Infrared intensities of polyatomic molecules calculated from SCEP dipole-moment functions and anharmonic vibrational wavefunctions. I. Stretching vibrations of the linear molecules HCN, HCP and C 2N 2

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Infrared intensities of transitions between a variety of stretching vibrational states of various isotopomers of HCN, HCP and C 2N 2 have been calculated from SCF and SCEP dipole-moment functions and anharmonic vibrational wavefunctions. Best agreement with experiment, usually with deviations of the order of 10% for the fundamentals, is obtained by CEPA when the dipole moment is calculated as an energy derivative. The performance of the familiar “double-harmonic approximation” is investigated and found to work well in the absence of vibrational resonances and strongly curved dipole-moment functions.