Density functional studies of the stepwise substitution of pyridine, pyridazine, pyrimidine, pyrazine, and 1,3,5-triazine with BCO.

The structures, stabilities, and aromaticities of a series of (BCO) n (CH)5-n N (n = 0-5), (BCO) n (CH)4-n N2 (n = 0-4), and 1,3,5-(BCO) n (CH)3-n N3 (n = 0-3) clusters were investigated at the B3LYP density functional level of theory. The most stable positional isomers of individual clusters were obtained. All of the calculated CO binding energies were positive, suggesting that the BCO-substituted species are stable. It was found that the BCO-substituted structures are much less strained than their carbocation counterparts. The negative nucleus-independent chemical shifts (NICSs) obtained show that all of the BCO-substituted species possess three-dimensional aromaticity, in good accord with the aromaticities of the corresponding hydrocarbon species.

Density functional studies of the stepwise substitution of pyrrole, furan, and thiophene with BCO.

The structures, stabilities, and aromaticities of a series of (BCO) n (CH)4-n NH (n = 0-4), (BCO) n (CH)4-n O (n = 0-4), and (BCO) n (CH)4-n S (n = 0-4) clusters were investigated at the B3LYP density functional level of theory. The most stable positional isomers of the individual clusters were obtained. All of the calculated CO binding energies were exothermic, suggesting that these BCO-substituted species are stable. Calculated differences in strain energy between the BCO-substituted structures and their corresponding hydrocarbon clusters were all exothermic, indicating that the BCO-substituted structures are less strained. The negative nucleus-independent chemical shift (NICS) values obtained show that these BCO-substituted clusters are aromatic compounds, in good agreement with the aromaticities of the corresponding hydrocarbon species. To aid further experimental investigations, CO-stretching frequencies were also computed.

Structures and aromaticity of Cationic closo-BnHn-3(CO)3+ (n = 5-12).

Structures and stabilities of tricarbonyl closo-boranes cation, BnHn-3(CO)3+ (n = 5-12), isolobal with cationic closo-carboranes C3Bn-3Hn+, have been investigated at the B3LYP/6-311+G** level of theory. The most stable positional isomers of individual cluster are in agreement with those of closo-C3Bn-3Hn+ clusters except for n = 8 and 10. Energetic analysis identifies closo-B6H3(CO)3+, closo-B10H7(CO)3+ and closo-B12H9(CO)3+ as the most stable cages. It is also found that closo-BnHn-3(CO)3+ is much less strained than closo-C3Bn-3Hn+. The negative nucleus independent chemical shifts (NICS) at the cage center reveal three-dimensional aromaticity of the closo-BnHn-3(CO)3+ cages. The CO stretching frequencies have been computed in advance to aid experimental study.

Boron nitride cages from B12N12 to B36N36: square-hexagon alternants vs boron nitride tubes.

The structures and stabilities of square-hexagon alternant boron nitrides (Bx Nx , x=12-36) vs their tube isomers containing octagons, decagons and dodecagons have been computed at the B3LYP density functional level of theory with the correlation-consistent cc-pVDZ basis set of Dunning. It is found that octagonal B20N20 and B24N24 tube structures are more stable than their square-hexagon alternants by 18.6 and 2.4 kcal mol(-1), respectively, while the square-hexagon alternants of other cages are more stable. Trends in stability as a function of cluster size are discussed.

Structures and energies of isolobal (BCO)n and (CH)n cages.

The structures and energies of isolobal (CH)n and (BCO)n polyhedral species, computed at the B3LYP density functional theory level, reveal contrasts in behavior. The strain energies of the (BCO)n cages are much smaller. Also unlike the (CH)n cages, the most stable (BCO)n polyhedra (n > or = 10) prefer structures with the largest number of three-membered rings. The planar (or nearly planar) faces of the cage systems were modeled by computations on planar, isoelectronic (CH2)n (Dnh) and (HBCO)n (Cnv) rings. While the strain energies of all the planar carbon rings, relative to the most stable D5h (CH2)5, were large, the strain energies of all the planar (HBCO)n (Cnv) rings were small. Remarkably, the three-membered (HBCO)3 (C3v) ring was the most stable. Finally, large (BCO)n systems prefer tubelike rather than cage structures.