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Electronic Structure of Singlet and Triplet Carbenes, Nitrenium Ions and Valence Isoelectronic Analogs from MCSCF and DFT Calculations

Worthington, S. E.; Cramer, C. J.; Dulles, F. J.; Storer, J. W.
in CD-ROM Proceedings of the First Electronic Computational Chemistry Conference, Bachrach, S. M., Boyd, D. B., Gray, S. K., Hase, W., Rzepa, H. S., Eds.; ARInternet: Landover, MD, 1996.

Aromatic nitrenium ions have been postulated as carcinogenic species derived from the in vivo catabolism of aromatic amines. Nitrenium ions incorporate a disubstituted, positively charged nitrogen atom, and may thus have either singlet or triplet spin multiplicity. The former spin state is implicated in carcinogenesis. We have calculated singlet-triplet gaps for a number of small to medium-sized nitrenium ions, in particular nitrenium, imenium, methylnitrenium, aziridenium, and phenylnitrenium; similar calculations have also been done for the analogous isoelectronic carbenes and for other valence isoelectronic analogs in the XH2 series. Several levels of theory have been employed, including multi-configuration self consistent field, multi-reference configuration interaction, coupled-cluster theory, and density functional theory. The electronic and steric factors that influence the singlet-triplet gaps for these species are discussed, and the levels of theory are compared. Density functional theory appears to hold promise for larger, biologically relevant nitrenium ions.