Bond formation and bond strength variation in I (X = Be, BH, CH2, NH, O, S) were calculated by MO and expressed quant. by bond strength parameters. In I and in Me2X, C-X bond strength increases as the kernel charge increases from X = Be to X = O. During cyclization, the stronger bonds are weakened and the weaker bonds are strengthened because of a-conjugation in 3-centered bonds in I sharing bond charge, similar to the p-conjugation effect. Between X = O and X = S in I, S with its larger atomic radius can contribute more charge to C-C for sharing than O, resulting weakened C-X and strengthened C-C to the greatest degree.

Key words: CALCULATION, BOND PARAMETER, THREEMEMBER HETEROCYCLICS COMPOUNDS, MOLECULAR ORBITALS CALCULATIONS