Vibrational anharmonicity in ethylenic compounds

Abstract Previous studies of the CH (and CD) stretching vibration manifolds of the series of molecules H 2 CCH 2 , H 2 CCD 2 , H 2 CCCl 2 , and H 2 CCF 2 have been extended to cover all spectroscopic observations made to date on excited state vibration levels. From these, sets of normal mode anharmonicity constants have been assembled over the four molecules. These are individually incomplete, but exhibit a considerable degree of transferability of constants which relate to the normal vibrations associated with the moiety. In turn, this allows the vibrations of the group to be characterized in terms of a set of 24 averaged normal mode anharmonicity constants, which reproduce 77 spectroscopic observations for ethylene-H 4 up to 16 500 cm −1 with an rms error of 4.30 cm −1 .