The intra- and intermolecular chemistry of phenylnitrene (PhN), its singlet-triplet energy separation, and its electronic spectra are interpreted with the aid of ab initio molecular orbital theory. The key to understanding singlet PhN is the recognition that this species has an open-shell electronic structure, in contrast to the related species, phenylcarbene, which has a closed-shell electronic structure. The thermodynamics of nitrenes, benzazirines, dehydroazepines, aminyl radicals, and their hydrocarbon analogues are also discussed.

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Laser flash photolysis (LFP, Nd:YAG laser, 35 ps, 266 nm, 10 mJ) of phenyl azide (PA) releases open-shell singlet phenylnitrene (1PN). 1PN 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 (3PN) is in very good agreement with the experimental spectrum. A sharp absorption band of 3PN 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 3PN tailing to 500 nm is assigned as a mixture of π−n x , n x −π* transitions. The decay of 1PN 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 1PN to benzazirine and k ISC is the absolute rate constant of intersystem crossing to the lower energy triplet state (3PN). 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 1PN, k ISC = 3.2 ± 0.3 × 106 s-1 and the Arrhenius parameters for k R are found to be E a = 5.6 ± 0.3 kcal mol-1 and A = 1013.1±0.3 s-1.

The photochemistry of ortho-biphenyl azide (1a) has been studied by laser flash photolysis (LFP), with UV-vis and IR detection of the transient intermediates formed. LFP (266 nm) of 1a in glassy 3-methylpentane at 77 K releases singlet ortho-biphenylnitrene (1b) (lambda(max) = 410 nm, tau = 59 +/- 6 ns), which under these conditions decays cleanly to the lower energy triplet state. In fluid solution at 298 K, 1b rapidly (tau < 10 ns) partitions between formation of isocarbazole (4) (lambda(max) = 430 nm, tau = 70 ns) and benzazirine (1e) (lambda(max) = 305 nm, tau = 12 ns). Isocarbazole 4 undergoes a 1,5-hydrogen shift, with k(H)/k(D) = 3.4 at 298 K to form carbazole 9 and smaller amounts of two other isocarbazoles (7 and 8). Benzazirine 1e ring-opens reversibly to azacycloheptatetraene (1f), which serves as a reservoir for singlet nitrene 1b. Azacycloheptatetraene 1f ultimately forms carbazole 9 on the millisecond time scale by the pathway 1f --> 1e --> 1b --> 4 --> 9. The energies of the transient intermediates and of the transition structures connecting them were successfully predicted by CASPT2/6-31G calculations. The electronic and vibrational spectra of the intermediates, computed by density functional theory, support the assignment of the transient spectra, observed in the formation of 9 from 1a.

The photochemistry of p-azidoaniline was studied in argon matrices in the absence and presence of oxygen. With the help of quantum chemical calculations we were able to characterize the triplet p-aminophenylnitrene as well as the cis-and trans-p-aminophenylnitroso oxides. It was found that the latter two isomers can be interconverted by selective irradiation and that they are ultimately converted into p-nitroaniline. Although restricted wavefunctions of the nitroso oxides are unstable, CASSCF calculations turned up no evidence for the claimed diradical character of these compounds. Also we found no evidence for dioxaziridines as intermediates of the conversion of the nitroso oxides to p-nitroaniline. IntroductionGreat progress has been made in understanding the photochemistry of aryl azides over the past few years. 1,2 Singlet phenylnitrene and a series of its simple derivatives have been detected directly and the effects of substituents on the spectra and the decay kinetics of singlet aryl nitrenes have been examined systematically. 2 Thereby it was found that groups which act as strong p donors dramatically accelerate the rate of intersystem crossing. This could be the reason why photolysis of p-azidoaniline (1) and p-azidodimethylaniline, unlike that of phenyl azide and most of its derivatives, gives only products of triplet nitrene reactions. 3,4 For instance, in the presence of oxygen at ambient temperature p-nitroaniline (2) and p-nitrosoaniline (3) are formed in quantitative yield upon photolysis of 1 (Scheme 1). 3,5 Since the early seventies it has been known that triplet arylnitrenes react with oxygen by formation of adducts. [5][6][7][8][9] In an early study on 1,4-diazidobenzene, two types of adducts (diamagnetic and paramagnetic) were detected in glassy matrices. 6 It was proposed that the diamagnetic adduct is an arylnitroso oxide, R-NQO 1 -O À , and the paramagnetic species is a ''spin isomer'' thereof, the triplet aryliminodioxy diradical, R-N-O -O . 6,10 Although subsequent studies did not confirm the formation of triplet adducts of arylnitrenes with oxygen, 3,5,7,8 speculations about the occurrence of aryliminodioxy diradicals in the photooxidation of aryl azides continue to surface occasionally in the literature. 11 This paper is devoted to a comprehensive experimental and computational study of the reactions of triplet arylnitrenes with oxygen in order to unambiguously identify the resulting adducts by their experimentally observed spectra, and to understand their properties. It should be noted that the earlier assignments of a strong near-UV absorption to arylnitroso oxides 3,5,7-9 were never supported by calculations. p-Azidoaniline was used in our matrix isolation study because, as noted above, it gives exclusively products of triplet nitrene reactions in solution, and we thought that this would also simplify the photochemistry in Ar matrices. In addition, the reaction of triplet p-aminophenylnitrene (4) with oxygen in solution and in glassy matrices has previously been studied. 5 2 Experim...

Laser flash photolysis (LFP, Nd:YAG laser, 35 ps, 266 nm, 10 mJ or KrF excimer laser, 10 ns, 249 nm, 50 mJ) of 2-fluoro, 4-fluoro, 3,5-difluoro, 2,6-difluoro, and 2,3,4,5,6-pentafluorophenyl azides produces the corresponding singlet nitrenes. The singlet nitrenes were detected by transient absorption spectroscopy, and their spectra are characterized by sharp absorption bands with maxima in the range of 300-365 nm. The kinetics of their decay were analyzed as a function of temperature to yield observed decay rate constants, k(OBS). The observed rate constant in inert solvents is the sum of k(R) + k(ISC) where k(R) is the absolute rate constant of rearrangement of singlet nitrene to an azirine and k(ISC) is the absolute rate constant of nitrene intersystem crossing (ISC). Values of k(R) and k(ISC) were deduced after assuming that k(ISC) is independent of temperature. Barriers to cyclization of 4-fluoro-, 3,5-difluoro-, 2-fluoro-, 2,6-difluoro-, and 2,3,4,5,6-pentafluorophenylnitrene in inert solvents are 5.3 +/- 0.3, 5.5 +/- 0.3, 6.7 +/- 0.3, 8.0 +/- 1.5, and 8.8 +/- 0.4 kcal/mol, respectively. The barrier to cyclization of parent singlet phenylnitrene is 5.6 +/- 0.3 kcal/mol. All of these values are in good quantitative agreement with CASPT2 calculations of the relative barrier heights for the conversion of fluoro-substituted singlet aryl nitrenes to benzazirines (Karney, W. L. and Borden, W. T. J. Am. Chem. Soc. 1997, 119, 3347). A single ortho-fluorine substituent exerts a small but significant bystander effect on remote cyclization that is not steric in origin. The influence of two ortho-fluorine substituents on the cyclization is pronounced. In the case of the singlet 2-fluorophenylnitrene system, evidence is presented that the benzazirine is an intermediate and that the corresponding singlet nitrene and benzazirine interconvert. Ab initio calculations at different levels of theory on a series of benzazirines, their isomeric ketenimines, and the transition states converting the benzazirines to ketenimines were performed. The computational results are in good qualitative and quantitative agreement with the experimental findings.

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Laser flash photolysis (LFP) of perfluorophenyl azide and perfluoro-4-biphenyl azide produces the corresponding singlet nitrenes which were detected by their transient absorptions at 330 and 350 nm, respectively. The absolute rate constants of the fundamental processes that consume the singlet nitrenes (intersystem crossing, k isc, rearrangement, k R; reaction with pyridine, k pyr) were determined by monitoring the decay of the singlet nitrene and by the growth of its reaction products (ketenimine, triplet nitrene, or pyridine ylide). In the case of singlet 4-perfluorobiphenylnitrene in CH2Cl2 k isc = (2.2 ± 0.1) × 106 s-1, k R = 1013.2±0.2 exp[−(9400 ± 400)/RT] s-1, and k pyr = 109.06±0.15 exp[−(2400 ± 200)/RT] M-1 s-1. In the case of singlet perfluorophenylnitrene in CH2Cl2 k isc = (1.05 ± 0.05) × 107 s-1, k R = 1013.8±0.3 exp[−(8800 ± 400)/RT] s-1, and k pyr = 109.00±0.13 exp[−(1600 ± 160)/RT] M-1 s-1.