GEOMETRY OF C-H...O HYDROGEN-BONDS IN CARBOHYDRATE CRYSTAL-STRUCTURES - ANALYSIS OF NEUTRON-DIFFRACTION DATA

Geometrical properties of C-H...O hydrogen bonds in carbohydrate crystal structures are analyzed on the basis of 30 neutron diffraction studies (395 H atoms bonded to C as potential donors, and 328 O atoms at potential acceptors). A total of 21% of the H atoms are engaged in C-H...O interactions with H...O separations of <2.5 angstrom (and 65% of the H's with H...O < 2.7 angstrom). Only 7% of the H atoms have no contact to O shorter than 3.0 angstrom. Correlations between hydrogen-bond distances and angles are studied in scatterplots. The shortest interactions tend to be close to linear, but the correlation between distances and angles is much less pronounced than in O-H...O hydrogen bonds. There is a continuous transition from stronger to weaker hydrogen bonds and to nonbonding arrangements; consequently, cutoffs based on van der Waals contacts should be discouraged. Intermoleclar and intramolecular interactions are treated separately. Short intramolecular contacts, where H and O are separated by only four covalent bonds, occur frequently due to steric restrictions. In beta-cyclodextrin inclusion complexes, host/guest C-H...O hydrogen bonds with H...O separations as short as 2.39 angstrom are observed; for water molecules that cannot arrange in the preferred tetrahedral O-H...O hydrogen-bond coordination, the resulting ''free'' acceptor potential is frequently satisfied by C-H...O interactions. C-H...O hydrogen bonds are not strong enough to significantly reduce the thermal vibrations of the engaged H atom. The lengthening of the covalent C-H bond due to the H...O interaction is at the limit of experimental accuracy: approximately 0.004 angstrom for d(H...O) < 2.4 angstrom.