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 inc...