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 1Au 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 3Au state but rather the cis 3B2 state. The predicted geometry of the ? 1Au state is re(CC)=1.384 Å, re(CH)=1.096 Å, ϑe(HCC)=121.7 °, in good agreement with available spectroscopic data. The predicted relative energies of the excited singlet states are 5.06 eV (1Au), 5.54 eV (cis 1A2), 6.87 eV (1B2), and 7.29 eV (1Bu). Thus the energetic ordering of the singlet states is Au<A2<B2<Bu, completely different from that predicted for the analogous triplets B2<Bu<Au<A2. Electronic structures are discussed in terms of Mulliken populations and dipole moments predicted for the two cis excited singlet states.