Speaker
Prof.
Garry McIntyre
(ANSTO)
Description
Hydrogen bonds are one of the most important classes of intermolecular interaction, and accurate H-atom positions are critical for analysis of the energy terms which determine the thermodynamic stability of molecular crystals. At ambient pressure and low temperatures, H atoms can often be located by X-ray diffraction, and X-ray data can provide an accurate picture of the intermolecular contacts. High-pressure experiments do not afford this luxury. The high systematic errors introduced by the pressure cell and low completeness mean that H-atom positions are not revealed in X-ray Fourier maps. In some compounds H-atom positions can be inferred from the positions of other atoms, but this is not possible in all cases.
Neutron diffraction data are much more sensitive to H than are X-ray data, and they are essential in cases where accurate H-atom location is important. Neutron powder patterns of complex molecular systems suffer from extensive peak overlap, and single-crystal diffraction therefore has a huge advantage; there is also no need to deuterate. The main disadvantage of neutron diffraction is that a large sample is usually required, which is at odds with the decreasing volumes possible with increasing pressure with existing pressure-cell materials. Modern neutron Laue diffraction and large moissanite anvil cells offer some respite [1], but complementing high-pressure X-ray data with high-pressure neutron data is still fraught with technical challenges to obtain identical conditions.
Initial developmental experiments using a miniature diamond-anvil cell with a single crystal of size typical for X-ray diffraction on the KOALA Laue diffractometer at OPAL have shown the feasibility of the Laue technique for single-crystal neutron studies at high pressure. Remarkably, data completeness is similar to ambient-pressure measurements, despite the presence of the pressure cell. It is now possible to perform joint X-ray and neutron studies on the same sample under identical conditions.
The implications of the high-pressure sample environment for neutron Laue diffraction will be explored and the results from experiments on hexamine presented.
[1] G.J. McIntyre, L. Mélési, M. Guthrie, C.A. Tulk, J. Xu and J.B. Parise, J. Phys.: Condens. Matter 17, S3017 (2006)
Primary author
Dr
Jack Binns
(ANSTO)
Co-authors
Prof.
Garry McIntyre
(ANSTO)
Prof.
Konstantin Kamenev
(School of Engineering and Centre for Science at Extreme Conditions, The University of Edinburgh)
Prof.
Simon Parsons
(EaStCHEM School of Chemistry and Centre for Science at Extreme Conditions, The University of Edinburgh)
Dr
Stephen Moggach
(EaStCHEM School of Chemistry and Centre for Science at Extreme Conditions, The University of Edinburgh)
