title: D(3h) [A-CE3-A]⁻ (E = Al and Ga, A = Si, Ge, Sn, and Pb): a new class of hexatomic mono-anionic species with trigonal bipyramidal carbon.
abstract: The non-classical trigonal bipyramidal carbon (TBPC) arrangement generally exists as transition states (TSs) in nucleophilic bimolecular substitution (S(N)2) reactions. Nevertheless, chemists have been curious about whether such a carbon bonding could be stable in equilibrium structures for decades. As the TBPC arrangement was normally realized as cationic species theoretically and experimentally, only one anionic example ([At-C(CN)3-At](-)) was computationally devised. Herein, we report the design of a new class of anionic TBPC species by using the strategy similar to that for stabilizing the non-classical planar hypercoordinate carbon. When electron deficient Al and Ga were used as the equatorial ligands, eight D(3h) [A-CE3-A](-) (E = Al and Ga, A = Si, Ge, Sn, and Pb) TBPC structures were found to be the energy minima rather than TSs at both the B3LYP and MP2 levels. Remarkably, the energetic results at the CCSD(T) optimization level further identify [Ge-CAl3-Ge](-) and [Sn-CGa3-Sn](-) even to be the global minima and [Si-CAl3-Si](-) and [Ge-CGa3-Ge](-) to be the local minima, only slightly higher than their global minima. The electronic structure analyses reveal that the substantial ionic C-E bonding, the peripheral E-A covalent bonding, and the axial mc-2e (multi center-two electrons) bonding play roles in stabilizing these TBPC structures. The structural simplicity and the high thermodynamic stability suggest that some of these species may be generated and captured in the gas phase. Furthermore, as mono-anionic species, their first vertical detachment energies are differentiable from those of their nearest isomers, which would facilitate their characterization via experiments such as the negative ion photoelectron spectroscopy.
authors.affiliation: Institute of Molecular Science, Key Laboratory of Chemical Biology and Molecular Engineering of Education Ministry, Shanxi University, Wucheng Road 92#, Taiyuan 030006, People's Republic of China.
authors.forename: Yan-Bo
authors.initials: YB
authors.lastname: Wu
authors.affiliation: Institute of Molecular Science, Key Laboratory of Chemical Biology and Molecular Engineering of Education Ministry, Shanxi University, Wucheng Road 92#, Taiyuan 030006, People's Republic of China.
authors.forename: Yan-Qin
authors.initials: YQ
authors.lastname: Li
authors.affiliation: Institute of Molecular Science, Key Laboratory of Chemical Biology and Molecular Engineering of Education Ministry, Shanxi University, Wucheng Road 92#, Taiyuan 030006, People's Republic of China.
authors.forename: Hui
authors.initials: H
authors.lastname: Bai
authors.affiliation: Institute of Molecular Science, Key Laboratory of Chemical Biology and Molecular Engineering of Education Ministry, Shanxi University, Wucheng Road 92#, Taiyuan 030006, People's Republic of China.
authors.forename: Hai-Gang
authors.initials: HG
authors.lastname: Lu
authors.affiliation: Institute of Molecular Science, Key Laboratory of Chemical Biology and Molecular Engineering of Education Ministry, Shanxi University, Wucheng Road 92#, Taiyuan 030006, People's Republic of China.
authors.forename: Si-Dian
authors.initials: SD
authors.lastname: Li
authors.affiliation: Institute of Molecular Science, Key Laboratory of Chemical Biology and Molecular Engineering of Education Ministry, Shanxi University, Wucheng Road 92#, Taiyuan 030006, People's Republic of China.
authors.forename: Hua-Jin
authors.initials: HJ
authors.lastname: Zhai
authors.affiliation: College of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Yuquan Road 19#, Beijing 100049, People's Republic of China.
authors.forename: Zhi-Xiang
authors.initials: ZX
authors.lastname: Wang