UV photochemistry of 1,3-cyclohexadiene isolated in solid parahydrogen

1,3-cyclohexadiene was isolated in solid parahydrogen, and its photochemical reactions were studied via irradiation with 193 nm photons from an ArF excimer laser. Photoproducts were identified using Fourier-transform infrared spectroscopy supported by density functional theory calculations. Photolysis with 193 nm light caused the formation of two well-characterized conformers of 1,3,5-hexatriene, tZt-1,3,5-hexatriene and tEt-1,3,5-hexatriene. A third initial photoproduct was identified as the conformer cZc-1,3,5-hexatriene, and its infrared spectrum was characterized for the first time. In addition, the bicyclo[2.2.0]hex‑2-ene and 1-methyl-bicyclo[2.1.0]pent‑2-ene were also found to be a minor products from photo-induced ring-closure reactions, where the latter further converts to 2-methyl-cyclopentadiene. The reaction mechanisms and stability of these products are discussed using infrared spectroscopy and density functional theory calculations.