Simplest neutral singlet C2E4 (E = Al, Ga, In, and Tl) global minima with double planar tetracoordinate carbons: equivalence of C2 moieties in C2E4 to carbon centers in CAl4(2-) and CAl5(+).

Ab initio and DFT calculations have been carried out to search for the simplest neutral singlet species with double planar tetracoordinate carbons (dptCs) [the "simplest" means the species containing the least number (six) and types (two) of atoms]. Under the restrictions to the possible models (M1-M4) with dptCs and to the singlet electronic states, the B3LYP/6-31+G* scanning on the candidates, C(2)E(4) (E = the second- and third-row main group elements), only led to two minima (D(2h) C(2)Al(4) and C(2h) C(2)Be(4)) with stable DFT wave functions. The extensions to the heavier elements after the fourth row in the IIA and IIIA groups revealed that the D(2h) C(2)E(4) (E = Ga, In, and Tl) are also minima with dptCs but C(2)Ca(4) (C(2h)) is a first-order saddle point. Extensive explorations at the DFT level on their potential energy surfaces (PESs) further confirmed that the D(2h) C(2)E(4) (E = Al, Ga, In, and Tl) are the global minima, but the C(2h) C(2)Be(4) is a local minimum. The optimizations at the MP2 level distorted the D(2h) C(2)E(4) (E = Ga, In, and Tl) slightly and the distortion energies are less than 0.02 kcal/mol. The C(2)E(4) (E = Al, Ga, In, and Tl) with dptCs are 18.0, 18.3, 13.4, and 12.2 kcal/mol energetically more favorable than their nearest isomers, respectively, at the CCSD(T)//MP2 level with aug-cc-pVTZ for C and Al and aug-cc-pVTZ-PP for Ga, In, and Tl basis set. The substantial energy differences suggest their promise to be experimentally realized. The strong peak on the C(2)Al(4)(-) component in the time-of-flight mass spectrum from laser vaporization of a mixed graphite/aluminum may relate to the D(2h) C(2)Al(4) global minimum. The analyses of the electronic structures of C(2)Al(4) (D(2h)), CAl(4)(2-) (D(4h)) and CAl(5)(+)(D(5h)) indicates that the C(2) moiety in C(2)Al(4) is the equivalence of carbon centers in CAl(4)(2-) and CAl(5)(+) and unveils the reasons for their stability. The electronic structures of C(2)Al(4) and ethene are compared. On the one hand, an Al atom functions like an H atom because the eight more valence electrons of C(2)Al(4) than C(2)H(4) occupy four nonbonding orbitals and are not effectively utilized for bonding. On the other hand, an Al atom is different from an H atom because an Al atom has p electrons available for peripheral bonding around the C(2) moieties in C(2)Al(4), which further rationalize the origins for C(2)E(4) to achieve double ptCs.