Comparative semiempirical and DFT study of styrylnaphthalenes and styrylquinolines and their photocyclization products

摘要

Semiempirical molecular orbital (PM3, PM6, and RM1) and density functional theory (DFT) (B3LYP/6-31G*) studies are carried out for 1- and 2-styrylnaphthalenes and their aza-derivatives-2- and 4-styrylquinolines. Relative stabilities of three isomeric forms: E- and Z-isomers and the closed-ring dihydrocyclophotoproduct (derivative of dihydrophenanthrene) are calculated. Compared to PM3, PM6 and especially RM1 understate heats of formation; in some cases, PM6 and RM1 even place Z-isomer in energy below E-isomer. PM3 rather close to DFT predicts heats of isomerization reaction, whereas PM6 and especially RM1 underestimate these values. Semiempirical methods in comparison with DFT markedly underestimate heats of cyclization reaction; however, reproduce trends in relative stabilities of different isomers in dependence on the structure of styrylnaphthalenes and styrylquinolines. Qualitative correlation is found between calculated relative stabilities of the closed-ring forms (heats of cyclization reaction) and experimental data: cyclized products with low heats of cyclization are observed in steady-state photolysis and those with high heats of cyclization are not. In the latter case, the closed-ring compounds, if formed in the excited state, due to thermal instability decompose rapidly with ring opening in the ground state that prevents their observation.