A Model Glycosidic Linkage: An ab Initio Geometry Optimization Study of 2-Cyclohexoxytetrahydropyran (original) (raw)

Quantum mechanical ab initio calculations have been performed to study regions close to minimum energy conformations of a pseudo-disaccharide, viz., 2-cyclohexoxytetrahydropyran. Dunning's double-c basis set was used for most of the calculations. The global minimum was obtained for the axial conformer, and a local minimum for the axial form was also observed at 0.5 kcdmol above the global minimum. The equatorial form showed local energy minima for two conformers about 2.5 k c d m o l above the global minimum. Analysis of the effect of electron correlation on the geometry optimization was also performed on the MP2 level of approximation using the DZ basis set. Geometry optimization calculations at various values of the ly dihedral angle were performed with the 4 dihedral angle restrained to the staggered conformation where the exoanomeric effect is contributing to energy stabilization. Barrier heights between the axial conformers and between the equatorial conformers, respectively, were well below kT at room temperature as identified from the ab initio calculations, thus indicating a conformationally averaged low-energy region. The barrier for the full rotation of the ly dihedral angle is estimated to be ca 4-5 kcdmol. The relative energies and the geometries obtained from the calculations at the Hartree-Fock level of approximation were compared to those from MM3 calculations. The optimal conformations calculated with the ab initio and MM3 methods, respectively, were similar for the global minimum, whereas significant differences were observed for the 4 and ly dihedral angles as well as in relative energies for low-energy conformers. The values of the ly dihedral angle in the optimal geometries were closer together when calculated by the ab initio method than obtained from MM3 calculations, thus indicating a more narrow low-energy region.