Laser Flash Photolysis and Computational Study of Singlet Phenylnitrene

Laser flash photolysis (LFP, Nd:YAG laser, 35 ps, 266 nm, 10 mJ) of phenyl azide (PA) releases open-shell singlet phenylnitrene ( 1 PN). 1 PN has a sharp absorption band at 350 nm and a very weak absorption band at 540 nm. These bands are assigned as n z −n y and a mixture of 52% π−n x and 28% n x −π* transitions, respectively, on the basis of CASSCF and CASPT2 calculations. A CASPT2 calculation of the spectrum of triplet phenylnitrene ( 3 PN) is in very good agreement with the experimental spectrum. A sharp absorption band of 3 PN at 308 nm and a broad, structured band at 370 nm are assigned as n z −n y and π−π transitions on the basis of CASSCF and CASPT2 calculations. The low intensity, long wavelength band of 3 PN tailing to 500 nm is assigned as a mixture of π−n x, n x −π* transitions. The decay of 1 PN in pentane was measured as a function of temperature to obtain observed rate constants of disappearance, k obs . The rate constant k obs is equal to k R + k ISC, where k R is the absolute rate constant of rearrangement of 1 PN to benzazirine and k ISC is the absolute rate constant of intersystem crossing to the lower energy triplet state ( 3 PN). When it is assumed that k ISC is independent of temperature, k obs can be dissected, and absolute values of k R and k ISC can be deduced. For 1 PN, k ISC = 3.2 ± 0.3 × 10 6 s - 1 and the Arrhenius parameters for k R are found to be E a = 5.6 ± 0.3 kcal mol - 1 and A = 10 13.1 ± 0.3 s - 1 .