APPLICATION OF ABINITIO MOLECULAR-ORBITAL CALCULATIONS TO THE STRUCTURAL MOIETIES OF CARBOHYDRATES .5. GEOMETRY OF THE HYDROGEN-BONDS

Ab initio RHF/4-31G level molecular orbital calculations and Boltzmann factors are used to calculate the room temperature probability distribution of the 0-H…0 hydrogen-bond angle using a model consisting of two methanol molecules. The maximum in the distribution curve occurs at 163°. The data available from 18 neutron diffraction studies of carbohydrates yield a mean value of 166°. The energetics of symmetrical and asymmetrical bifurcated hydrogen bonds, which are not uncommon in carbohydrate crystal structures, were modeled with three water molecules. It is found that a symmetrical bifurcated hydrogen bond with typical geometry is more stable, relative to 3H2O, than an equilibrium linear bond, relative to 2H2O, by 1-2 kcal/mol. For a typical asymmetrical bifurcated geometry, the addition of a third water molecule to a “linear” bond with an 0-H…0 angle of 150° can lead to further stabilization of 1-4 kcal/mol, depending upon the geometry of the system. © 1979, American Chemical Society. All rights reserved.