| Abstract
| - 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,φ, of 40°. In this conformation, the aromatic rings are oriented so as to maximize interaction with the unpairedelectron while minimizing repulsion between the 2- and 2‘-hydrogen atoms. This calculated ground statestructure of Ph2N• differs from that, which has been accepted for the past 15 years, which had the two ringsorthogonal (φ = 90°) with one ring conjugating with the nitrogen's lone pair and the other conjugating withthe unpaired electron. This structure was based on unexpected differences between the UV−vis absorptionspectra of Ph2N• and the diphenylmethyl radical. However, our calculations indicate that this orthogonalstructure lies 3.5 kcal/mol above the global minimum. Further support for the staggered conformation ofPh2N• is provided by the similarities between absorption transition wavelengths determined theoretically andthe 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 (Ph2)+N-, isred-shifted as compared to the related transition from Ph2CH• to (Ph2)+CH- due to the electronegativity ofthe N atom. The absorption bands for PhCH2•, PhNH•, and PhO• in the 300−450 nm region are similar inposition, which has been taken to indicate that for isoelectronic species the electronic transition energiesshould be little affected by heteroatom substitution. Our calculations show, however, that these sets of absorptionbands arise from different transitions. Therefore, the experimentally similar 300−450 nm absorption bandsfor these three radicals are fortuitous and do not reflect some common, unifying traits, a fact that furtherserves to emphasize the importance of theory in the assignment of bands due to electronic transitions.
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