THE RULES OF CHANGES OF BOND STRENGTHS IN 3 MEMBER HETEROCYCLIC-COMPOUNDS WITH HETEROATOMS BE, B, N, O AND S

The nature of the chemical bonds and the rules of the changes of their strengths in CH2-CH2-X (X-Be, BH, CH2, NH, O, S) may be expressed qua ntitatively by means of bond strength parameters f(i,A-B) and f(A-B). The rules are: 1) f(C-X) increases as Z of the heavy atom in X increas es when X = Be to X = O, both in 3MR and in CH3-X-CH3. This is clue to the fact that Z(eff) of the heavy kernel in X, and hence the ''bond c harge' in C-X, increases as Z increases. 2) During cyclization, the st ronger bonds are weakened and the weaker ones strengthened, the bond l engths are also averaged thereby from X = Be to O. The reason is: ther e are sigma-conjugated three-centered bonds in 3MR molecules, bonds C- X and C-C share ''bond charge'' together. The effect of this sigma-con jugation is similar to that of the pi-conjugation. 3) Since the radius of S atom is larger than that of O, S can contribute more charge to C -C for sharing than O during cyclization, so that in CH2-CH2-S, C-X is weakened and C-C is strengthened to the greatest degree.