The Effect of Pressure on Halogen Bonding in 4-Iodobenzonitrile
oleh: Nico Giordano, Sergejs Afanasjevs, Christine M. Beavers, Claire L. Hobday, Konstantin V. Kamenev, Earl F. O’Bannon, Javier Ruiz-Fuertes, Simon J. Teat, Rafael Valiente, Simon Parsons
| Format: | Article |
|---|---|
| Diterbitkan: | MDPI AG 2019-05-01 |
Deskripsi
The crystal structure of 4-iodobenzonitrile, which is monoclinic (space group <i>I</i>2/<i>a</i>) 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 <i>b</i> axis compressing by 3.3%, and the <i>a</i> and <i>c</i> axes by 12.3 and 10.9 %. Since the chains are exactly aligned with the crystallographic <i>b</i> 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 (<inline-formula> <math display="inline"> <semantics> <mrow> <mi>P</mi> <mover accent="true"> <mn>1</mn> <mo>¯</mo> </mover> </mrow> </semantics> </math> </inline-formula>) 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<sup>−1</sup> and the mixed C-I/I..N stretching frequency observed by Raman spectroscopy increases from 249 to 252 cm<sup>−1</sup>. 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.