The conformational space of D-glucose and D-mannose. The role of the hydrogen bonds.

The conformational space of D-glucose and D-mannose has been studied by the MM2* -SUMM method. The energy and molecular geometry of the 19 lowest ener gy conformers were analyzed at the HF/3-21G, 6-31G and GGA-DFT levels of th eory. The O...H interactions are analyzed by the electron density gradient vector field theory, and a suitably selected polar coordinate system, cente red on the oxygen atom, is proposed to describe the hydrogen bonds. The ene rgies and the molecular geometries of four conformers of C-1(4) and C-4(1) beta-D-glucose were calculated using HF/3-21G,HF/6-31G(d), and GGA-DFT meth ods.