Theoretical Studies On The Structures And Properties Of The Species With Trigonal Bipyramidal Pentacoordinate Carbon

The non-classical trigonal bipyramidal carbon (TBPC) arrangement widely exists as transition states (TSs) in the nucleophilic bimolecular substitution (SN2) reactions.The quest for trigonal bipyramidal carbon (TBPC) originated from the curiosity whether the TBP arrangement around the central pentacoordinate carbon at midpoint of Walden inversion process of SN2 reactions (i.e. the SN2 transition state) can be a metastable intermediate? Now, TBPC arrangement is known to exist in many positively charged species like carbocations, but we would like to note here that most of the small clusters with non-classical carbon bonding were experimentally realized in negative ion photoelectron spectroscopy. As far as we known, there is only one example that reports the negatively charged TBPC species, i.e. [At-C(CN)3-At]-, but its thermodynamic stability is not systematically studied and thus the possibility for its experimental realization could not be assessed.In this work, we tried to use the strategy with the non-classical planar hypercoordinate carbon to devise the TPBC anions molecular. Based on theoretical calculation, a series of simple and stable TBPC compounds [A-CE3-A]- (E=Al, Ga, In and Tl, A=Si, Ge, Sn and Pb) were predicted. Simultaneously the thermodynamic stability, the kinetic stability and the bond nature of the TBPC anions were calculated systematically.The main conclusions are listed as the followings:1. A series of isomers of [A-CE3-A]-(E=Al, Ga, In and Tl, A=Si, Ge, Sn and Pb) were generated through the Stochastic Search Algorithm. After the theoretical calculation at high level, we found that the hexatomic anionic TBPC species:D3h [A-CE3-A]-(E=Al, Ga, In, Tl; A=Si, Ge, Sn, Pb) are energy minima, in which [Ge-CAl3-Ge]- (2a), [Sn-CGa3-Sn]-(7a), [Ge-CIn3-Ge]-(10a), [Sn-CIn3-Sn]-(11a), [Pb-CIn3-Pb]’(12a) and [Pb-CTl3-Pb]-(16a)were the global minima. [Si-CA13-Si]-(la), [Sn-CAl3-Sn]-(3a), [Pb-CA13-Pb]-(4a), [Si-CGa3-Si]-(5a), [Ge-CGa3-Ge]-(6a), [Pb-CGa3-Pb]- (8a), [Si-CIn3-Si]-(9a), [Si-CT13-Si](13a), [Ge-CTl3-Ge]-(14a), [Sn-CT13-Sn]-(15a) are local minima, [Si-CA13-Si]" (la) and [Ge-CGa3-Ge]-(6a) were the lowest local minima very close to their global minima.2. The kinetic stabilities of la,2a,6a,7a,10a, 11a,12a and 16a are further evaluated by the Ab-initio Born-Oppenheimer molecular dynamic simulation (BOMD) at 298 and 373 K. The results revealed that the basic structures of la,2a,6a,7a,10a, 11a, 12a, and 16a were well-maintained during the BOMD simulations and their kinetic stability were proved to be very excellent.3. Natural bond orbital (NBO) analysis indicates that the total electron counts on the central carbon atom of [A-CE3-A]" obey the octet rule,except 13a and 14a. The detailed AdNDP analyses have revealed that of la-16a were not identical. They all have localized two-center-two-electron (2c-2e) C-A σ-bonds and delocalized four-center-two-electron (4c-2e) σ-bonds, and except for 4a and 8a, they all have three-center-two-electron (3c-2e) A-C-A π-bond, while in 4a and 8a, the 3c-2e π-bond becomes a 6c-2e Pb-C-Pb a-bond. Nevertheless, both the 3c-2e π-bond in TBPC species with axial Si, Ge, or Sn atoms and the 6c-2e σ-bond in those with axial Pb atoms play an important role in keeping the rigidity of A-C-A axis. The 4c-2e σ-bonds contribute to stabilize the peripheral cage refinement.4. The vertical detachment energies (VDE) of la,2a,6a,7a,10a, 11a,12a and 16a were calculated by the outer-valence Green’s function (OVGF) procedure. The PES peaks should be stronger in the PES spectra and it can be regarded as the spectroscopic fingerprints of these species since the VDEs of la,2a,6a,7a,10a, 11a,12a were relatively large and the HOMOs of these TBPC species are all degenerate.In summary, the global minima D3h TBPC anion structures 2a,7a,10a, 11a,12a and 16a, and local minima structures la and 6a may be generated by laser vaporization or arc discharge and captured in the gas phase, which would facilitate their characterization via experiments such as the negative ion photoelectron spectroscopy.