Conformational Complexity of Succinic Acid and Its Monoanion in the Gas Phase and in Solution: Ab Initio Calculations and Monte Carlo Simulations

Optimized structures and relative energies for conformers of succinic acid and its monoanion in the gas phase were obtained using ab initio molecular orbital calculations at the MP2/6-311+G**//HF/6-31G* and MP2/6-311+G**//HF/6-31+G* levels, respectively. The lowest energy conformer for succinic acid, designated ZsgsZ, has a gauche conformation about the central C2−C3 bond; the lowest energy conformer with an E-acid group and an internal hydrogen bond is ca. 3 kcal/mol higher in energy. The lowest energy structure for the monoanion, Ecgs, does have the expected internal hydrogen bond and is 15 kcal/mol more stable than any alternative. The ab initio results were used to determine corresponding torsional-energy parameters in the OPLS all-atom force field. This allowed application of statistical perturbation theory in Monte Carlo simulations to explore the effect of hydration on the conformational equilibria. The diacid and monoanion were both computed to be ca. 80% gauche in water at 25 °C. These results are in excellent agreement with NMR data. Though the conformational results are consistent with the gauche effect, their true origin requires a detailed understanding of the potential internal hydrogen bonding and solvation. Thus, in contrast to the monoanion's striking gas-phase preference, neither the diacid nor monoanion are computed to populate E conformers in aqueous solution.