A Comparison of the Energies of Double Bonds of Second-Row Elements with Carbon and Silicon

Theoretical π-bond energies (E_π) are evaluated for the double bond systems H₂Y=XH_n (Y = C, Si; X = B, C, N, O, Al, Si, P, S) employing the MP4SDTQ/6-31G*//6-31G* + ZPE level of theory. The difference in energy between two single bonds, X-Y, and a double bond, X= Y, is calculated by means of isodesmic equations. E_π is given by subtraction of this difference from the dissociation energies of the single bond system, D₀°. Si=X bonds are found to have significantly lower E_π energies than the corresponding C—X bonds; for each series, C~X and Si=X, the π-bond energies for both first-and second-row substituents correlate with the electronegativities of X. When electronegativity differences between carbon and silicon and among the X groups is taken into account, second- and first-row π-bond energies are similar. Families of linear correlations are also observed for E_π and Y=X bond lengths. Alternative procedures for estimating π bond energies (rotation barriers and diradical components) are criticized.