Substituent and solvent effects on the rotational barriers in selenoamides: A theoretical study

Theoretical calculations at HF, MP2 and B3LYP levels using 6-31+G* basis set have been performed on nine substituted selenoamides and their thio- and oxo-analogs (H2NC(=X)Y, X=Se, S, O and Y=F, Cl, Br, NO2, CN, NH2, CH3, CF3, SH). The variations of barrier heights for rotation around C-N bond in nine substituted selenoamides are studied in comparative context with series of substituted thioamides and amides. The change in bond length, atomic charges for the equilibrium conformations, orbital interactions and rotational barriers indicate that pπ-pπ type electron delocalization is affected by π donor nature of substituents at carbonyl carbon rather than electron withdrawing nature. In addition steric effects, hyperconjugation, anomeric effect and change in hybridization during rotation also play important role. Solvent effects predict increase of rotational barrier with increase in polarity of the solvent, consistent with the experimental results on amides that suggest retardation of C-N rotation with increasing polarity of the solvent.