The electronic and geometric structures of formyl, acetyl, and benzoyl azides were studied and fragments of the potential surfaces for the thermal Curtius rearrangement of these azides into the corresponding isocyanates were calculated by density functional theory at the PBE/TZ2P level. Acyl azides adopt two stable, conformations syn and anti, with respect to the C-N bond. The syn conformers are more stable than their anti analogs. The activation energies of the syn-anti isomerization in the series under study are 9.4, 7.0, and 9.2 kcal mol -1 , respectively, and the activation energies of the reverse reaction are 8.5, 6.1, and 2.5 kcal mol -1 . The rearrangement of syn acyl azides is a one step process, in which elimination of N 2 occurs synchronously with the rearrangement of atoms and bonds to form isocyanates. The activation energies of the rearrangements of syn HC(O)N 3 , syn MeC(O)N 3 , and syn PhC(O)N 3 are 28.0, 32.9, and 34.5 kcal mol -1 , respectively. The rearrangement of the anti conformers of the above mentioned azides involves the formation of singlet acylnitrene. The activation energies of the latter process are 34.6, 32.9, and 32.3 kcal mol -1 , respectively. The activation energies of the rearrangement of acylnitrenes into isocyanates are 20.9, 18.9, and 13.6 kcal mol -1 , respec tively. The energy characteristics of the process and the structural data for the starting com pounds, final products, and transition states provide evidence that the thermal Curtius rear rangement occurs predominantly by a concerted mechanism.

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