A REKS assessment of the face-diagonal bond in 1,3-didehydrocubane and a comparison with benzyne biradicals

The three isomeric cubane biradicals were studied using spin-restricted ensemble-referenced Kohn-Sham (REKS) density functional calculations at the B3LYP/6-31G(d) level of theory. The most stable biradical was found to be ortho-cubene with the meta-cubene and para-cubene biradicals 4.7 kcal mol(-1) and 17.8 kcal mol(-1) higher in energy, respectively. The singlet ground states are well separated from their lowest lying triplet states. These singlet-triplet energy differences mostly originate from through-space interactions for the ortho and meta isomers, whereas these interactions are rather weak for the para isomer. In contrast to para-benzyne, which was also considered for comparison with an unsaturated system, the singlet-triplet energy gap remains large in para-cubadiyl, mainly as a result of its much stronger through-bond interactions. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003).