Are there C-H...O Hydrogen Bonds in Dimethyloxalate? Huanwang Yang, R. K. McMullan & B. M. Craven, Crystallography Dept., University of Pittsburgh, Pittsburgh, PA 15260 and W. T. Klooster, Chemistry Dept., Brookhaven National Laboratory, Upton, NY 11973.

The crystal structure of dimethyloxalate (CH3-O-CO-CO-O-CH3) seems likely to contain C-H...O hydrogen bonds, as pointed out by Dougill & Jeffrey (1953), who noted that the melting point (327K) is much higher than other closely related esters, such as diethyloxalate (232K). They reported that the shortest intermolecular C...O distance was 3.36A.

We have collected neutron diffraction data at 15, 70 and 123K for this simple structure which is monoclinic and has each molecule on a crystallographic center of symmetry. We find that the methyl group has one hydrogen close to the plane of the heavy atoms in the molecule and that this has a C-H bond slightly shorter than the other two [1.063(3) vs 1.077(3) and 1.077(3)A]. The two C-H groups with longer bonds form shorter H...O intermolecular distances (2.45 and 2.63A) than the other (2.90A), although we believe none of these is unusually short.

We then collected high resolution X-ray data at 123K and carried out a charge density refinement. The resulting net atomic charges [-0.04(4), +0.13(3) and +0.20e] indicate that the two H atoms forming longer (1.077A) C-H bonds are also more electropositive. The most electro- positive H-atom is the one that forms the shortest H...O distance. Thus we are led to conclude that at least one of the C-H...O interactions can be described appropriately as a weak hydrogen bond.

Work supported by a grant GM-39513 from the NIH. The neutron study was carried out at Brookhaven under contract DE-AC02-76CH00016 from the US Department of Energy Sciences.