The isomerization of the highly strained benzvalyne structure to o-benzyne has been investigated using MCSCF and CCSD(T) levels of theory. Two reaction channels were modeled: the disrotatory one which leads...
Benzvalyne (C6H4) is a bicyclic structural isomer of o-benzyne that some typically trusted levels of theory do not report as a minimum on the potential energy surface (PES). The structure was found to be a C2v minimum at the MCSCF, MP2, coupled-cluster single double, coupled-cluster single double triple (CCSDT)-1b, and CCSDT-2 levels of theory. Density functionals at the B3LYP-D3, B2PLYP-D3, and M06-D3 levels also produced a minimum structure. On the other hand, the CCSD(T), CCSD(T)-F12, and CCSDT-1a methods produced a single imaginary frequency for benzvalyne. However, the increase in the correlation for the CCSDT-1b and CCSDT-2 methods implies that benzvalyne is, in fact, a true, if highly strained, minimum on the PES. The C–C≡C bond angle was found to be only 108°; this angle is 180° for an unstrained C–C≡C triple bond moiety. As a result, the strain energy is notably high at 159 kcal mol−1. Comparing the strain energy of the rest of the molecule gives a strain energy of 92 kcal mol−1 for this triple bond region alone. The computed harmonic frequencies contain normal modes consisting of two hindered rotations of the C≡C diatomic part of the molecule, suggesting that the dissociation of this diatomic from the bicylobutane moiety may be important in the chemistry of this molecule. Because the putative C2v minimum is predicted to have a significant dipole moment (2.6 D), benzvalyne may be detectable in TMC-1, where the related o-benzyne molecule has recently been observed by radio astronomy.
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.
The isomerizations of 3,4-diazatricyclo[4.1.0.02,7]hept-3-ene and 3,4-diazatricyclo[4.1.0.02,7]heptane to their corresponding products were studied by ab initio calculations.
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