Experimental Charge Densities and Intermolecular Interactions:  Electrostatic and Topological Analysis of dl-Histidine

A high-resolution, low-temperature X-ray diffraction data set on dl-histidine, collected with a CCD detector, is used in the analysis of molecular bonding and intermolecular interactions. The molecular dipole moment in the crystal is enhanced relative to that from HF and DFT calculations. Topological properties of the molecular electron density differ from theory for the polar bonds but generally agree well for the C−C bonds in the molecule. A major aim of the study is the evaluation of the electrostatic contribution to the intermolecular interactions from the experimental density. The electrostatic interaction energies between pairs of neighboring molecules, as calculated from the experimental density, compare reasonably well with the total interaction energies from supermolecule calculations. The agreement is somewhat improved by the addition of nonelectrostatic repulsion and dispersive terms, which together contribute much less than the electrostatic energy. The electrostatic interaction energy calculated from the CHARMM point-charge force field is often close to the values derived from the experimental charge density, though exceptions occur. In an alternative approach, the topology of the intermolecular charge density is related to the intermolecular interaction energy. The latter approach makes use of a density functional by Abramov (Acta Crystallogr. 1997, A53, 264−272) and a relation between the potential energy density at the bond critical point and the hydrogen bond dissociation energy (Espinosa, et al. Chem. Phys. Lett. 1998, 285, 170−173).