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Degenerate hydrogen atom exchange reactions have been studied using calculations, based on density functional theory (DFT), for (i) benzyl radical plus toluene, (ii) phenoxyl radical plus phenol, and (iii) methoxyl radical plus methanol. The first and third reactions occur via hydrogen atom transfer (HAT) mechanisms. The transition structure (TS) for benzyl/toluene hydrogen exchange has C(2)(h)() symmetry and corresponds to the approach of the 2p-pi orbital on the benzylic carbon of the radical to a benzylic hydrogen of toluene. In this TS, and in the similar C(2) TS for methoxyl/methanol hydrogen exchange, the SOMO has significant density in atomic orbitals that lie along the C-H vectors in the former reaction and nearly along the O-H vectors in the latter. In contrast, the SOMO at the phenoxyl/phenol TS is a pi symmetry orbital within each of the C(6)H(5)O units, involving 2p atomic orbitals on the oxygen atoms that are essentially orthogonal to the O.H.O vector. The transferring hydrogen in this reaction is a proton that is part of a typical hydrogen bond, involving a sigma lone pair on the oxygen of the phenoxyl radical and the O-H bond of phenol. Because the proton is transferred between oxygen sigma orbitals, and the electron is transferred between oxygen pi orbitals, this reaction should be described as a proton-coupled electron transfer (PCET). The PCET mechanism requires the formation of a hydrogen bond, and so is not available for benzyl/toluene exchange. The preference for phenoxyl/phenol to occur by PCET while methoxyl/methanol exchange occurs by HAT is traced to the greater pi donating ability of phenyl over methyl. This results in greater electron density on the oxygens in the PCET transition structure for phenoxyl/phenol, as compared to the PCET hilltop for methoxyl/methanol, and the greater electron density on the oxygens selectively stabilizes the phenoxyl/phenol TS by providing a larger binding energy of the transferring proton.

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.

The rearrangement of singlet phenylnitrene (1a) to 1-azacyclohepta-1,2,4,6-tetraene (3a) has been studied computationally, using the CASSCF and CASPT2N methods in conjunction with the 6-31G*, cc-pVDZ, and 6-311G(2d,p) basis sets. Ring expansion from the 1A2 state of 1a is predicted to occur in two steps via 7-azabicyclo[4.1.0]hepta-2,4,6-triene (2a) as an intermediate. The rearrangement of 1a to 2a is estimated to have a barrier of ca. 6 kcal/mol and to be rate-determining. Azirine 2a is unlikely to be detected, because of the small calculated barrier (ca. 3 kcal/mol) to its rearrangement to 3a. At the CASPT2N/6-311G(2d,p)//CASSCF(8,8)/6-31G* + ZPE level of theory, the reaction 1a → 3a on the lowest singlet potential energy surface is calculated to be exothermic by 1.6 kcal/mol. This reaction is predicted to be ca. 19 kcal/mol less exothermic, but to have a barrier ca. 9 kcal/mol lower than the analogous ring expansion of 1A‘ phenylcarbene (1b) to cyclohepta-1,2,4,6-tetraene (3b). Factors which contribute to these and other energetic differences between the ring expansion reactions of 1a and 1b are discussed. The lowest singlet state of planar 1-azacyclohepta-1,3,5-trien-7-ylidene (4a) is an open-shell singlet (11A‘‘), which is calculated to be ca. 20 kcal/mol above 3a and to be the transition state for enantiomerization of 3a. Unlike cycloheptatrienylidene (4b), 4a is predicted to have a triplet ground state and an energy difference between 11A‘‘ and the lowest triplet (13A‘‘) of ca. 1 kcal/mol. The similar geometries of and small adiabatic energy difference between these two states of 4a is probably the reason why triplet 4a has not been detected by EPR.

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We observed ring expansion of 1-methylcyclobutylfluorocarbene at 8 kelvin, a reaction that involves carbon tunneling. The measured rate constants were 4.0 x 10(-6) per second in nitrogen and 4 x 10(-5) per second in argon. Calculations indicated that at this temperature the reaction proceeds from a single quantum state of the reactant so that the computed rate constant has achieved a temperature-independent limit. According to calculations, the tunneling contribution to the rate is 152 orders of magnitude greater than the contribution from passage over the barrier. We discuss environmental effects of the solid-state inert-gas matrix on the reaction rate.

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.

High levels of ab initio (MP2, CASSCF, CASPT2N) and density functional (Becke3LYP) theory have been used to assess the homoaromatic character of some strained semibullvalenes. Based on geometric, energetic, and magnetic criteria (magnetic susceptibility exaltations and the nucleus independent chemical shifts, NICS), C s semibullvalene itself is not aromatic, but the C 2 v transition state for its Cope rearrangement is highly bishomoaromatic. Appropriate annelations destabilize the C s geometries, and the bishomoaromatic structures are the only minima for several semibullvalenes:  1,5-methano; 2,8:4,6-bisethano and -bismethano; and 2,8-monoethano. In contrast, the 4,6-ethanosemibullvalene is predicted to be localized and not homoaromatic. Inclusion of dynamic electron correlation is very important for computing the geometries and relative energies of the delocalized structures. To aid experimental investigations, the UV and 13C NMR spectra of some of the semibullvalenes have been predicted. Long-wavelength UV absorptions and down-field 13C NMR chemical shifts for C2,8,4,6 are characteristic.