W0084

Factors Affecting the Variation in Hydrogen-Bonding Patterns of Keto Carboxylic Acids. Andrew P. J. Brunskill, Roger A. Lalancette & Hugh W. Thompson, Rutgers University, Department of Chemistry, 73 Warren St., Newark, NJ 07102

Keto carboxylic acids offer opportunities for variation of the dimeric hydrogen bonding pattern most often found for functionally unelaborated carboxylic acids. The addition of the ketone functionality creates a molecule that has one donor (the carboxylic acid OH) and two acceptors (ketone oxygen and the acid's carbonyl oxygen). The disparity in the number of donors and acceptors creates a situation in which one acceptor will usually have its hydrogen bonding capacity unfulfilled. Solid state structural studies of many simple keto carboxylic acids reveal that there is a great deal of variation in hydrogen bonding motifs. The simplest division of the motifs is made by considering whether the acceptor is the ketone function or the carbonyl oxygen of the carboxylic acid. From this subdivision, further categorization can be made. For molecules in which the acid carbonyl is the hydrogen bond acceptor, two distinct hydrogen bonding modes have been found: acid-to-acid cyclic dimers and acid-to-acid infini.

Examples of the various hydrogen bonding motifs of keto carboxylic acids (both from our work and a survey of the CSD) and the frequency of occurrence of each motif will be given. These studies raise several questions. "What factors govern the variation of motif?" "What is the effect of the geometry of the molecule itself?" "Do the small differences in electropotential of the acceptors (within the molecule) influence the overall structure?" "Does the need for efficient close packing affect the frequency of a particular motif?" "How do symmetry constraints (e.g. lack of inversion center for single enantiomers) govern the distribution of the motifs?" Results of our studies will be presented, and some possible answers to these questions will be proposed.