Knowledge Bank

Transition dipole moments and band intensities of asymmetric-top molecules are derived based on a perturbation theory approach that includes anharmonicity $effects1$. Potential energy and dipole moment expansions together with group theoretical analyses for vibrational selection rules are employed. Calculations are carried out using ab initio SCF and CI potential energy surfaces combined with ab initio dipole moment surfaces for the H and D istopes of formaldehyde. Dipole moment expansion coefficients for the three components of the molecule-fixed coordinates, transition dipole moments of fundamental, first overtone, combination and second overtone transitions, and integrated absorption band intensities within the ratio 1:10000 to that of the highest absorption band are reported. Intensity results in terms of band shapes of conventional molecular infrared spectra are also derived and compared to experimental results.