequency correlation between the initially excited hydroxyl stretch and the frequency shifted hydroxyl stretch formed by hydrogen bond breaking.

Accidental vibrational degeneracy in vibrational excited states observed with ultrafast two-dimensional IR vibrational echo spectroscopy

The coupling between the OD stretch v=2 level and benzene-ring modes in 2-methoxyphenol-OD (hydroxyl H replaced by D) is observed with ultrafast two-dimensional (2D) IR vibrational echo spectroscopy. Because of this coupling, the 1-2 transition peak in the 2D spectrum is split into a doublet with peaks of approximately equal amplitudes. Several molecules and solvents were used to study this phenomenon. Near-IR (NIR) spectroscopy measurements and density-functional theory calculations (B3LYP6-31+G(d,p) level) were also applied. Experimental results and calculations show that the OD stretch 1-2 transition is coupled to a combination band related to the benzene-ring motions. A simple quantum-mechanical model indicates that the combination band has a frequency of 5172 and 5176.5 cm(-1) in CCl4 and hexane, respectively. The transition between this combination band and the ground state is too weak to detect by NIR. The transition between this band and the OD stretch first excited state is also so weak that most of the intensity of the doublet comes from the oscillator strength produced by coupling to the OD stretch. The model gives the coupling strengths as 6.5 and 7 cm(-1) in CCl4 and hexane, respectively.

Vibrational Echo Correlation Spectroscopy

Multidimensional vibrational echo correlation spectroscopy with full phase resolution is used to measure hydrogen bond dynamics in water and methanol. The OD hydroxyl stretches of methanol-OD oligomers in CCl4 and HOD inH2O are studied using the shortest mid-IR pulses ( 400 cm-1) spectrum of the 0-1 and 1-2 vibrational transitions. Hydrogen bond population dynamics are extricated with exceptional detail in MeOD oligomers because the different hydrogen-bonded species are spectrally distinct. The experimental results along with detailed calculations indicate the strongest hydrogen bonds are selectively broken through a non-equilibrium relaxation pathway following vibrational relaxation of the hydroxyl stretch. Following hydrogen bond breaking, the broken MeOD oligomers retain a detailed structural memory of the prior intact hydrogen bond network. The correlation spectra are also a sensitive probe of the structural fluctuations in water and provide a stringent test of water models that are widely used in simulations of aqueous systems. The analysis of the 2D band shapes demonstrates that different hydrogen-bonded species are subject to distinct (wavelength-dependent) ultrafast (∼ 100 fs) local fluctuations and essentially identical slower (0.4 ps and ∼ 2 ps) structural rearrangements. Observation of wavelength-dependent dynamics demonstrates that standard theoretical approaches assuming Gaussian fluctuations cannot adequately describe water dynamics.