ACETYLENE - SYNERGY BETWEEN THEORY AND EXPERIMENT

Six anomalous vibronic feature states [approximately 2 cm-1 full-width at half-maximum (FWHM), each consisting of approximately 20 partially resolved eigenstates] have been observed in stimulated emission pumpi ng (SEP) spectra of C2D2. Of the two plausible assigmnents for these f eatures, the one most consistent with spectroscopic observations would imply that the lowest energy cis-bent triplet state of acetylene has T0 less-than-or-equal-to 25 820 cm-1, which is inconsistent with previ ous ab initio predictions. New higher level ab initio quantum mechanic al methods have been used to predict the energy difference between X 1 SIGMA(g)+ ground state and the cis-bent a3 B2 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 = DELTAE(a3 B2-X 1SIGMA(g)+) = 30 500 cm-1. The true value of T0 for th e a3 B2 state is estimated to be approximately 500 cm-1 higher. At the same level of theory the zero-point levels of the lowest triplet stat e of the trans-bent (a3 B(u)) and vinylidene (a3 B2) isomers lie at st ill higher energies. This result conclusively rules out any triplet as signment for the anomalous feature states. The alternative assignment, as highly excited vibrational levels of the X 1SIGMA(g)+ state, is su rprising 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 implausibl e in the absence of the present ab initio calculations. Previous exper imental observations [Lisy and Klemperer, J. Chem. Phys. 72, 3880 (198 0) and Wendt, Hippler, and Hunziker, J. Chem. Phys. 70, 4044 (1979)] o f acetylene triplet states are discussed and shown to be completely co nsistent with each other and with the present ab initio ordering of th e cis and trans isomeric minima on the T1 potential energy surface: ci s a3B2 below trans a3B(u).