Analysis of the compression of molecular crystal structures using Hirshfeld surfaces

The literature and unpublished work have been searched for examples of small molecule single crystal compression studies, for which 13 such studies have been found. These pressure series have been analysed using Hirshfeld surfaces in order to investigate the effect of compression on packing for a range of different organic crystal structures. Examination of the Hirshfeld surfaces during compression has shown a number of consistently recurring features within the data set. The areas which relate to long contacts or voids are seen to decrease and more short contacts appear at elevated pressures. The most striking observation found, however, was that the H···H contacts seen in the fingerprint plots for the surfaces become extremely prominent at high pressures, but do not appear to compress below a H···H distance of 1.7 Å. For the range of compounds studied, the structures in which the H···H contacts were compressed near to this limit were seen to undergo a phase transition or else the crystal fell apart. This value of 1.7 Å for the limit of H···H contacts is consistent with results in the literature and PIXEL calculations, which both show a substantial increase in the repulsive energy of the contact between 1.9 and 1.7 Å. In ambient pressure organic structures, studies of the Cambridge Structural Database show that the frequency of H···H contacts as a function of distance drops off drastically between 1.9 and 1.7 Å.