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 conform er for succinic acid, designated ZsgsZ, has a gauche conformation abou t the central C2-C3 bendy the lowest energy conformer with an E-acid g roup and an internal hydrogen bond is ca. 3 kcal/mol higher In energy. The lowest energy structure for the monoanion, Ecgs, does have the ex pected internal hydrogen bond and is 15 kcal/mol more stable than any alternative. The ab initio results were used to determine correspondin g torsional-energy parameters in the OPLS all-atom force field. This a llowed application of statistical perturbation theory in Monte Carlo s imulations to explore the effect of hydration on the conformational eq uilibria. The diacid and monoanion were both computed to be ca. 80% ga uche in water at 25 degrees C. These results are in excellent agreemen t with NMR data. Though the conformational results are consistent with the gauche effect, their true origin requires a detailed understandin g of the potential internal hydrogen bonding and solvation. Thus, in c ontrast to the monoanion's striking gas-phase preference, neither the diacid nor monoanion are computed to populate E conformers in aqueous solution.