Spin isomerisation of para-substituted phenyl cations

Abstract

The singlet and triplet potential energy surfaces of a series of p-X-substituted aryl cations (X ‫؍‬ H, CN, CH 3 , F, OH, NH 2 ) are investigated computationally at the B3LYP/6-31G(d) level of theory. The first four species are found to be ground state singlets, the last has a triplet ground state, and the spin states of the OH derivative are almost isoenergetic. The minimum energy crossing points (MECPs) between the two surfaces are found to lie very little above the higher of the two minima in all cases, and the spin-orbit coupling is significant at those points. Therefore, it is expected that aryl cations will rapidly convert to their most stable spin state, and that in cases of near degeneracy such as for p-HO-C 6 H 4 ؉ , the states may interconvert rapidly enough to both be accessible in thermal reactions.

Homolytic cleavages in pyridinium ions, an excited state process

“…Yield 0.7 g, 89%. 4-(4-Phenyl-bytyl)-pyridine (19) was prepared by hydrogenation of (0.086 g, 0.41mmol) of the free base of 2 in 3 ml of EtOH at 1 atm and room temperature, in the presence of 0.015 g of 5% Pd/C catalyst. After 24 h stirring the catalyst was filtered off, the resulting solution was concentrated in vacuum, affording a crystalline colorless product in 93% yield (0.080g).…”

“…Thus it was found that in the case of phenyl cation the singlet-triplet splitting is relatively small and with a right choice of substituents at the phenyl ring can even be reversed [19]. The geometry for the cross-point between singlet and triplet has been calculated as well as spin orbit coupling for this transient structure [20].…”

The Reductive Elimination of Methane from ansa‐Hydrido(methyl)metallocenes of Molybdenum and Tungsten: Application of Hammond’s Postulate to Two‐State Reactions

“…Although there are, a priori, no reasons why the principle should not apply in such cases, this does not appear to be a generally appreciated phenomenon. We previously discussed Hammond's postulate to rationalize trends in spin-isomerization barriers for a series of para-substituted phenyl cations, [24] but there do not appear to have been other mentions in the context of spin-forbidden processes.…”

Singlet-triplet excitation energies of substituted benzenes: A G4 theoretical study

“…Similarly, the presence of G4 level singlet state imaginary frequencies for the p-hydroxy and thiophenyl cations not only prevents comparison with the E S-T estimates for these compounds in ref. [18,19], but also questions whether they have a gas phase singlet state potential energy well.…”