Ab Initio Characterization of Phenylnitrenium and Phenylcarbene: Remarkably Different Properties for Isoelectronic Species

Cramer, C. J.; Dulles, F. J.; Falvey, D. E.
J. Am. Chem. Soc. 1994, 116, 9787.

The S-T gaps for phenylcarbene and phenylnitrenium are predicted to be 4.3 and -21.2 kcal/mol, respectively, at the BVWN5/cc-pVTZ//BVWN5/cc-pVDZ (density functional theory (DFT), 294 basis functions) level of theory including zero-point corrections. Triplet phenylcarbene is a planar species (ip) with a barrier to rotation about the exocyclic bond of 2.7 kcal/mol; however, the corresponding structure for triplet phenylnitrenium is a rotational transition state (TS) structure. The local minimum instead has the N-H bond perpendicular to the aromatic ring (oop) and the rotational barrier is 1.8 kcal/mol. High spin contamination causes the unrestricted Hartree-Fock (UHF) Hessian matrix to erroneously have no negative eigenvalues for the planar structure. Post-HF methods all agree that the perpendicular isomer is lower in energy than the planar, as does DFT, which moreover predicts a single imaginary frequency for the latter and none for the former. Singlet phenylcarbene is a planar species with a barrier to rotation about the exocyclic bond of 11.0 kcal/mol; however, singlet phenylnitrenium does not rotate about this bond, but instead inverts through nitrogen, resulting in a high-energyC2v linear TS structure; the inversion barrier is 26.0 kcal/mol. MP4 energies are very untrustworthy because of the high spin contamination of the UHF wavefunctions. Projected MP4 (PMP4) energies are more useful, but significant quantitative differences remain in comparison to the DFT results; the latter are quite consistent for three different sets of correlation functionals and appear converged for BVWN5 with respect to basis set.

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