AB-INITIO CALCULATIONS OF IR-SPECTRA IN IDENTIFICATION OF PRODUCTS OFMATRIX-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 SCF/6-31G* levels of theory. These were compared with the experim ental spectrum of the photoproduct that emerged upon UV (308 nm) irrad iation of 3-methyl-4(3H)-pyrimidinone isolated in a low-temperature ar gon matrix. The agreement between the spectrum simulated at the MP2 le vel and the experimental spectrum was remarkable and enabled positive assignment of the photoproduct structure. Photoreactions of matrix-iso lated 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 structu re formation. The relative probabilities of the three competing reacti on directions and their dependencies on ring substitutions have been i nvestigated, and results are presented.