A COMPUTATIONAL EXAMINATION OF THE ANOMERIC EFFECT IN 1,3-DIAZANES

The anomeric effect was examined for several 1,3-diazane molecules, wi th computations carried out at the HF/6-31G, HF/6-31G, MP2-FC/6-31G*, and HF/6-31 IG* levels. Results were analyzed using the NBO method, and concepts of steric hindrance. The effect of n --> sigma donation to C-N antibonds appears to control the conformation of diazane system s in which nitrogens are monosubstituted. The computed and experimenta l results show that specific n --> sigma donation to C-N antibonds is irrelevant to the conformations of 1,3-diazane systems in which the n itrogens are trisubstituted. Lone pair electron repulsion would normal ly operate in the same sense as n --> sigma donation, but extensive d elocalization of the lone pairs in-the sigma orbitals appears to dimin ish this effect to insignificant levels. In the methyl substituted mol ecules, a generalized and subtle type of steric hindrance seems to be dominant over n --> sigma electron donation in determining conformati onal preference.