Electron spin resonance characterization of ground state triplet aryl cations substituted at the 4 position by dialkylamino groups

Abstract

UV irradiation at 77 K of arenediazonium salts substituted at the 4 position by dialkylamino groups yields ESR spectra comprised of three principal, broad peaks at ca. 800, 1500, and 4400 G, together with a resonance at g -2.0. The low-and high-field absorptions are assigned to the aryl cation, Ar+, which appears to be stabilized in its triplet configuration by the -NR2 group in accordance with recent ab initio calculations. The D parameter for these unusual triplet states is ca. 0.24 cm-l, indicating a high K spin density on C1. The triplet states were produced from powders and solutions in both polymer films and a 9 M LiCl aqueous glass, and constitute irrefutable evidence for the existence of these carbonium ions.The aryl cation Ar+ has long been regardedZa as the probable key intermediate in both the thermal2" and photochemicalZb decomposition of arenediazonium salts in aqueous solution:L o r hw ArN*+ -Ar+ + Nz While there have been suggestions (summarized in Zollinger's review2c) that the departure of N2 may coincide with arrival of a water molecule or other n~cleophile,~ this view has been strongly c o n t e~t e d .~~.~ The extremely high reactivity of the supposed Ar+ is implied both by its lack of selectivity toward nucleophile^^^ and its apparent ability to reverse reaction 1 as evinced both by scrambling of I5N-l4N in partially decomposed ArN*+X-and by incorporation of N * when dediazoniation is carried out under high pressures of n i t r~g e n .~ The recent intensified interest in Ar+ has focused upon (1) its detection and role as an intermediate in gas-phase studies involving mass spectrometry,6a ion cyclotron resonance,6b and crossed molecular beams,6c and (2) theoretical approaches to its structure, dimensions, and spin m~l t i p l i c i t y .~~~ In the latter context, Taft's early proposal9 (modified by AbramovitchIo) that Ar+ is a triplet ground state, based on anomalously large accelerating effects of substituent groups such as m-OMe on the thermal decomposition of ArN*+ (an argument since criticized4"), has been developed by several groups using semiempirical m e t h o d~~~,~ and most recently by a b initio calculations of Pople etThe latter approach suggests that of the two alternative electronic configurations for Ar+, the "Taft configuration" ( x )~ (sp2)I is especially stabilized8c by a p-NH2 group; otherwise the ( T )~ (sp*)O configuration, albeit highly distorted with L C~C I C~ -145O, is the more stable form by a considerable margin (e.g., 20.1 kcal mol-] for C6Hs+ itself).8b Other "triplet-stabilizing'' influences are substitution a t the ortho positions (by NH2 or OH) and a t the para position by OH (although only the latter feature gave per se a resultant triplet ground state apart from p-NH2, and then only as a small difference between two large figures). Another group" has argued that the greater selectivity of p-O2NC6H4+ toward attack on arenes (compared with C6H5+) is best understood in terms of both species existing as triplets, with the NO2 substituent exercising additional stabi...

Singlet–triplet excitation energies of substituted phenyl cations: a G4(MP2) and G4 theoretical study

“…Other researchers have observed this via theoretical studies [15,16]. Cox et al [54] experimentally observed a triplet ground state for the p-dimethylaminophenyl cation. At the G4 level, we calculate AE S−T of −30.4, 18.0, and −36.6 kJ/mol for the ortho-, meta-, and para-substituted dimethylaminophenyl cation derivatives, in support of the experimental claims for a para-substituted triplet ground state.…”

Specific photoaffinity labeling induced by energy transfer: application to irreversible inhibition of acetylcholinesterase.

“…The pronounced protection against enzyme inactivation observed in the presence of tetraethylammonium (Table 1) indicates that the binding of DDF very likely occurs at the anionic subsite. By irradiation, DDF loses nitrogen and produces a triplet diradical (24,25). It is possible that covalent labeling occurred at the residue responsible for cation binding because of the extreme reactivity of the photogenerated diradical.…”

Sterically hindered aromatic compounds. XI. Spectral and product studies of the decomposition of N-nitrosoacetanilides

“…To provide The aromatic dediazoniation reaction has been Some answers 10 this question We selected N-nithe subject of renewed interest by a number of ~~OSO-2,4,6-tri-tert-b~tyla~etanilide (1) for dediazinvestigators (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12). The variety of reactive inter-oniation studies.…”

“…The variety of reactive inter-oniation studies. This compound will undergo the mediates involved: diazonium ions (5), aryl cations reaction without added reagents and is therefore a (6)(7)(8), nitrogen -aryl cation molecule -ion pairs ( 1 , "cleaner" system for studying the effect of varying 9, lo), aryl radicals (2), and benzynes (3, 4, 11), conditions such as the Presence of an electron have attracted considerable interest as have newer transfer medium on the reactive intermediates methods of deamination of arylamines for synthetic involved and products formed. Some sPectroscoPpurposes (12).…”