Acetylene: Synergy between theory and experiment

Abstract

Six anomalous vibronic feature states [∼2 cm−1 full-width at half-maximum (FWHM), each consisting of ∼20 partially resolved eigenstates] have been observed in stimulated emission pumping (SEP) spectra of C2D2. Of the two plausible assignments for these features, the one most consistent with spectroscopic observations would imply that the lowest energy cis-bent triplet state of acetylene has T0≤25 820 cm−1, which is inconsistent with previous ab initio predictions. New higher level ab initio quantum mechanical methods have been used to predict the energy difference between X̃ 1Σg+ ground state and the cis-bent ã 3B2 lowest triplet state of acetylene. In conjunction with a triple zeta plus double polarization plus f function (TZ2Pf) basis set, the coupled cluster including single, double, and linearized triple excitations CCSD(T) method yields T0=ΔE(ã 3B2–X̃ 1Σg+)=30 500 cm−1. The true value of T0 for the ã 3B2 state is estimated to be ∼500 cm−1 higher. At the same level of theory the zero-point levels of the lowest triplet state of the trans-bent (ã 3Bu) and vinylidene (ã 3B2) isomers lie at still higher energies. This result conclusively rules out any triplet assignment for the anomalous feature states. The alternative assignment, as highly excited vibrational levels of the X̃ 1Σg+ state, is surprising in view of the Franck–Condon selectivity, dynamical stability, and nonselective relaxation of this special class of ‘‘bright states’’ observed in the SEP spectra. Such an assignment would be implausible in the absence of the present ab initio calculations. Previous experimental observations [Lisy and Klemperer, J. Chem. Phys. 72, 3880 (1980) and Wendt, Hippler, and Hunziker, J. Chem. Phys. 70, 4044 (1979)] of acetylene triplet states are discussed and shown to be completely consistent with each other and with the present ab initio ordering of the cis and trans isomeric minima on the T1 potential energy surface: cis ã 3B2 below trans ã 3Bu .