MRMP2, CCSD(T), and DFT Calculations of the Isomerization Barriers for the Disrotatory and Conrotatory Isomerizations of 3-Aza-3-ium-dihydrobenzvalene, 3,4-Diaza-3-ium-dihydrobenzvalene, and 3,4-Diaza-diium-dihydrobenzvalene.

The isomerizations of 3-aza-3-ium-dihydrobenzvalene, 3,4-diaza-3-ium-dihydrobenzvalene, and 3,4-diaza-diium-dihydrobenzvalene to their respective cyclic-diene products were studied using electronic structure methods with a multiconfigurational wave function and several single reference methods. Transition states for both the allowed (conrotatory) and forbidden (disrotatory) pathways were located. The conrotatory pathways of each structure all proceed through a cyclic intermediate with a trans double bond in the ring: this trans double bond destroys the aromatic stabilization of the π electrons due to poor orbital overlap between the cis and trans π bonds. The 3,4-diaza-3-ium-dihydrobenzvalene structure has C symmetry, and there are four separate allowed and forbidden pathways for this structure. The 3-aza-3-ium-dihydrobenzvalene structure is C symmetric, and there are two separate allowed and forbidden pathways for this structure. For 3,4-diaza-3,4-diium-dihydrobenzvalene, there was a single allowed and single forbidden pathway due to the C symmetry. The separation of the barrier heights for all three molecules was studied, and we found the difference in activation barriers for the lowest allowed and lowest forbidden pathways in 3,4-diaza-3-ium-dihydrobenzvalene, 3-aza-3-ium-dihydrobenzvalene, and 3,4-diaza-diium-dihydrobenzvalene to be 9.1, 7.4, and 3.7 kcal/mol, respectively. The allowed and forbidden barriers of 3,4-diaza-diium-dihydrobenzvalene were separated by 3.7 kcal/mol, which is considerably less than the 12-15 kcal/mol expected based on the orbital symmetry rules. The addition of the secondary ammonium group tends to shift the conrotatory and disrotatory barriers up in energy (∼12-14 kcal/mol (conrotatory) and 5-10 kcal/mol (disrotatory) per secondary NH group) relative to the barriers of dihydrobenzvalene, but there is negligible effect on E,Z to Z,Z isomerization barriers, which remain in the expected range of greater than 4 kcal/mol.