A theoretical study of the structure, energy, and shape of novel boron-nitrogen helices

Szakacs, Csaba E. (2009) A theoretical study of the structure, energy, and shape of novel boron-nitrogen helices. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Ab initio self-consistent field molecular orbital and density functional theory calculations have been performed on a series of helical structures comprised of boron-nitrogen analogues of extended hydrogenated helicenes, with helically arranged N fused benzene rings, N fused methylenylnaphthalene fragments, and alternating N benzene units fused to N -1 cyclobutadiene rings as reference structures. -- The electronic structure of boron-nitrogen analogues of angular [ N ]helicenes (N = 5, 6, 7, 12) and [N ]phenylenes (N = 5, 6, 7, 13) were investigated at the HF/6-31G(d), B3LYP/6-31G(d), and MP2/6-31G(d)//HF/6-31G(d) levels of theory. It was observed that the presence of an even number N of rings in the boron-nitrogen [ N ]helicenes leads to the possibility of angular isomers. Their energetics and relative stability was discussed. -- Furthermore, the electronic structure and relative stability of three isomeric non-hydrogenated boron-nitrogen helices were investigated at the HF/6-31G(d), B3LYP/631G(d), and MP2/6-31G(d)//HF/6-31G(d) levels of theory. According to this study, some of the initially assumed regular helical structures are unstable; two types of the isomeric structures convert to characteristically different equilibrium geometries. -- In addition, laterally extended boron-nitrogen analogues of [ N ]polymethylenylnaphthalenes (N = 6, 8, and 12) were also investigated at the HF/6-31G(d,p) and B3LYP/6-31G(d,p) levels of theory. By alternating the positions of the boron and nitrogen atoms, two very similar structures are possible, the Nx B y Hz type helix and the B x Ny Hz (y = x- 1 ) type helix, which are comprised of the odd number of fused rings. The inclusion of one more ring results in an even number of fused rings, which leads to helical isomerism in these extended boron-nitrogen helices. Their geometries and energetics are also discussed. -- Electron density contours were calculated in order to interpret the existing bonding patterns. These structures may provide supramolecnlar building blocks and macromolecular springs with potential use in nanotechnology.

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