Effect of a Heteroatom on Bonding Patterns and Triradical Stabilization Energies of 2,4,6-Tridehydropyridine versus 1,3,5-Tridehydrobenzene

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 113; no. 11; pp. 2591 - 2599
• Main Authors: Manohar, Prashant U, Koziol, Lucas, Krylov, Anna I
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 19.03.2009
• Subjects: A: Molecular Structure, Quantum Chemistry, General Theory
• ISSN: 1089-5639; 1520-5215; 1520-5215
• DOI: 10.1021/jp810522e

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 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 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.