σ and π Relaxation of Strain in Three-Membered Ring Molecules (CH2)2X, (NH)2X, and (SiH2)2X (X = SiH2, PH, S)

The σ and π relaxation of ring strain by electron delocalization from bonds to bonds was investigated to understand or predict the strains of (CH2)2X, (NH)2X, and (SiH2)2X (X = SiH2, PH, S). The σ relaxation by the delocalization between the geminal ring bonds is found to be significant and general for these molecules as well as for the homoatomic molecules. Delocalization of the lone pair on the heteroatom to the neighboring σ bonds contributes to the π relaxation of ring strain in (CH2)2X and (SiH2)2X except for X = SiH2 but contributes little to that of (NH)2X. However, the cyclic delocalization of lone pairs is generally not significant except in (SiH2)2S.