Early events in the photochemistry of 2-naphthyl azide from femtosecond UV/Vis spectroscopy and quantum chemical calculations: direct observation of a very short-lived singlet nitrene.
Exposure of 2-naphthyl azide in acetonitrile at ambient temperature to femtosecond pulses of 266 nm light produces a transient absorption with maxima at 350 and 420 nm. The carrier of the 350 nm band decays more rapidly than that of the 420 nm band which has a lifetime of 1.8 ps. Analogous experiments with 1-chloro-2-naphthyl azide in methanol allow the assignment of the 350 nm band to a singlet excited state of 2-naphthyl azide and the carrier of the 420 nm band to singlet 2-naphthylnitrene. This reactive intermediate has the shortest lifetime of any singlet nitrene observed to date and is a true reactive intermediate. Computational studies at the RI-CC2 level of theory support these conclusions and suggest that initial excitation populates the S2 state of 2-naphthyl azide. The S2 state, best characterized as a pi --> (pi*, aryl) transition, has a geometry similar to S0. S2 of 2-naphthyl azide can then populate the S1 state, a pi --> (in-plane, pi*, azide) excitation, and in the S1 state, electron density is depleted along the proximal N-N bond. S1 is dissociative along that N-N coordinate to form the singlet nitrene, and with a barrier of only approximately 5 kcal/mol for N2 extrusion.
Wang
Jin
J
Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210, USA.
Kubicki
Jacek
J
Burdzinski
Gotard
G
Hackett
John C
JC
Gustafson
Terry L
TL
Hadad
Christopher M
CM
Platz
Matthew S
MS
D001386:Azides / Q000737:chemistry*; D007097:Imines / Q000737:chemistry*; D007700:Kinetics; D008958:Models, Molecular; D009281:Naphthalenes / Q000737:chemistry*; D010777:Photochemistry; D011789:Quantum Theory; D013056:Spectrophotometry, Ultraviolet; D013816:Thermodynamics