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Second-order Moller-Plesset perturbation theory investigations indicate that the highly strained three- and four-membered semiconductor ring systems A[sub 3]R[sup +, sub 3], A[sub 2]BR[sup +, sub 3], GeSiCR[sup +, sub 3] A[sub 3]R[sub 3]Ha, and A[sub 4]R[sup 2+, sub 4] (A,B=C,Si,Ge; R=H, SiH[sub 3]; Ha=F, Cl, Br) exhibit aromaticities featured with the delocalized π orbitals perpendicular to the ring planes. [ABSTRACT FROM AUTHOR]

Abstract: Closo-B n H n−1CO− (n =5–12), isolobal with , has been computed at the B3LYP/6-311+G∗∗ level. The most stable positional isomers of closo-B n H n−1CO− have the same structural patterns as those of , and closo-B n H n−1CO− are much less strained than . Energetic analysis identifies closo-B6H5CO−, closo-B8H7CO− and closo-B12H11CO− as the most stable cages. The negative nucleus-independent chemical shifts (NICS) show that closo-B n H n−1CO− are aromatic compounds. [Copyright &y& Elsevier]

Abstract: Closo-B n H n−2(CO)2 (n =5–12), isolobal analogues of closo-C2B n−2H n , have been investigated at the B3LYP/6-311+G**density functional level of theory. The most stable isomers of closo-B n H n−2(CO)2 are similar to those of closo-C2B n−2H n in geometric patterns apart from closo-B6H4(CO)2, and closo-B n H n−2(CO)2 is much less strained than closo-C2B n−2H n . Energetic analysis identifies closo-B6H4(CO)2, closo-B12H10(CO)2 and closo-B10H8(CO)2 to be most stable, of which the latter two cages have been prepared experimentally. On the basis of the negative and rather large nucleus independent chemical shifts (NICS), closo-B n H n−2(CO)2 are aromatic. To aid further experimental study, the CO stretching frequencies have been computed. [Copyright &y& Elsevier]