THE ELECTRONIC STATES OF CIS- AND TRANS-ACETYLENE

Abstract

The electronic energy levels of cis- and trans-bent planar centrosymmetric acetylene molecules, with and r cc = 1.383 Å, have been calculated by the LCAO/ASMO/CI procedure. The lowest energy state that can spectroscopically combine with the linear ground state is found to be of symmetry class A_u belonging to the trans-bent molecule and to lie at 4.58 ev above the ground state. This compares favorably with the experimentally observed electronic transition of lowest energy which is of type (¹A_u−¹∑_g⁺) and at 5.24 ev. The lowest-energy allowed transition to a cis-bent state of these dimensions is at 9.39 ev, and hence this may not form part of the unanalyzed system of bands in the 2000–1500 Å region, as has been suggested. However, a transition to a trans-bent state of type (¹B_u−¹∑_g⁺) is predicted to fall in this region. The energies of other electronic states are discussed in relation to the observed absorption systems of acetylene.