Vibrational frequencies from anharmonic ab initio/empirical potential energy functions. III. Stretching vibrations of hydrogen cyanide and acetylenes

Abstract

Stretching vibrational states are calculated for various isotopes of hydrogen cyanide, acetylene, fluoroacetylene, chloroacetylene and cyanoacetylene by diagonalizing an approximate vibrational hamiltonian which neglects the anharmonic coupling between stretching and bending motions. Empirically corrected ab initio potential energy functions are constructed from which the lower lying vibrational transition frequencies are obtained with an overall accuracy of about 20 cm −1. The influence of electron correlation on different potential energy terms is investigated for hydrogen cyanide and acetylene by means of the coupled electron pair approximation (CEPA) within the framework of the self-consistent electron pairs theory (SCEP). Equilibrium geometries are estimated for the substituted acetylenes and quartic and sextic centrifugal distortion constants are calculated for all molecules under investigation.