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

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Abstract
Molecular electronic structure theory has been used to predict the equilibrium geometries and energies of acetylene in its ground state and lowest triplet electronic states. Both double zeta (DZ) and double zeta plus polarization (DZ+P) basis sets of Gaussian functions were used, in conjunction with self‐consistent‐field (SCF) and large scale configuration interaction (CI) wavefunctions. The predicted 1Σ+g ground state structure is re(C–C) =1.213 Å, re(C–H) =1.066 Å, in good agreement with experiment, re(C–C) =1.203 Å, re(C–H) =1.060 Å. The first excited state of acetylene is the cis 3B2 state, predicted to lie 3.43 eV above the 1Σ+g ground state. The predicted equilibrium geometry of this cis state is re(C–C) =1.343 Å, ϑe(HCC) =127.8°, re(C–H) =1.090 Å. The three other lowest triplet states are trans 3Bu (Te=3.78 eV), trans 3Au (4.13) eV, and cis 3A2 (4.50 eV). The predicted ordering and CCH bond angles for these states are not entirely consistent with Walsh’s rules.