The Role of Electrical Anharmonicity in the Association Band in the Water Spectrum

The origin of the intensity of the feature in the spectrum of liquid water near 2100 cm–1 is investigated through calculations of the spectra of water clusters based on low-order expansions of the potential and dipole surfaces in internal and normal mode coordinates. The intensity near 2100 cm–1 is attributed to combination bands involving the HOH bend and intermolecular vibrations that break the hydrogen bonding network. Further, the leading contribution to the intensity reflects large second derivatives of the dipole moment with respect to the internal coordinates that are excited, or electrical anharmonicity. This picture changes if the derivatives of the potential and dipole surfaces are taken with respect to normal modes. In the normal mode representation, the second derivatives of the dipole moment are often vanishingly small, while the mixed third and fourth derivatives of the potential become quite large. On the basis of this result, mechanical anharmonicity appears to be responsible for the intensity in the 2100 cm–1 region. This strong dependence of the interpretation of the origins of the intensity in the 2100 cm–1 region of the water spectrum is investigated and discussed.