Bimodal Distribution of the Shortest Intermolecular Contacts in Crystals of Organic Compounds

There are clearly two maxima in the distribution of the shortest intermolecular contacts, referred to the atomic van der Waals radii, in crystals of organic compounds. Accordingly, the crystals of organic compounds can be classified into those governed by strong cohesion forces (such as OH center dot center dot center dot O, NH center dot center dot center dot O, and NH center dot center dot center dot N hydrogen bonds) and weak ones (CH-O, CH center dot center dot center dot pi, halogen bonds, and dispersion forces). In about 1/3 of all known structures of organic compounds, there are strong cohesion interactions, while in 2/3 of structures the shortest contacts are associated with weak interactions. Characteristic properties of organic compounds depend on these either strong or weak cohesion forces. The distributions of the shortest intermolecular contacts of specific types, such as OH center dot center dot center dot O, NH center dot center dot center dot O, CH center dot center dot center dot N, and Br center dot center dot center dot Br can be approximated by the Gaussian functions. These Gaussian functions, with mean distance and standard deviation characteristic of specific interactions, can be used for predicting molecular arrangements and for validating crystal structures.