The Bent Excited State of Acetylene

Abstract

THE band systems of C₂H₂ and C₂D₂ at 2400–2100 A. have been studied in absorption, and the vibrational and resolved rotational structures fully analysed. It emerges that, in its excited state, acetylene is a plane centrosymmetric zig-zag (like ethylene from which two trans-related hydrogen atoms have been removed). It is therefore an asymmetric top. Its dimensions in the lowest vibrational level of that state are CC = 1.383 A., CH = 1.08 A., ∠CCH = 120.2°. The frequency of the CC stretching vibration is 1,385 cm.⁻¹ in C₂H₂ and 1,300 cm.⁻¹ in C₂D₂, the derived mean force-constant being F = 7.2 × 10⁵ dyne/cm.; while the vibration which together opens the angles has the frequency 1,049 cm.⁻¹ in C₂H₂ and 835 cm.⁻¹ in C₂D₂, the corresponding mean bending-force-constant being δ = 8.5 × 10⁻¹² dyne.-cm./radian. The electronic wave function has the two-fold axis and plane, but not the centre of symmetry of the zig-zag molecule. The lowest vibrational level of this bent excited state lies at 42,209 cm.⁻¹ in C₂H₂ and at 42,280 cm.⁻¹ in C₂D₂, above that of the linear normal state, the difference of 71 cm.⁻¹ arising from differences in zero-point vibrational energy. In the transition from the normal to the excited state, the electrons oscillate perpendicularly to the plane of the bent molecule. The resultant electron-spin remains zero. The transition probability, expressed as an oscillator strength, is 8 × 10⁻⁵. All this follows from the analysis of the spectra.