The shapes of some excited states of acetylene

Abstract

Parts of the potential surfaces for the ground and excited states (below 11 eV) of acetylene are computed with an ab initio frozen core method. Various planar and non-planar geometrics are investigated. Rydberg states, including the 1 1Σ + u V ππ * state, are found to be linear and to retain their Rydberg character upon bending. Valence ·(1π u) 3(1π * g) states are cis and trans bent if they result chiefly from excitation to the component of the 1π * g MO in the molecular plane. They are further distorted to gauche, non-planer, geometries if they arise from excitation to the out-of-plane component of the 1π * g MO. Walsh diagrams are used to rationalize these trends and the computed results are compared with the available experimental data.