Chapter 4 - Vibrational Potential Energy Surfaces in Ground and Excited Electronic States

Abstract

Infrared and Raman spectra of molecules, often of vapor samples at high temperatures and/or utilizing special sample cells, have been utilized to investigate molecular structures, conformations, and vibrational potential energy surfaces (PESs) in electronic ground states. Similarly, laser-induced fluorescence (LIF) and ultraviolet spectra have provided the data for investigating electronic excited states. Theoretical ab initio and density functional theory (DFT) calculations have complemented the experimental work. The focus of the work has been on cyclic and bicyclic molecules that possess large-amplitude motions such as the ring-puckering and ring-twisting vibrations. We first discuss molecules with intramolecular π-type hydrogen bonding in cyclic and bicyclic alcohols and amines. This type of bonding is achieved only when the ring puckering or ring twisting and OH or NH2 torsional coordinates are at their optimal positions. Second, we review the Raman spectra of 1,3-butadiene that allowed us to calculate the torsional potential energy function for this molecule. Third, we examine the ultraviolet electronic spectra of pyridine and several fluoropyridines and discuss their potential energy functions in electronic excited states. Fourth, we present our experimental data and theoretical calculations for the two-dimensional PESs of three unusual molecules. In each case, the characteristics of the energy levels and their corresponding wave functions are discussed in detail. Fifth, the PESs for the torsional vibrations of stilbenes are reviewed. Sixth, we summarize our results for the ground and excited states of several bicyclic aromatics including the remarkable 1,3-benzodioxole molecule, which possesses the anomeric effect. Lastly, we describe our results on the LIF spectra of cyclic ketones that generally possess double-minimum PESs for the carbonyl inversion vibrations in their S1(n, π⁎) excited states.