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 mole
cular energies and optimized structures of 36 conformers of ascorbic acid w
ere obtained. the potential energy surface is mainly dependent on the rotat
ion 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 l
evels of theory, respectively. The three lowest energy, fully optimized con
formations are the same for the four levels of theory. (C) 1998 Elsevier Sc
ience B.V. All rights reserved.