title: Computational studies on stable triplet states of heteroacetylenes and the effects of halogen substituents.
abstract: This paper describes theoretical studies of halogen-substituted heteroacetylenes (XCMY, 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 pi bond is less favorable energetically than the triplet state with a sigma bond. A related phenomenon occurs for the homonuclear heavy acetylenes in singlets in which the lead compound RPbPbR switches to a Pb-Pb sigma bond from the pi bonds observed for the lighter acetylenes.
authors.affiliation: Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan.
authors.forename: Mu-Jeng
authors.initials: MJ
authors.lastname: Cheng
authors.forename: Hsin-Mei
authors.initials: HM
authors.lastname: Cheng
authors.forename: San-Yan
authors.initials: SY
authors.lastname: Chu
mesh_terms: D000480:Alkynes / Q000737:chemistry*; D003198:Computer Simulation; D005857:Germanium / Q000737:chemistry*; D006219:Halogens / Q000737:chemistry*; D011789:Quantum Theory; D012825:Silicon / Q000737:chemistry*
chemical_list: D000480:Alkynes; D006219:Halogens; D005857:Germanium; D012825:Silicon