OVERTONE INTENSITIES AND DIPOLE-MOMENT SURFACES FOR THE ISOLATED CH CHROMOPHORE IN CHD3 AND CHF3 - EXPERIMENT AND ABINITIO THEORY

The band strengths of fundamentals (N = 1) and overtones (up to N = 6) of the strongly coupled CH stretching and bending vibrations in CHD3 and CHF3 are calculated using high level ab initio (SCF-CI) dipole moment functions and potential surfaces in one and two (three) dimensions. The calculations are performed in approximate normal coordinate and internal coordinate subspaces, the former giving generally superior results. The overall prediction of relative and absolute intensities ranging over many orders of magnitude is often accurate to within a factor of 2, but not to within experimental accuracy. Different dipole model functions and potential surfaces are investigated and an empirical adjustment of the dipole function to experiment is proposed for CHF3. The comparison of experimental and ab initio overtone intensities for the Fermi resonance system is discussed in some detail, as well as the importance of the results for IR spectroscopy and IR multiphoton excitation. © 1990 American Institute of Physics.