The o-, m-, and p-benzyne radical cations: a theoretical study

Abstract

On the basis of the CASPT2 (multiconfigurational second-order perturbation theory) geometry optimization calculations, the ground states of the o-C₆H₄⁺ (C_2v), m-C₆H₄⁺ (C_2v), and p-C₆H₄⁺ (D_2h) radical cations were determined to be 1 ²B₁, 1 ²A₂, and 1 ²B_1u, respectively. For o-C₆H₄⁺ and m-C₆H₄⁺, the first excited states (1 ²A₂ and 1 ²A₁, respectively) lie very close to the respective ground states. The small distance value of 1.419 Å between the two dehydrocarbons in the ground-state geometry of m-C₆H₄⁺ indicates that there is a real chemical bond between the two dehydrocarbons (the distance in the 1 ²A₁ geometry of m-C₆H₄⁺ is very long as in the m-C₆H₄ molecule). The (U)B3LYP isotropic proton hfcc (hyperfine coupling constant) calculation results imply that the ground and first excited states of o-C₆H₄⁺ will have similar ESR spectrum patterns while the ground and first excited states of m-C₆H₄⁺ will have completely different ESR spectrum patterns.