AM1 CALCULATIONS OF THE POTENTIAL-ENERGY SURFACE OF THE PHOTOISOMERIZATION OF 1-IMINOPYRIDINIUM YLIDES TO DIAZEPINES

AMI semiempirical molecular orbital calculations have been performed on a simple derivative of 1-iminopyridinium ylides (1) with the aim of elucidating the potential energy surfaces relevant to the known photoinduced ring expansion to the isomer, 1,2-diazepine (2). AM1 reveals that the ground-state reaction coordinate linking 1 with 2 possesses an intermediate bicyclic species (3), which is the 1,7-diazanorcaradiene species postulated earlier. These calculations also reveal that, in the relaxed excited singlet state of 1, the amidate group is considerably twisted with respect to the six-membered ring; moreover, a low-lying, relaxed excited state of 3 is also shown by AM1 to possess this geometry and to be nearly degenerate with that of 1. The photoconversion of 1 to 2 is postulated to occur through the excited singlet surface that links 1 with 3 by a common excited state; the subsequent formation of ground-state 3, formed via internal conversion, then leads to the product, 2, along the ground-state surface. The triplet state of 1, T1, is shown by AMI to have a very similar geometry relative to the ground state, and it is implied that the other known photochemical process, N-N bond scission, forming pyridine and the nitrene, occurs via the ground-state surface subsequent to intersystem crossing from T1. These calculations show that the transition state of this process reveals the three-centered H-C-N species that accounts for the 1,2-H shift to form the nitrene, CONH. A comparison of the AM1 results with experimental data indicates the importance of solvent and subsituent effects on the ground-state potential energy surface. © 1990 American Chemical Society.