Oxepins molecular geometry

The oxepin ring structure (8) appeared to be formed in preference to the arene oxide tautomer according to the NMR spectrum, and the oxepin form was unequivocally established by X-ray structure analysis (78AG(E)12l). Unfortunately the molecular dimensions of the oxepin ring in structure (8) were not included in the original report. In contrast, the valence isomeric arene oxide form of oxepin (9) was found to predominate and detailed information about the arene oxide molecular geometry was provided by the X-ray diffraction method (Table 3) (80LA1889). 

Preferred geometry of the benzene oxide-oxepin system can be predicted by molecular orbital methods. Thus benzene oxide la is predicted to be markedly non-planar (with the epoxide ring at an angle of 73° to the benzene ring), while the oxepin lb has been predicted to prefer a shallow boat structure (MINDO/3) or a planar structure ab initio) As previously mentioned, the proportion of each tautomer present at equilibrium is both temperature and solvent-dependent. Molecular orbital calculations have been used to rationalize the solvent effects, both in terms of the more polar character of the arene oxide that is favored in polar solvents and the strengthening of the oxirane C-C bond upon coordination of the oxygen atom lone pair in polar solvents. Thus values in the range 1.5-2.0 D and 0.76-1.36 D for the dipole moments of arene oxide la and oxepin lb have been calculated. 

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