Benzvalyne: Real or imaginary?

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 C_2v 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 C_2v 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.