Modern Molecular Mechanics and ab Initio Calculations on Benzylic and Cyclic Delocalized Cations

Abstract

Calculations of optimized force field (MMP2 extended to carbocations) and ab initio (MP2/6-31G*) geometries as well as π-electron densities of various benzyl and cyclic delocalized cations agree well. The MMP2 heats of formation reproduce the available experimental values. MMP2 π-resonance energies are consistent with those obtained by isodesmic equations from experimental and ab initio data. When carbon π-charges are lower than 0.2, the influence of phenyl substituents is attenuated. Thus, the triphenylmethyl cation resonance stabilization value (−41.6 kcal/mol average for each phenyl ring) is much less than that of the benzyl cation (−76.4 kcal/mol) and the benzhydryl cation (average stabilization value of −51.4 kcal/mol). MMP2 aromatic stabilization energy estimates of the benzyl and tropylium cations as well as benzene agree well with the assessments of aromaticity by the nucleus independent chemical shift (NICS) criterion, which is based on the magnetic shieldings computed at ring centers. The MMP2 method allows quantitative evaluations of homoconjugative interactions. The stabilization in the homotropylium cation due to 1,7 homoconjugative overlap is estimated to be quite appreciable, −13.4 kcal/mol.

Gas-Phase Chemistry of Benzyl Cations in Dissociation ofN-Benzylammonium andN-Benzyliminium Ions Studied by Mass Spectrometry

“…Studying the structural properties and reactivities of benzyl cations is a subject of fundamental importance. Experimental observation supported by theoretical calculations indicates that two major resonance forms, as shown in Scheme 1, contribute significantly to the total electronic structure of the benzyl cation [1][2][3][4][5][6]. The well-known reactivity of benzyl cation is electrophilicity due to the phenylmethyl cation character.…”

Electronic absorptions of the benzylium cation

“…1-17 The tropylium isomer (Tr + ), a seven membered ring, is predicted to be extremely stable due to its aromatic electronic structure with six π electrons (satisfying Hückel's aromaticity rule). The benzylium cation (Bz + ), a six membered ring with an attached methylidene group, is stabilised through resonance interactions, 12 but is predicted to lie slightly higher in energy than Tr + (by ∼0.4 eV) with rearrangement requiring passage over a substantial ∼3 eV barrier. [14][15][16][17] In mass spectrometric studies, the two isomers are usually distinguished by their reactivity: the Bz + isomer transfers CH + 2 to toluene forming C 8 H + 9 , whereas the more stable Tr + molecule is unreactive.…”

Carbocations in Superacid Systems

“…230,1010 At low temperature, the methylene protons of the ion 578 exhibit nonequivalence, indicating ring puckering. 346,1026 Both MP2(full)/6-31G* and MMP2 geometries give almost perfect CÀC bond equalization in the seven-membered ring moiety. 1016 Subsequent computational studies by Schleyer and co-workers 1017 have confirmed the bent structure of 578.…”

“…Reindl et al 346 used force field (MMP2) and ab initio (MP2/6-31G*) methods to show, however, that stabilization by the phenyl group is attenuated. Strong contribution from para-(and ortho-) quinonoidal resonance forms are responsible for much of the reactivity of the ion.…”