Exploring the Absorption Spectrum of Simulated Water from MHz to Infrared

Abstract

Absorption spectra of liquid water at 300 K are calculated from both classical and density functional theory molecular dynamics simulation data, which together span from 1 MHz to hundreds of THz, agreeing well with experimental data qualitatively and quantitatively over the entire range, including the IR modes, the microwave peak, and the intermediate THz bands. The spectra are decomposed into single-molecular and collective components, as well as into components due to molecular reorientations and changes in induced molecular dipole moments. These decompositions shed light on the motions underlying the librational and translational (hydrogen-bond stretching) bands at 20 and 5 THz, respectively; interactions between donor protons and acceptor lone pair electrons are shown to be important for the line shape in both librational and translational regimes, and in- and out-of-phase librational dimer modes are observed and explored.

Time-dependent friction effects on vibrational infrared frequencies and line shapes of liquid water

“…The d OHs and d OHa spectra overlap significantly, with a small red shift of the d OHs spectrum relative to the d OHa spectrum, in accordance with previous observations [52]. The spectrum of the φ HOH mode overlaps perfectly with the spectrum from the total (nuclear and electronic) dipole moment, which is due to the fact that the bending angle vibrations of neighboring water molecules are only weakly (and in fact anti-) correlated, as shown in the SI section II [51]. We conclude that the spectra calculated from the total system polarization (including nuclear and electronic polarization from all water molecules and their correlations) match the spectra based on the single-water nuclear-coordinatebased vibrational modes rather faithfully, which is at the heart of the common molecular interpretation of IR spectra and also validates our further approach.…”

“…IR spectra can therefore be calculated straight-forwardly from sufficiently long trajectories from aiMD simulation data using eq. ( 7) and the Wiener-Kintchine relation [51,64], derived in SI section XIV. Quantum corrections have previously been addressed, but were not applied here [65].…”

“…The symmetric stretch mode in the gas phase shows a much smaller intensity than the anti-symmetric stretch mode, in agreement with experiment [50], which is caused by electronic polarization effects. The OH-stretch peak in the liquid is significantly red-shifted and enhanced compared to gas phase, which is typically rationalized by the softening of the OH bond potential and the constructive collectivity of OH-stretching vibrations in the liquid (see SI section II) [2,3,51]; the significant enhancement is noteworthy, since one could expect the friction acting on the OH bond to be much stronger in the liquid and thus to reduce the vibrational amplitude. In contrast, the HOH-bending mode in the liquid is slightly blue-shifted and not enhanced, which can be rationalized by collective effects that are slightly destructive (see SI section II).…”

Time-Dependent Friction Effects on Vibrational Infrared Frequencies and Line Shapes of Liquid Water

“… 54 The vibrational spectrum of the ϕ HOH mode overlaps perfectly with the spectrum from the total (nuclear and electronic) dipole moment, which is due to the fact that the bending angle vibrations of neighboring water molecules are only weakly (and in fact anti-) correlated, as shown in section II in the Supporting Information . 53 We conclude that the absorption spectrum calculated from the total system polarization (including nuclear and electronic polarization from all water molecules and their correlations) match the power spectra based on the single-water nuclear-coordinate-based vibrational modes rather faithfully. This good agreement lies at the heart of the common interpretation of IR absorption spectra in terms of molecular vibrations; it also validates our approach, because it means that the conclusions from our GLE analysis, which in the present formulation can be applied only on one-dimensional reaction coordinates that are derived from nuclear positions, can also be used to interpret simulated and experimental absorption spectra.…”

“…The symmetric stretch mode in the gas phase shows a much smaller intensity than the antisymmetric stretch mode, in agreement with experiment, 52 which is caused by electronic polarization effects. The OH-stretch peak in the liquid is significantly red-shifted and enhanced compared to gas phase, which is typically rationalized by the softening of the OH bond potential and the constructive collectivity of OH-stretching vibrations in the liquid (see section II in the Supporting Information ); 2 , 3 , 53 the significant enhancement is noteworthy, because one could expect the friction acting on the OH bond to be much stronger in the liquid and thus to reduce the vibrational amplitude. In contrast, the HOH-bending mode in the liquid is slightly blue-shifted and not enhanced, which can be rationalized by collective effects that are slightly destructive (see section II in the Supporting Information ).…”