The Effect of Pressure on Halogen Bonding in 4-Iodobenzonitrile

Abstract

The crystal structure of 4-iodobenzonitrile, which is monoclinic (space group I2/a) under ambient conditions, contains chains of molecules linked through C≡N···I halogen-bonds. The chains interact through CH···I, CH···N and π-stacking contacts. The crystal structure remains in the same phase up to 5.0 GPa, the b axis compressing by 3.3%, and the a and c axes by 12.3 and 10.9 %. Since the chains are exactly aligned with the crystallographic b axis these data characterise the compressibility of the I···N interaction relative to the inter-chain interactions, and indicate that the halogen bond is the most robust intermolecular interaction in the structure, shortening from 3.168(4) at ambient pressure to 2.840(1) Å at 5.0 GPa. The π∙∙∙π contacts are most sensitive to pressure, and in one case the perpendicular stacking distance shortens from 3.6420(8) to 3.139(4) Å. Packing energy calculations (PIXEL) indicate that the π∙∙∙π interactions have been distorted into a destabilising region of their potentials at 5.0 GPa. The structure undergoes a transition to a triclinic ( P 1 ¯ ) phase at 5.5 GPa. Over the course of the transition, the initially colourless and transparent crystal darkens on account of formation of microscopic cracks. The resistance drops by 10% and the optical transmittance drops by almost two orders of magnitude. The I···N bond increases in length to 2.928(10) Å and become less linear [<C−I∙∙∙N = 166.2(5)°]; the energy stabilises by 2.5 kJ mol−1 and the mixed C-I/I..N stretching frequency observed by Raman spectroscopy increases from 249 to 252 cm−1. The driving force of the transition is shown to be relief of strain built-up in the π∙∙∙π interactions rather than minimisation of the molar volume. The triclinic phase persists up to 8.1 GPa.

A Short Review of Current Computational Concepts for High-Pressure Phase Transition Studies in Molecular Crystals

“…In contrast to FF methods, DFT calculations for systems with periodic boundary conditions prove to give very accurate energies if multiple parameters are used correctly [78][79][80][81][82]. Discussing any phase transition, we assume that Gibbs energy change should be negative for spontaneous phase transition, which can be estimated from enthalpy and entropy terms.…”

“…Discussing any phase transition, we assume that Gibbs energy change should be negative for spontaneous phase transition, which can be estimated from enthalpy and entropy terms. Entropy term is frequently neglected (especially if space group preserves after phase transition), supposing to have a small impact on Gibbs energy, while enthalpy can be calculated relatively easy [61,67,80,81,83]. Nevertheless, a lack of entropy calculations can lead to significant mistakes in the prediction of phase stability, which was shown for many inorganic materials [84,85].…”

“…Enthalpy is a sum of internal energy and PV term, where internal energy can be described as lattice energy-a sum of intermolecular and intramolecular (conformational) interactions. The thing that can be calculated using FF methods is intermolecular interactions only, while conformational (relaxation) energies should be calculated using DFT methods [80,81]. Constant-pressure (or, more precisely, fixed-stress) geometry optimization has to be carried out in DFT energy calculations to accurately define static pressure.…”

“…Following Gibbs energy contributions can lead to a new understanding of reasons for phase transitions for any crystalline material. In some cases, it is possible to claim volume change (PV), energy (E inter or E intra ), enthalpy (H) or entropy (S) as a driving force for phase transition [61,67,81,89] DFT methods can be applied to experimental structures, obtained at some pressure, and all the above-mentioned parameters can be calculated. If one needs thermodynamic parameters for crystal structure at a pressure where no experimental data available, there are two main possibilities.…”

Variations of quantum electronic pressure under the external compression in crystals with halogen bonds assembled in Cl3-, Br3-, I3-synthons

“…Characterization and prediction of mechanical properties, such as stiffness, brittleness, elasticity or plasticity of crystal structures, are directly related to the changes at the level of noncovalent interactions (Coudert & Fuchs, 2016;Mishra et al, 2020;Feng et al, 2016). Research of bonding effects in molecular crystals in ambient conditions and under external stress has been performed in both experimental studies (Bag et al, 2012;Casati et al, 2017;Mishra et al, 2017;Saha et al, 2018;Arkhipov et al, 2019) and those using density functional theory (DFT) calculations (Lin et al, 2017;Matveychuk et al, 2018;Colmenero, 2019a,b;Giordano et al, 2019). The popular targets in the investigation of mechanical properties were the hexahalobenzene molecular crystals, C 6 Cl 6 , C 6 Br 6 and C 6 I 6 , including their treatment under external compression.…”