Conformationally Induced Changes in the Electronic Structures of Some Flexible Benzenes. A Molecular Orbital Model

Abstract

High-resolution electronic spectroscopy experiments on several substituted benzenes in the gas phase have revealed a remarkable sensitivity of the orientation of the S1−S0 transition moment (TM) in the molecular frame to the conformation of the attached group. Here, we explore the origins of this effect by performing ab initio calculations on five conformationally flexible benzenes (toluene, ethylbenzene, n-propylbenzene, 2-phenethyl alcohol, and 3-phenylpropionic acid) using the configuration interaction singles method (6-31G* basis set). Comparisons of the theoretical predictions with the experimentally determined parameters show that there are two principal sources of the observed sensitivity: a "through-bond" effect caused by rotation of the substituent about the bond connecting it to the ring, which breaks the Cs symmetry of the parent molecule, and a "through-space" effect, apparently caused by interactions of the side chain orbitals with the π orbitals of the ring. "Purely electrostatic" interactions between the ring and the tail, while significant in some cases, result in TM orientations that do not agree with experiment. All three types of interactions produce mixings of the ring π* orbitals, which in turn can lead to 1La/1Lb excited electronic state mixing in the isolated molecule. Possible photochemical and photophysical consequences of this effect are discussed.