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

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.