Electronic spectrum of ethylene. I. The 1744 Å Rydberg transition

Abstract

The vibrational structures of the 1744 Å Rydberg transitions of C2H4 and C2D4 have been examined in detail. For both isotopes the relative intensities of the principal bands have been measured, and various 'hot' bands appearing in high temperature spectra have been assigned. Vibrational analysis has shown that the ethylene molecule is non-planar at equilibrium in the 1B3(u) upper electronic state, being twisted by 25° ± 1°, with a potential barrier of 290 ± 20 cm−1 to inversion through the planar configuration. The C–C bond length in the upper state is 1.41 ± 0.01 Å and the hydrogen atoms lie 0.03 Å further from the axis of least moment of inertia (through the C–C bond) than they do in the ground state. It is concluded that C2H4 in its higher Rydberg states and the C2H4+ ion in its ground state are also twisted at equilibrium.The correct form for the torsional potential function for the 1B3(u) electronic state (which becomes one component of a degenerate 1E state when the molecule is twisted by 90°) has been considered; it is found that although the 1E state suffers a comparatively large static Jahn–Teller distortion, there are no observable effects resulting from coupling of the electronic and torsional motions (dynamic Jahn–Teller effect).