Excited singlet electronic states of acetylene: c i s and t r a n s structures and energetics

Abstract

Molecular electronic structure theory has been used to predict the equilibrium geometries and energies of acetylene in its excited singlet electronic states. A double zeta plus polarization basis set of contracted Gaussian functions was used in conjunction with self‐consistent field and large scale configuration interaction wave functions. The first excited singlet state of acetylene is the trans ¹A_u state, in agreement with the experimental studies of King, Ingold, and Innes. This result is particularly interesting because the lowest triplet state of C₂H₂ is not the ³A_u state but rather the cis ³B₂ state. The predicted geometry of the ? ¹A_u state is r_e(CC)=1.384 Å, r_e(CH)=1.096 Å, ϑ_e(HCC)=121.7 °, in good agreement with available spectroscopic data. The predicted relative energies of the excited singlet states are 5.06 eV (¹A_u), 5.54 eV (cis ¹A₂), 6.87 eV (¹B₂), and 7.29 eV (¹B_u). Thus the energetic ordering of the singlet states is A_u<A₂<B₂<B_u, completely different from that predicted for the analogous triplets B₂<B_u<A_u<A₂. Electronic structures are discussed in terms of Mulliken populations and dipole moments predicted for the two cis excited singlet states.