Effect of a heteroatom on bonding patterns and triradical stabilization energies of 2,4,6-tridehydropyridine versus 1,3,5-tridehydrobenzene

P.U. Manohar, L. Koziol, and A.I. Krylov
J. Phys. Chem. A 113, 2591 – 2599 (2009)

Electronic structure of 2,4,6-tridehydropyridine and isoelectronic 1,3,5-tridehydrobenzene is characterized by the equation-of-motion spin-flip coupled-cluster calculations with single and double substitutions and including perturbative triples corrections. Equilibrium geometries of the three lowest electronic states, vertical and adiabatic states ordering, and triradical stabilization energies are reported for both triradicals. In 1,3,5-tridehydrobenzene, the ground 2A1 state is 0.016 eV below the 2B2 state, whereas in 2,4,6-tridehydropyridine the heteroatom reverses adiabatic state ordering bringing 2B2 below 2A1 by 0.613 eV. The doublet-quartet gap is also larger in 2,4,6-tridehydropyridine as compared to 1,3,5-tridehydrobenzene, the respective adiabatic values are 1.223 and 0.277 eV. Moreover, the heteroatom reduces bonding interactions between the C2 and C6 radical centers, which results in the increased stabilizing interactions between C4 and C2/C6. Triradical stabilization energies corresponding to the separation of C4 and C2 are 19.7 and -0.2 kcal/mol, respectively, in contrast to 2.8 kcal/mol in 1,3,5-tridehydrobenzene. Similarly weak interactions between C2 and C6 are also observed in 2,6-didehydropyridine resulting in a a nearly zero singlet-triplet gap, in contrast to m-benzyne and 2,4-didehydropyridine. The total interaction energy of the three radical centers is very similar in 1,3,5-tridehydrobenzene and 2,4,6-tridehydropyridine, and is 19.5 and 20.1 kcal/mol, respectively.

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