IR and Raman spectra of liquid water: Theory and interpretation

Abstract

IR and Raman (parallel- and perpendicular-polarized) spectra in the OH stretch region for liquid water were measured some years ago, but their interpretation is still controversial. In part, this is because theoretical calculation of such spectra for a neat liquid presents a formidable challenge due to the coupling between vibrational chromophores and the effects of motional narrowing. Recently we proposed an electronic structure/molecular dynamics method for calculating spectra of dilute HOD in liquid D(2)O, which relied on ab initio calculations on clusters to provide a map from nuclear coordinates of the molecules in the liquid to OH stretch frequencies, transition dipoles, and polarizabilities. Here we extend this approach to the calculation of couplings between chromophores. From the trajectories of the fluctuating local-mode frequencies, transition moments, and couplings, we use our recently developed time-averaging approximation to calculate the line shapes. Our results are in good agreement with experiment for the IR and Raman line shapes, and capture the significant differences among them. Our analysis shows that while the coupling between chromophores is relatively modest, it nevertheless produces delocalization of the vibrational eigenstates over up to 12 chromophores, which has a profound effect on the spectroscopy. In particular, our results demonstrate that the peak in the parallel-polarized Raman spectrum at about 3250 wavenumbers is collective in nature.

Dissecting the THz spectrum of liquid water from first principles via correlations in time and space

“…This finding clearly supports that the observed dipole correlations in the first solvation shell are largely imprinted by correlated electronic polarization effects and not due to correlated particle motion at intramolecular frequencies. This result not only supports earlier findings (39,40) concerning the OH-stretching IR band (where this mode, however, is termed delocalized or collective) but also provides the space-resolved decay of both the underlying dipole and particle correlations.…”

Revealing structural properties of the marine nanolayer from vibrational sum frequency generation spectra

“…A comparison with selected literature data is presented in Figure S3 in the auxiliary material. Relying on ab initio calculations of water clusters (up to n = 10 atoms), the two-peak feature has been assigned to hydrogen bonds in different coordination geometries [Sadlej et al, 1999;Buch, 2005;Auer and Skinner, 2008]. This conclusion has been sharpened for a water surface layer to an interpretation as differently ordered structures termed "ice like" (≈3200 cm −1 ) and "liquid like" (≈3400 cm −1 ) [Shen and Ostroverkhov, 2006].…”

Coherent infrared multidimensional spectra of the OH stretching band in liquid water simulated by direct nonlinear exciton propagation

“…The coupling between the two OH stretching modes in a water is J ϳ 30 cm −1 . 43 ͑For the amide-I vibrations in peptide we have 0 ϳ 1650 cm −1 and J ϳ 10 cm −1 .͒ By taking the time step ⌬ to be small compared to the bath dephasing ⌬ Ӷ 1 / ⌫, we can assume that the Hamiltonian is constant over this period of time. For each t such as Յ t Ͻ + ⌬ , we have H͑t͒ = H͑ ͒, the Schrödinger equation for the one-exciton wave function can then be solved during this interval, Expanding Green's function up to the order ⌬H 2 and using a trapezoidale rule 44 to numerically evaluate the integrals, we find In a similar fashion, we can obtain the propagation of the two-exciton wave function, …”