UNDERSTANDING BARRIERS TO INTERNAL ROTATION IN SUBSTITUTED TOLUENES

In substituted toluenes, the potential energy barrier to internal methyl rotation and the preferred methyl conformation depend on the position of the fluorine, amino, or methyl substituents and also on the electronic state, either S0, S1, or ground state cation. We present a unified picture of the electronic factors controlling these effects. In S0 and cation, ab initio electronic structure calculations of modest scale produce rotor potentials in good agreement with experiment. The methyl group provides a sensitive probe of local ring geometry. When the geometry of the ring in the vicinity of the rotor has good local C2v symmetry, the barrier is invariably small. In S0 ortho-substituted toluenes, we use natural steric analysis to show that repulsive steric interactions between the halogen lone pair and the methyl CH bonds dominate over attractive donor–acceptor interactions to favor the pseudo-trans conformation. When steric interactions are unimportant, the key determinant of rotor barrier height is the difference in π-bond order between the two ring CC bonds nearest methyl.

The barrier height is proportional to the calculated bond order difference, with slope of 950 cm−1 per bond. Attractive donor–acceptor interactions favor the conformation that places the rotor CH bond cis to the ring CC bond of higher order, analogous to the localized case of 2-methylpropene. In toluene cations, π-ionization creates a pattern of long and short ring CC bonds. Simple molecular orbital theory readily explains the coarse bond-order patterns calculated for ortho- and meta-substituted toluene cations. A localized picture of π-bonding from natural resonance theory explains more subtle details of the distribution of CC bond orders about the ring. When π-ionization places the methyl group between ring CC bonds of quite different order, a substantial barrier results. This explains the strong preference of m-fluorotoluene+ for the pseudo-cis conformation and contributes to the preference of o-fluorotoluene+ for the pseudo-trans conformation. Finally, we speculate that a similar molecular orbital argument applied to S1 might explain the observed characteristic changes in barrier height on S1–S0 excitation of ortho- and meta-substituted toluenes.