Theoretical bond energies: A critical evaluation

The recently proposed scheme of Grimme (BE scheme) (J. Ant. Chem. Sec. 1996 , 118, 1529) to calculate intrinsic bond energies (BE's) of hydrocarbons, w hich define seminal equilibrium quantities of chemical structures, is evalu ated critically. CH and regular CC bonds are treated well; the correspondin g BE's are reliable and self-consistent. In contrast, the performance of th e method is markedly reduced for bonds of unusual length, if the bond lengt h is not determined by bond bending or by conjugation. Differences between BE's for CH bonds, which Lie within the remarkably narrow range from ca. 10 3 to 110 kcal mol(-1), and CH bond dissociation energies (BDE's, ca. 86-132 kcal mol(-1), linear correlation, R-c = 0.9291) give a measure of radical (de)stabilization. BE's of sp(x)-sp(y) CC single bonds correlate linearly w ith the respective BDE's (R, = 0.9987) and can be used for a reliable predi ction of BDE's at almost no computational cost. Individual intrinsic bond e nergies are used to establish CC and CH bond length-bond energy-bond order correlations. In extension of Grimme's original report, the performance of the model is tested thoroughly for anions, cations, and radicals of hydroca rbons and it is shown that these species are treated less satisfactorily. A ttempts to treat non-hydrocarbon compounds by the same procedure are also l ess successful with the exception of saturated silicon hydrides. Results of this work show that the relationships between bond length-bond order-bond energy as described by established models of the chemical bond can be relat ed to the properties of the electron density at bond critical points. Despi te the much greater angle distortion, cyclopropane has a strain energy only slightly larger than cyclobutane. This problem of the nearly equivalent st rain energies is readdressed, leading to new estimates for the stabilizatio n of cyclopropane due to CH bond strengthening (11.7 kcal mol(-1)) and to s igma -aromaticity (11.3 kcal mol(-1)).