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

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 triple 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 (2)A(1) state is 0.016 eV below the (2)B(2) state, whereas in 2,4,6-tridehydropyridine the heteroatom reverses adiabatic state ordering bringing (2)B(2) below (2)A(1) by 0.613 eV. The adiabatic doublet-quartet gap of 2,4,6-tridehydropyridine is smaller than that of 1,3,5-tridehydrobenzene by 0.08 eV; the respective values are 1.223 and 1.302 [corrected] eV. Moreover, the heteroatom reduces bonding interactions between the C(2) and C(6) radical centers, which results in the increased stabilizing interactions between C(4) and C(2)/C(6). Triradical stabilization energies corresponding to the separation of C(4) and C(2) 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 C(2) and C(6) are also observed in 2,6-didehydropyridine resulting in a nearly zero singlet-triplet energy 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.