Conformational ab initio study of ascorbic acid

In this paper, conformations of ascorbic acid have been determined from ab initio calculations with full geometry optimization using different levels of theory and basis sets: RHF/6-31G, RHF/6-31G(d,p), RHF/6-311 + G(d,p) and MP2/6-31(d,p). The starting point for the optimizations were the A and B X-ray structures of ascorbic acid as determined by crystallography. The molecular energies and optimized structures of 36 conformers of ascorbic acid were obtained. the potential energy surface is mainly dependent on the rotation of the side chain structure. One optimized gas phase conformer is very close to the X-ray structure of the crystal. The largest energy difference between the 36 local minima are 18.641, 6.1 and 7.97 kcal/mol at the four levels of theory, respectively. The three lowest energy, fully optimized conformations are the same for the four levels of theory. (C) 1998 Elsevier Science B.V. All rights reserved.