Ab Initio Calculations of IR Spectra in Identification of Products of Matrix Isolation Photochemistry: Dewar Form of 4(3H)-Pyrimidinone

Ab initio calculations of the infrared spectrum of the Dewar isomer of 3-methyl-4(3H)-pyrimidinone have been carried out at the MP2/6-31G** and SC F/6- 31G** levels of theory. These were compared with the experimental spectrum of the photoproduct that emerged upon UV (308 nm) irradiation of 3-methyl-4(3H)-pyrimidinone isolated in a low-temperature argon matrix. The agreement between the spectrum simulated at the MP2 level and the experimental spectrum was remarkable and enabled positive assignment of the photoproduct structure. Photoreactions of matrix-isolated 4(3H)-pyrimidinones not methylated at the N3 nitrogen atom have also been studied. For these compounds, three types of photoreactions were observed, i.e., phototautomerism, ring opening, and Dewar structure formation. The relative probabilities of the three competing reaction directions and their dependencies on ring substitutions have been investigated, and results are presented.