Role of Intramolecular Electron Delocalization in the C-X Bond Strength in CH4-nX n ( n = 0-4, X = F, Cl, CN, OCH3).

Most recently, Wiberg and Rablen examined a few substituted methanes and identified the polar effect, which is associated with the atomic charge at the central carbon, as a possible factor for bond energy changes while the exact role of the hyperconjugation effect is unsettled. In this work, we revisited a series of substituted methanes CH4- nX n ( n = 0-4, X= F, Cl, CN, OCH3) by explicitly computing the intramolecular electron delocalization energies using the simplest variant of ab initio valence bond (VB) theory, namely the block-localized wave function (BLW) method. This BLW method is designated to derive electron-localized states where intramolecular electron delocalization is "turned off". Computations show that the deactivation of the intramolecular electron delocalization in these molecules only slightly increases the charge (i.e., reduces the atomic population) at the central carbon in all systems. This seems consistent with the polar effect therein as proposed by Wiberg and Rablen. But apart from the stabilizing polar effect, there should be steric (including both Pauli and electrostatic) repulsion among the substituent groups. This is evidenced by the stretched C-X bonds in CX4 compared with the same bonds in H3CX (X = Cl, CN) when the specific nX' → σCH/CX* orbital interactions are "turned off", though there is indeed notable polar effect in fluoromethanes considering fluorine is the most electronegative atom. By focusing on one C-X' bond, we show that its involved orbital interactions enhance the bond strength from H3C-F' (by 11.6 kcal/mol) to F3C-F' (by 18.7 kcal/mol) but weaken from H3C-CN' (by 11.6 kcal/mol) to (CN)3C-CN' (by 9.3 kcal/mol).