UNDERSTANDING BARRIERS TO INTERNAL ROTATION IN SUBSTITUTED TOLUENES

Abstract

In recent years microwave, resonant two-photon ionization, and threshold photoionization spectroscopies as well as ab initio calculations have determined the dependence of the barrier to internal methyl rotation on electronic state for a variety of substituted toluenes. The barrier changes from $S_{0}$ to $S_{1}$ to $D_{0}$ (ground state cation) in a characteristic fashion for ortho-, meta-, and para-substitution. In this paper we summarize recent experimental and computational work (from our group and others) that explains the underlying causes of the intriguing pattern of barriers vs electronic state and substituent location. Two effects are important: repulsive steric interactions and attractive donor-acceptor interactions. The sequence of molecules o-fluorotoluene, o-chlorotoluene, and 2-fluoro-6-chlorotoluene demonstrates additivity of steric effects in a particularly clear fashion. The molecule m-fluorotoluene illustrates the importance of donor-acceptor interactions mediated by the pattern of $\pi-bond$ orders of the two ring CC bonds adjacent to methyl. In a certain view, the methyl rotor provides a uniquely sensitive structural probe of local ring geometry.