Quantum Chemical Analysis of para-Substitution Effects on the Electronic Structure of Phenylnitrenium Ions in the Gas Phase and Aqueous Solution

Sullivan, M. B.; Brown, K.; Cramer, C. J.; Truhlar, D. G.
J. Am. Chem. Soc. 1998, 20, 11778.

Ab initio calculations for para-substituted phenylnitrenium ions predict larger singlet-triplet splittings, shorter singlet C-N⁺ bond lengths, and higher singlet aromatic ring stretching frequencies for substituents with greater electron donating character. Trends in these properties correlate linearly with para substituent constants sigma_R+, indicating that phenylnitrenium ions closely resemble other electron-deficient aromatic systems where resonance interactions with substituents are dominant. Sensitivity to substitution is large as judged by the slope of the correlating line for aqueous singlet-triplet splittings, r = 6.4. For 13 of 15 substituted cases, aqueous solvation preferentially stabilizes the singlet state by 0.9 to 4.4 kcal/mol; for the p-CO₂H and p-CF₃ cases, the triplet state is better solvated by less than 1 kcal/mol.

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