  From ab initio simulations of liquid water, the time-dependent friction
functions and time-averaged non-linear effective bond potentials for the OH
stretch and HOH bend vibrations are extracted. The obtained friction exhibits
adiabatic contributions at and below the vibrational time scales, but also much
slower non-adiabatic contributions, reflecting homogeneous and inhomogeneous
line broadening mechanisms, respectively. Compared to the gas phase, hydration
softens both stretch and bend potentials, which by itself would lead to a
red-shift of the corresponding vibrational bands. In contrast, non-adiabatic
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 non-adiabatic
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
non-adiabatic friction contributions for both, the stretch and bend modes in
liquid water. Thus, the understanding of infrared vibrational frequencies and
line shapes is considerably advanced by the quantitative analysis of the
time-dependent friction that acts on vibrational modes in liquids.
