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

The electronic absorption spectrum in the gas phase of the metastable 13B2 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 13A2-13B2 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→C2H 2(13B2) + H2. © 1979 American Institute of Physics.