Revised structure for the diphenylaminyl radical:: The importance of theory in the assignment of electronic transitions in Ph2X• (X = CH, N) and PhY• (Y = CH2, NH, O)

Density functional theory indicates that the minimum energy structure of the diphenylaminyl radical, Ph2N., has a "staggered" conformation in which the two phenyl rings are twisted relative to each other by an angle, phi, of 40degrees. In this conformation, the aromatic rings are oriented so as to maximize interaction with the unpaired electron while minimizing. repulsion between the 2- and 2'-hydrogen atoms. This calculated ground state structure of Ph2N. differs from that, which has been accepted for the past 15 years, which had the two rings orthogonal (phi = 90degrees) with one ring conjugating with the nitrogen's lone pair and the other conjugating with the unpaired electron. This structure was based on unexpected differences between the UV-vis absorption spectra of Ph2N. and the diphenylmethyl radical. However, our calculations indicate that this orthogonal structure lies 3.5 kcal/mol above the global minimum. Further support for the staggered conformation of Ph2N. is provided by the similarities between absorption transition wavelengths determined theoretically and the experimental absorption bands of Ph2N. and other diarylaminyl radicals generated by laser flash photolysis. The long wavelength transition of Ph2N., resulting in a structure that can be represented as (Ph-2)N-+(-), is red-shifted as compared to the related transition from Ph2CH. to (Ph-2)+CH- due to the electronegativity of the N atom. The absorption bands for PhCH2., PhNH., and PhO. in the 300-450 nm region are similar in position, which has been taken to indicate that for isoelectronic species the electronic transition energies should be little affected by heteroatom substitution. Our calculations show, however, that these sets of absorption bands arise from different transitions. Therefore, the experimentally similar 300-450 nm absorption bands for these three radicals are fortuitous and do not reflect some common, unifying traits, a fact that further serves to emphasize the importance of theory in the assignment of bands due to electronic transitions.