Computational studies on stable triplet states of heteroacetylenes and the effects of halogen substituents

This paper describes theoretical studies of halogen-substituted heteroacetylenes (XC=MY, M = Si and Ge; X, Y = H, Cl and F) performed at the QCISD(T)/6-311G**//QCISD/6-31G* level of theory. The electronegative halogen substituents destabilize the singlet state such that the triplet state tends to become favorable. The triplet state has the bifunctional electronic structure of a triplet carbene joined to a heavy singlet carbene. We found that the substituents effectively reduce the energy of the donor-acceptor interactions (E_D-A) between the two in-plane lone pairs of electrons of the singlet state; therefore, the remaining π bond is less favorable energetically than the triplet state with a σ bond. A related phenomenon occurs for the homonuclear heavy acetylenes in singlets in which the lead compound RPbPbR switches to a Pb-Pb σ bond from the π bonds observed for the lighter acetylenes.