Theoretical Study of the Low-Lying Electronically Excited States of Diacetylene

The complete-active-space SCF and CASMP2 methods are used to characterize the low-lying excited singlet and triplet states of diacetylene. With the exception of the 1 ¹Δ<INF>u</INF> state, all excited states considered are found to have geometrical structures distorted away from linear. Three of the triplet states are predicted to have non-planar C<INF>2</INF>-symmetry structures. The other states are predicted to be planar, existing as both cis (C<INF>2</INF><INF>v</INF><INF></INF>) and trans (C<INF>2</INF><INF>h</INF><INF></INF>) isomeric forms. Vibrational frequencies are calculated for the low-lying electronically excited states, and vertical and adiabatic excitation energies are reported.