The CN Mode of HCN: A Comparative Study of the Variation of the Transition Dipole and Herman–Wallis Constants for Seven Isotopomers and the Influence of Vibration–Rotation Interaction

Abstract

The electric dipole transition moments and vibration-rotation constants of the CN stretching bands of H12C14N and six other isotopomers are reported. Hot-band measurements show the vibrational dependence of the constants. Infrared absorption spectra were measured with a Fourier transform interferometer and a multipass absorption cell with a path length of up to 240 m. The transition wavenumber measurements were used to derive vibration–rotation constants, and the intensities of individual lines were used to determine the transition dipoles and Herman–Wallis factors. The unusually small transition dipole coupled with the large dipole moment of HCN (2.98 D) results in very large values for the first Herman–Wallis factorA1and distorts the shape of the intensity envelope of the band. These measurements show how changes in the masses of the heavy atoms in the molecule have a systematic effect on the transition dipole and on the intensity pattern of the CN stretching band.