Atomic volumes and charges in a system with a strong hydrogen bond: L-tryptophan formic acid.

A high-resolution X-ray diffraction data set was collected within 48 h at 100 K with synchrotron radiation and area detection. A full topological analysis was applied to the resulting electron-density model. This analysis was followed by a Bader partitioning making use of the zero-flux surfaces of the electron-density gradient vector field. The atomic and bonding properties obtained were compared with the results of previous experimental studies, and with theoretical calculations for the title complex and free tryptophan as reported in the literature. The agreement between experiment and theory is similar to the agreement between different theoretical calculations. There is no charge transfer via the strong hydrogen bond; however, its strength is indicated by the very small atomic volume of the H atom involved.