A theoretical approach to the factorization of the effects governing the barrier for internal rotation around the C(sp(2))-C(sp(3)) bond into alpha-substituted toluenes

The effects governing the barrier for internal rotation in a number of alpha-substituted toluenes C6H5CH2X (X = Cl, F, CH3, C(CH3)(3), CF3, CCl3) and alpha,alpha-disubstituted toluenes C6H5CHX2 (X = Cl, Fl were interpreted using a model which factorizes the energy barrier into three components, namely, a) hyperconjugation, b) electrostatic effects and c) van der Waals interactions. The potential energy profiles for internal rotation of the CH2X and CHX2 rotors were calculated at molecular orbital MO ab initio level with a 6-31G* basis set and analysed by means of a truncated Fourier series in the V-2 and V-4 terms. The hyperconjugative contributions were estimated employing natural bond orbitals (NBO) derived from the 6-31G* wave functions in a scheme of acceptor-donor intramolecular interactions. The donor and acceptor hyperconjugative contributions, with respect to the pi system of the benzene ring, of each bond constituting the CH2X and CHX2 rotors were found to contribute additively to the hyperconjugative effect of the whole rotating group. Electrostatic effects and van der Waals interactions were tentatively estimated with empirical formulas. The separate contributions of these effects were compared, albeit at a qualitative level, with the total molecular energy and their relative weight discussed. The rotational barriers of benzylchloride and benzalchloride are mainly controlled by hyperconjugative effects. In benzylfluoride and benzalfluoride, the hyperconjugative effects are active to the same extent as in chlorine derivatives but the barrier is mainly controlled by electrostatic effects. In the compounds with bulky X groups (X = C(CH3)(3), CF3 and CCl3), hyperconjugation plays a less important role than van der Waals interactions and electrostatic effects, and the relative weight of these effects differs for the substituents examined.