Figure 3.

(On the opposite page.) (A) Ground-state PES showing both primary and secondary C-Br bond-breaking reaction paths for 1,4-dibromobenzene. The energies are calculated at the B3LYP/6-311+G(2df,p)//6–31G(d) level of theory with zero-point energy corrections. The spin-orbit splitting of each Br atom (10.5 kcal⋅mol⁻¹) is taken into account (see Discussion). (B) General ground-state PES for benzynes characterized at the B3LYP/aug-cc-pVTZ//6-31G(d) level of theory with zero-point energy corrections. The values shown in parentheses were calculated by using the gaussian 2 additive scheme at the G2M(RCC6) level. Other reaction pathways, such as those involving diradical and carbene intermediates, have also been considered.