New assignment of the vibrational structure of the V ← N transition in ethylene- d4

Abstract

The diffuse absorption bands of the valence (V ← N) transition of ethylene- d 4 (and by implication the more diffuse bands of all other ethylene isotopomers) are assigned on the basis of both ab intio MO calculations and spectroscopic modelling. Only two modes are found to contribute: the torsional mode ( v 4) producing cold- and hot-band progressions and a nontotally symmetric inducing mode of 703 cm −1 in the V state. The latter mode is active in intensity borrowing, probably from the lowest Rydberg transition (R ← N) in which case it is tentatively identified as v 8 (b 2g, CH 2 pyramidalization). All observed bands are assigned accurately by our model which locates the V ← N origin at 44300 cm −1.