A computational examination of the anomeric effect in 1,3-diazanes

The anomeric effect was examined for several 1,3diazane molecules, with computations carried out at the HF/ 6-31 G, HF/6-31 G*, MP2-FC/6-31 G*, and HF/6-311 G** levels. Results were analyzed using the NBO method, and concepts of steric hindrance. The effect of n → σ* donation to C−N antibonds appears to control the conformation of diazane systems in which nitrogens are monosubstituted. The computed and experimental results show that specific n → σ* donation to C−N antibonds is irrelevant to the conformations of 1,3-diazane systems in which the nitrogens are trisubstituted. Lone pair electron repulsion would normally operate in the same sense as n → σ* donation, but extensive delocalization of the lone pairs in the σ orbitals appears to diminish this effect to insignificant levels. In the methyl substituted molecules, a generalized and subtle type of steric hindrance seems to be dominant over n → σ* electron donation in determining conformational preference