Acetylene: Synergy between theory and experiment

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 X 1Σg+ ground state and the cis‐bent a 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(a 3B2–X 1Σg+)=30 500 cm−1. The true value of T0 for the a 3B2 state is estimated to be ∼500 cm−1 higher. At the same level of theory the zero‐point levels o...

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