Triplet acetylene: Near infrared electronic absorption spectrum of the c i s isomer, and formation from methylene

Abstract

The electronic absorption spectrum in the gas phase of the metastable 1 ³B₂ state of acetylene has been observed. Its’ features, including rotational and vibronic structure as well as deuterium isotope effects, agree remarkably well with ab initio theoretical predictions for the 1 ³A₂–1 ³B₂ electronic transition of the lowest triplet state in its cis geometry. The metastable species was generated in the Hg‐photosensitized reactions of acetylene, ketene, and diazomethane. In the latter two cases it is probably formed by the reaction ³CH₂+³CH₂ →C₂H₂(1 ³B₂)+H₂.