Triplet vinylidene, first predicted to have a sizeable barrier to unimolecular rearrangement in 1978 by theory, has now been observed under three different sets of experimental conditions. In order to quantitatively characterize the potential energy hypersurface of triplet vinylidene and triplet acetylene, highhyphen;levelabinitioquantum mechanical methods have been employed. Basis sets as large as triple zeta plus two sets of polarization functions augmented with higher angular momentum functions lsqb;TZ(2df,2pd)rsqb; have been utilized in conjunction with correlated methods as sophisticated as the coupled cluster approach including all single, double, and perturbative triple excitations lsqb;CCSD(T)rsqb;. Of particular interest are predictions of the zerohyphen;point vibrational energy corrected barriers for rearrangement ofatilde;thinsp;3B2vinylidene tobtilde;thinsp;3Butranshyphen;bent acetylene and ofcishyphen;bentatilde;thinsp;3B2acetylene totranshyphen;bentbtilde;thinsp;3Buacetylene. At the highest level of theory used here, TZ(2df,2pd) CCSD(T), these are predicted to be 47.9 and 13.0 kcal/mol, respectively. The physical properties ofatilde;thinsp;3B2vinylidene, including the dipole moment, harmonic vibrational frequencies, and the infrared intensities, have also been reported.
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