Absorption spectra of gaseous, liquid and solid water

The water absorption spectrum is very complex. Water's vapor spectroscopy has been recently reviewed. The water molecule may vibrate in a number of ways. In the gas state, the vibrations involve combinations of symmetric stretch (v1), asymmetric stretch (v3) and bending (v2) of the covalent bonds. The stretch vibrations of HD16O refer to the single bond vibrations, not the combined movements of both bonds. Gas phase rotations are complex and are combined with these vibrations. Rotations in the liquid phase are totally dominated by hydrogen bonding.
 
Main vibrations of water isotopologues
The water absorption spectrum is very complex. Water's vapor spectroscopy has been recently reviewed. The water molecule may vibrate in a number of ways. In the gas state, the vibrations involve combinations of symmetric stretch (v1), asymmetric stretch (v3) and bending (v2) of the covalent bonds with absorption intensity (H216O) v1;v2;v3 = 0.07;1.47;1.00. The stretch vibrations of HD16O refer to the single bond vibrations, not the combined movements of both bonds. Gas phase rotations are complex and are combined with these vibrations. Rotations in the liquid phase are totally dominated by hydrogen bonding.
 
Shown opposite are the main vibrations occurring in water. The movements are animated using the cursor. The dipole moments change in the direction of the movement of the oxygen atoms as shown by the arrows. As the H-atoms are light, the vibrations have large amplitudes.
 
The water molecule has a very small moment of inertia on rotation which gives rise to rich combined vibrational-rotational spectra in the vapor containing tens of thousands to millions of absorption lines. In the liquid, rotations tend to be restricted by hydrogen bonds, giving the librations. Also, spectral lines are broader causing overlap of many of the absorption peaks.

Opposite is shown a comparison of the gas, liquid and solid spectra of the same amount of H2O.

The main stretching band in liquid water is shifted to a lower frequency (v3, 3490 cm-1 and v1, 3280 cm-1) and the bending frequency increased (v2, 1644 cm-1) by hydrogen bonding. As seen, increased strength of hydrogen bonding typically shifts the stretch vibration to lower frequencies (red-shift) with greatly increased intensity in the infrared (but not Raman) due to the increased dipoles.