Bond angle variations in XCY fragments and their relationship to the anomeric effect

The crystal structures of 2-substituted heterocyclohexanes containing exocyclic X and endocyclic Y exhibit systematic variations in their XCY bond angles. When X is in the more stable axial orientation, corresponding to the anomeric effect, the XCY angle is larger than tetrahedral; when X is in the equatorial orientation the XCY angle is smaller than tetrahedral. These geometrical effects are predicted by the perturbational molecular orbital analysis employed previously to account for the existence of the anomeric effect and its variation with changes in X and Y. Abinitio molecular orbital calculations, with full geometry opimization, of selected conformations of XCH₂YH molecules also reproduce this geometrical effect.