posted on 2000-08-11, 00:00authored bySanjay Srivastava, Patrick H. Ruane, John P. Toscano, Michael B. Sullivan, Christopher J. Cramer, Dominic Chiapperino, Elizabeth C. Reed, Daniel E. Falvey
A series of para-substituted N-methyl-N-phenylnitrenium ions (N-(4-biphenylyl)-N-methylnitrenium
ion, N-(4-chlorophenyl)-N-methylnitrenium ion, N-(4-methoxyphenyl)-N-methylnitrenium ion, and N-(4-methylphenyl)-N-methylnitrenium ion) were generated through photolysis of the appropriately substituted
1-aminopyridinium salt. Laser flash photolysis using UV−vis detection as well as photoproduct analysis verified
that the expected nitrenium ions were formed cleanly and rapidly following photolysis. Laser flash photolysis
with time-resolved infrared detection allowed for structural characterization of the nitrenium ions through
observation of a symmetrical aromatic CC stretch in the region 1580−1628 cm-1. The specific frequencies
reflect the degree of quinoidal character present in each phenylnitrenium ion (i.e., the degree to which the
nitrenium ion resembles a 4-iminocyclohexa-2,5-dienyl cation). The 4-methoxy derivative shows the highest
frequency CC stretch, indicating that this strongly π-electron-donating substituent imparts more quinoidal
character, and the 4-chloro derivative shows the lowest frequency CC stretch, suggesting that it possesses
the least quinoidal character. Quantum calculations using density functional theory (BPW91/cc-pVDZ) were
carried out on the same nitrenium ions. The theoretically derived IR frequencies showed excellent quantitative
agreement with the experiment. The computed structures show significant bond length alternation in the phenyl
rings, shortened C−N bond lengths, and substantial positive charge delocalization into the phenyl rings. All
of these effects are more pronounced with increasing π-donating character of the ring substituent. Arylnitrenium
ions are well described as 4-iminocyclohexa-2,5-dienyl cations.