Photolysis of (3-Methyl-2H-azirin-2-yl)-phenylmethanone: Direct Detection of a Triplet Vinylnitrene Intermediate

The photoreactivity of (3-methyl-2H-azirin-2-yl)-phenylmethanone, 1, is wavelength-dependent (Singh et al. J. Am. Chem. Soc. 1972, 94, 1199-1206). Irradiation at short wavelengths yields 2P, whereas longer wavelengths produce 3P. Laser flash photolysis of 1 in acetonitrile using a 355 nm laser forms its triplet ketone (T-1K, broad absorption with lambda(max) similar to 390-410 nm, tau similar to 90 ns), which cleaves and yields triplet vinylnitrene 3 (broad absorption with lambda(max) similar to 380-400 nm, tau = 2 mu s). Calculations (B3LYP/6-31+G(d)) reveal that T-1K of 1 is located 67 kcal/mol above its ground state (S-0) and has a long C-N bond (1.58 angstrom), and the calculated transition state to form 3 is only 1 kcal/mol higher in energy than T-1K of 1. The calculations show that 3 has significant 1,3-carbon iminyl biradical character, which explains why 3 reacts efficiently with oxygen and decays by intersystem crossing to the singlet surface. Photolysis of 1 in argon matrixes at 14 K produced ketene imine 7, which presumably is formed from 3 intersystem crossing to 7. In comparison, photolysis of 1 in methanol with a 266 nm laser produces mainly ylide 2 (lambda(max) similar to 380 nm, tau similar to 6 mu s, acetonitrile), which decays to form 2P. Ylide 2 is formed via singlet reactivity of 1, and calculations show that the first singlet excited state of the azirine chromophore (S-1A) is located 113 kcal/mol above its So and that the singlet excited state of the ketone (S-1K) is 85 kcal/mol. Furthermore, the transition state for cleaving the C-C bond in 1 to form 2 is located 49 kcal/mol above the So of 1. Thus, we theorize that internal conversion of S-1A to a vibrationally hot S-0 of 1 forms 2, whereas intersystem crossing from S-1K to T-1K results in 3.