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.

Water's ion pairs?
The water molecule is often described in school and undergraduate textbooks of as having four, approximately tetrahedrally arranged, sp3-hybridized electron pairs, two of which are associated with hydrogen atoms leaving the two remaining lone pairs. In a perfect tetrahedral arrangement the bond-bond, bond-lone pair and lone pair-lone pair angles would all be 109.47° and such tetrahedral bonding patterns are found in condensed phases such as hexagonal ice.

Ab initio calculations on isolated molecules, however, do not confirm the presence of significant directed electron density where lone pairs are expected. The negative charge is more evenly concentrated along the line between where these lone pairs would have been expected, and lies closer to the center of the O-atom than the centers of positive charge on the hydrogen atoms.