A computational evaluation of the steric and electronic contributions to the stability of the structures of α− and β−D‐glucopyanose Part 1: conformational analysis of monosubstituted cyclohexanes and pyrans (593.2)

Geometry optimizations at a variety of computational levels were carried out on a series of substituted cyclohexanes (‐CH3 ‐OH and ‐CH2OH) and similarly 2‐substituted tetrahydro‐2H‐pyrans. By comparing the relationship between structure and energy of the cyclohexane series to the tetrahydro‐2H‐pyran series, it is possible to isolate the steric factors that are the principle contributor to the energy in the cyclohexanes from the combined steric and electronic factors that are present in pyrans. The optimized geometries and minimum energies for each compound were determined using HF, DFT and MP2 methods with increasingly complex basis sets. Thermal correction factors were applied to the conformational energies and the conformational free energy from the B3LYP/6‐311G++(d,p) optimizations closely reflected the available experimental values. Relaxed scans (also at the B3LYP/6‐311G++(d,p) level) of rotations of the exocyclic substituents helped to identifiy the steric contributions for different orientations of the rotamers. Analysis of the optimized geometries non‐bonded distances bond lengths, and bond angles contributed to an understanding of the steric contributions. Additionally, analysis of bond lengths and bond angles showed changes that reflected possible hyper‐conjugation (electronic) contributions to the stability of some configurations. The series of compounds selected in this study provide a series of model compounds that lead to modeling the steric and electronic contributions to the free energies energies of D‐glucopyranose.