METHYLENE GROUP DEFORMATIONAL COORDINATES IN 4-MEMBERED RINGS

The dependence of methylene group deformational coordinates upon the puckering amplitude q has been analyzed for a set of ab initio geometries of the four-membered ring (CH2)3X. For small values of the puckering amplitude, the rocking and twisting deformational coordinates are proportional to q, whereas for q > 0.3 angstrom, the inclusion of a cubic term in q is necessary in order to describe the dependence of those deformational coordinates with the puckering amplitude. The wagging deformational coordinate shows a linear correlation with the square of q, although the effect is so weak that, for small variations in q, the wagging coordinate can be considered constant.