Hybrid-DFT study and NBO interpretations of conformational behaviors of 2-methoxy-1,2,4,5-tetrahydro-benzo[d]oxepine, -thiepine and -selenepine

NBO analysis and density functional theory (DFT: B3LYP/6-311+G**) based method were used to study the impacts of the anomeric effects (AE) on the conformational properties of 2-methoxy-1,2,4,5-tetrahydro-benzo[d]oxepine (1), -thiepine (2) and -selenepine (3). The Gibbs free energy difference (Geq–Gax) values (i.e. ΔGeq-ax) at 298.15 K and 1 atm between the axial and equatorial conformations increase from compound 1 to compound 2 but decrease from compound 2 to compound 3. The NBO analysis showed that the AE associated with donor-acceptor (LP→σ*) interactions increases from compound 1 to compound 2 but decreases from compound 2 to compound 3. Also, the calculated dipole moment difference [Δ(μeq - μax)] values between the axial and equatorial conformations increase from compound 1 to compound 2 but increase from compound 2 to compound 3. Based on the results obtained, there is no conflict between the AE and the electrostatic interactions [i.e. Δ(μeq - μax)] on the conformational behaviors of compounds 1-3.