Simulation and analysis of virtual tensile of elastic and plastic crystals of dihalophenols

In order to check the stability of tensile deformation modeling results of molecular crystals, a test for virtual stretching of plastic 3,4-dichlorophenol and elastic 4-bromo-3-chlorophenol has been carried out. Modeling for all considered crystal structures has been carried out by density functional theory methods with allowance for periodic boundary conditions and localized basis sets. It has been confirmed that the previously proposed virtual tensile test of molecular crystals makes it possible to explain and predict their mechanical properties from the characteristic behavior of changes in the energy of a crystal cell and its volume depending on tensile strains. In the present study the tensile deformations of the crystal 3,4-dichlorophenol cell have been modeled with different steps (1% and 3%) of the values of the cell parameters. This has allowed us to establish that the trends in the crystal structure change and properties under consideration depend but little on the value of the stretching step within the studied limits. Only small differences in the crystal cell energy per 15% stretching have been found; they have later leveled out. For the studied crystal structures, it has been found that four O-H…O hydrogen bonds hold the formed synthons of four molecules with greater strength than the Cl…Cl or Br…Br interstack interactions, which are capable of switching or breaking during deformation. It has turned out that the interaction of type I halogens for both elastic and plastic crystals is lost during stretching, and instead of one van der Waals interaction, two halogen bonds, Cl…Br or Сl…Cl, respectively, are formed. Depending on the tensile strains at the initial stage of stretching, similar changes in the behavior of the structure and energy of a crystal cell, as well as its volume are discovered for both plastic 3,4-dichlorophenol and elastic 4-bromo-3-chlorophenol. However, plastic 3,4-dichlorophenol is characterized by slower changes in the structure and a shorter period of deceleration of the crystal cell energy growth with the appearance of interstack gaps. For an elastic crystal this period is much longer and is accompanied by “oscillations” in the energy change. Thus, the replacement of the chlorine atom by bromine in the 4th position of the phenol cycle leads to formation of reasonably large cavities if tensile strains reach a certain percentage, which contributes to significant changes in the structure without significant energy input.