Photodetachment of Zwitterions:  Probing Intramolecular Coulomb Repulsion and Attraction in the Gas Phase Using Pyridinium Dicarboxylate Anions

Abstract

Zwitterions are critically important in many biological transformations and are used in numerous chemical processes. The consequences of electrostatic effects on reactivity and physical properties, however, are largely unknown. In this work, we report the results of negative ion photoelectron spectra of nine isomeric pyridinium dicarboxylate zwitterions and three nonzwitterionic methoxycarbonylpyridine carboxylate isomers (-O(2)CPyrCO(2)CH(3)). Information about the intramolecular electrostatic interactions was directly obtained from the photoelectron spectra. The adiabatic and vertical detachment energies were measured and understood in terms of intramolecular Coulombic forces. Calculations at the B3LYP and CCSD(T) level were performed and compared to the experimental electron binding energies. Structures, relative stabilities, and the electron detachment sites also were obtained from the calculations.