Geometry and Energy of Substituted Phenyl Cations

Abstract

The geometry and the energy of a number of substituted phenyl cations have been calculated for both spin states at the UB3LYP/6-31G(d) level (o-, m-, p-Me, OMe, NH(2), CN, NO(2)) or at the UB3LYP/6-311++G(2d,p) level (o-, m-, p-SiMe(3), SMe). The geometric differences were assessed by means of a self-organizing neural network. The triplets maintain a regular hexagonal structure that is minimally affected by substituents, while in the singlets C1 puckers inward and, when an electron-donating group is present, shifts out of the plane. The triplets have the character of aromatic radical ions and are strongly stabilized by electron-donating substituents, independently of the position of the latter. In the case of singlets, the effect of substituents on the energy is weaker and depends on the position (the largest effect is exerted when the group is in meta). A two-parameter correlation of all of the triplet energies shows the predominant mesomeric effect of the substituents. In the case of singlets, linear correlations are obtained only when each position is treated separately and when the predominant effect is inductive for the ortho and, less markedly, the para position, whereas at the meta position, mesomeric and inductive effects are comparable. The ground state is determined to be the singlet for the parent cation and for electron-withdrawing substituted ions. With electron-donating substituents, the triplet is the ground state for ortho and para derivatives, while the two spin states are roughly isoenergetic when the donating group is in the meta position. These data allow predicting the reactivity of each cation.

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

“…Given the range in E S-T differences between our G4 results and the DFT data of Lazzaroni et al [17], although neither G4MP2 nor G4 E S-T calculations could be completed on the o-methoxyphenyl cation due to the presence of imaginary frequencies, there is ambiguity regarding the ground state of this cation as well (i.e., the UB3LYP/6-31G(d) E S-T underestimate for the o-methoxyphenyl cation may be greater than 32 kJ/mol, in which case the ground state would be a singlet). Alternatively, the presence of the imaginary frequencies at the G4MP2 and G4 levels may not be artifacts of the computational methods, and these compounds may not exist in the gas phase at ambient temperature for a finite lifetime.…”

“…At the G4 level, we are able to reproduce the gas phase findings of Bondarchuk and Minaev [16] for the o-nitrophenyl cation (see Supplementary Material for structure), while the mand p-nitrophenyl cations give converged structures using the G4 method with no imaginary frequencies and absent any intramolecular rearrangements. However, the triplet states of the o-, m-, and p-nitrophenyl cations all yield one imaginary frequency at the G4 level, preventing comparison with the E S-T estimates of Lazzaroni et al [17] for these cations (the o-nitrophenyl cation E S-T data was not provided in the Supporting Information of ref. [17], but values of 79.0 and 65.4 kJ/mol for the m-and p-isomers is available from these authors).…”

“…However, the triplet states of the o-, m-, and p-nitrophenyl cations all yield one imaginary frequency at the G4 level, preventing comparison with the E S-T estimates of Lazzaroni et al [17] for these cations (the o-nitrophenyl cation E S-T data was not provided in the Supporting Information of ref. [17], but values of 79.0 and 65.4 kJ/mol for the m-and p-isomers is available from these authors).…”

“…Of greater import, the G4 E S-T are in qualitative disagreement with the DFT data regarding the ground state multiplicity of some phenyl cations with electron donating substituents. Lazzaroni et al [17] predicted that the o-, m-, and p-amino and thiomethoxyphenyl cations would all be triplet ground states, whereas the G4 calculations predict only the o-and p-isomers will be triplet ground states, whereas the m-isomers will clearly (+18 kJ/mol for the m-amino and +17 kJ/mol for the mthiomethoxy) be singlet ground states. Similarly, Lazzaroni et al [17] predicted the p-methoxyphenyl cation to be a triplet ground state (by almost 20 kJ/mol), while the G4 results predict a singlet ground state (by about 4-6 kJ/mol).…”

“…n/a n/a n/a n/a n/a -Cl n/a n/a n/a n/a n/a -C≡CH n/a n/a n/a n/a n/a -NH n/a n/a n/a n/a n/a n/a n/a n/a The thiomethoxy\ phenyl cation E S-T in ref. [17] were calculated at the UB3LYP/6-311++G(2d,\ p) level.…”