The concerted trimerization of ethyne to benzene revisited

CCSD(T)/6-311G∗∗//QCISD/6-311G∗∗ calculations on the concerted [2+2+2] trimerization of ethyne to benzene yield ΔHtrimo (HCCH)=−140.2 kcal/mol and ΔHacto(HCCH)=53.1 kcal/mol. The corresponding transition state (TS) possesses C2 symmetry, although both the planar D3h and nonplanar D3 structures are negligibly higher in energy, indicating extreme flatness of the potential energy hypersurface along the distortion paths. The analogous trimerizations of HCCCl and ClCCCl are predicted to be considerably more exothermic. As the respective TSs cannot be located and the planar pseudo-TSs that possess several imaginary vibrational frequencies are associated with high reaction barriers, the concerted mechanism can be ruled out for these reactions.