UV photochemistry of benzene and cyclohexadienyl radical in solid parahydrogen

Matrix-isolation spectroscopy in solid parahydrogen is a powerful technique for the study of photochemical reactions in detail. It is especially suited for the detection of chemical intermediates and unstable molecular products of photochemical reactions that are otherwise difficult to observe using other conventional spectroscopic techniques. Here, we have revisited UV photochemistry of benzene induced by 193 and 253.7 nm excitations by matrix-isolation infrared spectroscopy using solid parahydrogen as a host matrix. In addition to the formation of benzvalene, fulvene, and Dewar benzene as photoproducts of the UV photolysis of benzene, we have confirmed the production of cyclohexadienyl radical in solid parahydrogen as an intermediate species for a ring-opening reaction to 1,3,5-hexatriene. Moreover, we have identified 1,3-hexadien-5-yne and o-benzyne as minor products of the photochemical reaction. The reaction mechanisms of these species based on the analysis of FTIR spectra and quantum chemical calculations are discussed.