A matrix isolation spectroscopy and laser flash photolysis study of 2-pyrimidylnitrene

Photolysis (254 nm) of 2-azidopyrimidine (AP) in glassy ethanol (EtOH) at 7 7 K produces the EPR spectrum of 2-pyrimidylnitrene (D = 1.15 cm(-1)) in it s triplet ground state. Photolysis (254 nm) of AP in EtOH at 77 K leads to bleaching of the absorption of the azide (lambda (max) = 242 nm) and format ion of a broad absorption band between 300 and 400 nm and a highly structur ed band between 400 and 450 nm. A more highly resolved but similar spectrum was observed by photolysis of AP in argon at 14 K. The appearance of these bands in argon is accompanied by the formation of a series of IR transitio ns. The experimentally observed IR spectrum was consistent with the spectru m of triplet 2-pyrimidylnitrene ((PN)-P-3) predicted by density functional theory with the 6-31G* basis set. The UV-vis spectrum is attributed to 3PN based on the IR and EPR results. Laser flash photolysis (LFP) of AP in dich loromethane at ambient temperature produced 3PN with its characteristic str uctured absorption between 400 and 450 nm. The triplet nitrene was formed i n an exponential process (k(OBS) = 8 +/- 2 x 10(7) s(-1), tau similar to 13 ns, lambda (max) = 429 nm) following the laser flash. The transient absorp tion observed at 455 nm decays with the same time constant and is attribute d to singlet 2-pyrimdylnitrene ((PN)-P-1). Simple expectations based on ant i-aromaticity arguments and density functional theory calculations agree th at cyclization of singlet 2-pyrimidylnitrene to form a 1H-benzodiazirine is more endothermic than the corresponding process in unsubstituted singlet p henylnitrene if the singlet-triplet gaps of the two nitrenes are comparable . The rate constant of intersystem crossing of (PN)-P-1 is more than 200 ti mes faster than that of parent singlet phenylnitrene. Cyclization of (PN)-P -1 to the benzo 1H-diazirine is not observed, and the hypothetical process is at least 13 times slower than that of singlet phenylnitrene to a benzazi rine at ambient temperature. (PN)-P-3 decays over tens of microseconds in a second order process, presumably to form the azo dimer, and reacts with mo lecular oxygen.