π*-σ* hyperconjugation mechanism on the rotational barrier of the methyl group (I):: Substituted toluenes in the ground, excited, and anionic states

We theoretically investigate the internal rotations of the methyl group in substituted toluenes such as fluorotoluene (-F), toluidine (-NH2), cresol (-OH), and tolunitrile (-CN) in the ground, excited, and anionic states. The calculated rotational barriers reproduce well the experimental data. Orbital pictures are given for the barrier variations by excitation and electron attachment. An idea of pi*-sigma* hyperconjugation is introduced for a comprehensive interpretation of the barrier variations. The pi*-sigma* hyperconjugation mechanism clarifies the differences among ortho-, meta-, and para-systems, between pi-electron donating and accepting substituents, and between first and second excited (anionic) states. (C) 2000 American Institute of Physics. [S0021-9606(00)30930-8].