title: The effect of temperature and pressure on the crystal structure of piperidine.
abstract: BACKGROUND
  The response of molecular crystal structures to changes in externally applied conditions such as temperature and pressure are the result of a complex balance between strong intramolecular bonding, medium strength intermolecular interactions such as hydrogen bonds, and weaker intermolecular van der Waals contacts. At high pressure the additional thermodynamic requirement to fill space efficiently becomes increasingly important.
  
  
  RESULTS
  The crystal structure of piperidine-d11 has been determined at 2 K and at room temperature at pressures between 0.22 and 1.09 GPa. Unit cell dimensions have been determined between 2 and 255 K, and at pressures up to 2.77 GPa at room temperature. All measurements were made using neutron powder diffraction. The crystal structure features chains of molecules formed by NH…N H-bonds with van der Waals interactions between the chains. Although the H-bonds are the strongest intermolecular contacts, the majority of the sublimation enthalpy may be ascribed to weaker but more numerous van der Waals interactions.
  
  
  CONCLUSIONS
  Analysis of the thermal expansion data in the light of phonon frequencies determined in periodic DFT calculations indicates that the expansion at very low temperature is governed by external lattice modes, but above 100 K the influence of intramolecular ring-flexing modes also becomes significant. The principal directions of thermal expansion are determined by the sensitivity of different van der Waals interactions to changes in distance. The principal values of the strain developed on application of pressure are similarly oriented to those determined in the variable-temperature study, but more isotropic because of the need to minimise volume by filling interstitial voids at elevated pressure. Graphical AbstractThough H-bonds are important interactions in the crystal structure of piperidine, the response to externally-applied conditions are determined by van der Waals interactions.
authors.affiliation: EaStCHEM School of Chemistry and Centre for Science at Extreme Conditions, The University of Edinburgh, King's Buildings, West Mains Road, Edinburgh, EH9 3FJ UK.
authors.forename: Laura E
authors.initials: LE
authors.lastname: Budd
authors.affiliation: ISIS Pulsed Neutron and Muon Facility, STFC Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Harwell Oxford, Didcot, OX11 0QX UK ; Present address: Neutron Sciences Directorate, Oak Ridge National Laboratory, PO Box 2008 MS6475, Oak Ridge, TN 37831-6457 USA.
authors.forename: Richard M
authors.initials: RM
authors.lastname: Ibberson
authors.affiliation: ISIS Pulsed Neutron and Muon Facility, STFC Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Harwell Oxford, Didcot, OX11 0QX UK.
authors.forename: William G
authors.initials: WG
authors.lastname: Marshall
authors.affiliation: EaStCHEM School of Chemistry and Centre for Science at Extreme Conditions, The University of Edinburgh, King's Buildings, West Mains Road, Edinburgh, EH9 3FJ UK.
authors.forename: Simon
authors.initials: S
authors.lastname: Parsons