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

Michael B. Sullivan, Kenneth Brown, Christopher J. Cramer, and Donald G. Truhlar

J. Am. Chem. Soc., 120 (45), 11778-11783, 1998.

Contribution from the Department of Chemistry and Supercomputer Institute, University of Minnesota, 207 Pleasant St. SE, Minneapolis, Minnesota 55455-0431

ABSTRACT:

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.

DOI: 10.1021/ja982542a