Extended ab initio and theoretical thermodynamics studies of the Bergman reaction and the energy splitting of the singlet o-, m-, and p-benzynes

Bergman reaction and the energy splitting of the transformations of (Z)-hex-3-ene-1, 5-diyne (enediyence) into the singlet coupled biradical p-benzyne are reinvestigated at the CCSD(T) and CASPT2(g1) levels of theory for various basis sets using ab-initio. Underestimation of 5 Kcal/mol and 7 Kcal/mol is found for heat of formation on using small basis sets and CASPT2(g1) methods as compared to CCSD(T) level and method respectively. There are no significant differences in CCSD(T) and CASPT2(g1) methods for the energy splitting.