Conformational change as a Franck–Condon electronic process

Abstract

Conformational changes are described as Franck–Condon electronic processes, thereby providing an alternative perspective for understanding molecular processes. The theory is based on a variant of the Born–Oppenheimer approach, wherein the nuclear coordinates are not used to define reference frames. The electronic wave function is derived as a solution to a unique stationary arrangement of external Coulomb sources (SACS) and, consequently, does not depend upon the instantaneous position of the nuclei. The electronic wave function and the SACS act as attractors by providing a potential energy function for the nuclear system. The attractor is invariant to the operations of the molecular point symmetry group to which a molecule belongs. A direct correspondence between chemical species and quantum electronic state notions follows. The energetics of a conformer, in its ground electronic state, is determined by a unique electronic stationary wave function that is orthogonal to other conformer states. In general, the electronic wave function shows a definite parity under coordinate inversion operation. A change of electronic state must fulfil parity selection rules. In the present approach, each conformer has a different electronic wave function, and the conformational change between molecules in their closed shell electronic states cannot be a direct process due to parity selection rules. Butadiene is used as an example to illustrate the new situation.