The ground state vinylidene-acetylene isomerization was investigated by ab initio molecular electronic structure theory. The coupled-cluster method with single, double, and noniterative inclusion of triple excitations ͓CCSD͑T͔͒; with single, double, and noniterative inclusion of triple and quadruple excitations ͓CCSD͑TQ͔͒; and with full single, double, and triple excitations ͑CCSDT͒ were used to treat the effect of electron correlation. Several correlation-consistent polarized valence basis sets, cc-pVXZ, were employed. Theoretical limiting values of the energetics of the reaction were then deduced from the series of computations. With zero-point energy correction, the energy of reaction is Ϫ42.95 kcal/mol and the reaction barrier is 1.5 kcal/mol. Both agree excellently with experimental values.
Ethylthio (C2H5S) radicals were formed on laser photolysis at 248 nm of diethyl disulfide (C2H5SSC2H5) or ethyl mercaptan (C2H5SH) in a free-jet expansion. The fluorescence excitation spectrum was recorded in the spectral region 398–432 nm. The origin lies at 23 519.6 cm−1, approximately 799 cm−1 greater than previously reported. Two main progressions with spacings near 420.5 cm−1 (C–S stretch) and 256.0 cm−1 (CCS bend) are dominant. Additional active fundamental vibrational modes of the B̃ state are at 718.4, 862.8, 1054.6, 1158.9, and 1203.3 cm−1. Observation of hot bands enables accurate determination of four low-lying vibrational modes of the ground state at 271.9, 296.0, 478.3, and 672.4 cm−1. The dispersed fluorescence was recorded in the spectral region 415–525 nm. We identified several additional vibrational modes of the X̃ state at 890, 957, 1075, 1257, 1290, 1470, 2950, and 3050 cm−1. Theoretical calculations at the MP2 level were performed to predict vibrational frequencies of both B̃ and X̃ states, and for the latter state were also with the B3-LYP density functional theory; the results agree satisfactorily with experimental observations.
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