Freie Universität Berlin, 14195 Berlin, Germany.
J Phys Chem B. 2022 Feb 24;126(7):1579-1589. doi: 10.1021/acs.jpcb.1c09481. Epub 2022 Feb 15.
From ab initio simulations of liquid water, the time-dependent friction functions and time-averaged nonlinear effective bond potentials for the OH stretch and HOH bend vibrations are extracted. The obtained friction exhibits not only adiabatic contributions at and below the vibrational time scales but also much slower nonadiabatic contributions, reflecting homogeneous and inhomogeneous line broadening mechanisms, respectively. Intermolecular interactions in liquid water soften both stretch and bend potentials compared to the gas phase, which by itself would lead to a red-shift of the corresponding vibrational bands. In contrast, nonadiabatic friction contributions cause a spectral blue shift. For the stretch mode, the potential effect dominates, and thus, a significant red shift when going from gas to the liquid phase results. For the bend mode, potential and nonadiabatic friction effects are of comparable magnitude, so that a slight blue shift results, in agreement with well-known but puzzling experimental findings. The observed line broadening is shown to be roughly equally caused by adiabatic and nonadiabatic friction contributions for both the stretch and bend modes in liquid water. Thus, the quantitative analysis of the time-dependent friction that acts on vibrational modes in liquids advances the understanding of infrared vibrational frequencies and line shapes.
从液态水的从头算模拟中，提取了 OH 伸缩和 HOH 弯曲振动的时变摩擦函数和时间平均非线性有效键势。所得到的摩擦不仅在振动时间尺度上表现出绝热贡献，而且还表现出慢得多的非绝热贡献，分别反映了均匀和非均匀线展宽机制。与气相相比，液态水中的分子间相互作用软化了伸缩和弯曲势，这本身会导致相应振动带的红移。相比之下，非绝热摩擦贡献导致光谱蓝移。对于伸缩模式，势能效应占主导地位，因此从气相到液相会导致明显的红移。对于弯曲模式，势能和非绝热摩擦效应具有可比的大小，因此会导致轻微的蓝移，这与众所周知但令人困惑的实验发现一致。观察到的线宽展宽对于液体中的伸缩和弯曲模式，由绝热和非绝热摩擦贡献大致相等引起。因此，对作用于液体中振动模式的时变摩擦的定量分析，推进了对红外振动频率和谱线形状的理解。
