Shishkina S. The molecular structure and conformational analysis of derivatives and heterocyclic analogues of cyclohexene

The thesis is devoted to study of conformational characteristics and intramolecular interactions in cyclohexene, its derivatives and heterocyclic analogues. The special attention was focused on the ring inversion process and the character of transition states. Unusual top-flattened character of the potential energy surface for cyclohexene was disclosed in the work. This leads to the transition between two symmetric conformations of twist-boat via boat conformation without change in energy. Analysis of the energy profile for ring inversion process for cyclohexene derivatives containing an exocyclic double bond or methyl substituents and for heterocyclic analogues demonstrates that the flattened character of the potential energy surface is kept for all rings where methylene groups in a-position relatively endocyclic double bond are non-substituted. The investigation of intramolecular interactions using NBO theory reveals the influence of hyperconjugation interactions between lone pair of heteroatom and s-orbital of double bond or s-antibonding orbitals of neighboring C-C bond on the geometric characteristics of the tetrahydrocycle. At the same time the hyperconjugation interactions between s-orbital of the endocyclic double bond and pseudo-pi-fragment orbital of neighboring methylene groups influences the energy profile of ring-inversion process in cyclohexene and its derivatives with non-substituted methylene groups in a-position relatively double bond. The combined study by experimental and theoretical methods demonstrates that the half-chair equilibrium conformation of six-membered tetrahydroaromatic rings is conformational flexible. The half-chair conformation may be deformed easily toward a sofa conformation under the influence of different factors. Key words: tetrahydroaromatic ring, conformational characteristics, the ring-inversion process, pi-pi conjugation, hyperconjugation, pi- and s*-antibonding orbitals.