Knowledge Bank

Canonical Van Vleck perturbation theory (CVPT) is used to calculate electric dipole intensities for one, two, and three-dimensional models of HCN. Lehmann and $Smith1$ have illustrated that the intensities of overtone transitions are sensitive to the details of the inner wall of the potential. Dipole intensities calculated for several, similar one-dimensional CH stretch potentials demonstrate that perturbation theory correctly predicts this sensitivity. The perturbative intensities of a two-dimensional ab initio dipole surface indicate the importance of selected stretch-stretch resonance interactions in interpreting the CH stretch overtone spectra of HCN. The inclusion of the bend confirms that this degree of freedom plays a significant role in weakening the intensity of the CN overtones. In all these calculations the perturbative results are in excellent agreement with the corresponding variational $results.1$K. K. Lehman and A. M. Smith, J. Chem. Phys. 93, 6140 (1990)