Michael Sullivan Online

The majority of my research efforts in graduate school were directed toward the study of open-shell organic molecules, particularly arylnitrenium ions. Nitrenium ions are isoelectronic with carbenes and are thought to be carcinogenic in the singlet state; nitrenium ions are formed in vivo from aromatic amines, a common pharmaceutical moiety. Although isoelectronic with carbenes, nitrenium ions can exhibit substantially different properties as a consequence of their positive charge. Specifically, pi-conjugation stabilizes the positive charge to so large an extent that phenylnitrenium ions exhibit substantial quinoidal character.

Since pi-conjugation plays so important a role, we explored systematic substitution at the para position of phenylnitrenium and analyzed trends in the singlet-triplet splittings and geometries. We found that electron donating groups stabilize the singlet state relative to the triplet. We also found that the quinoidal character of the arylnitrenium ion is sensitive to substitution. Specifically, in the singlet, pi-donors shorten the C-N⁺ bond length and increase the frequency of the normal mode corresponding to a symmetric combination of two C-C stretches parallel to the C-N⁺ bond in the singlet.

We studied the chemistry of azaaryl nitrenium ions. Synthetic work suggested that for 2-pyrimidylnitrenium ion, the singlet and triplet states are isoenergetic; we confirmed this surprising result and found other large effects associated with other nitrogen substitution. For example, triazinylnitrenium ion triplet is lower in energy than the singlet by 10 kcal/mol, which is a significant change from phenylnitrenium where the singlet is favored by 21 kcal/mol.

We also identified a novel low-barrier self-destruct pathway in certain azaaryl nitrenium ions whereby the 6-membered ring contracts to a 5-membered ring in a process that is highly exothermic. Lastly, we characterized another previously unknown pathway wherein N-acetyl arylnitrenium ions unimolecularly form a substituted benzoxazole. Unimolecular decomposition pathways for nitrenium ion singlet, may mitigate carcinogenicity by preventing the nitrenium ion from reacting with DNA.

This work was done in conjunction with Chris Cramer and the rest of the Cramer Group.