Prof. Judith A. K. Howard, CBE, FRS
(email at email@example.com)
Single Crystal X-ray Crystallography at Durham
The group is led by Professor Judith A. K. Howard and has a wide range of materials-based research interests. These include experimental charge density analysis, in-situ crystallisation of liquids, ultra-low temperature crystallography, high pressure crystallography, software development, solid-state reactions the study of non-linear optical materials and magnetically interesting materials. In addition,Dr. D.S.Yufit is the contact point for the departmental crystallography service, which is run with the help of all members of the group
Structural Chemistry research spans a wide range of activities, but pivotal is the elucidation of molecular structure by diffraction techniques. In the laboratory this means by X-ray diffraction using single crystal or powder techniques. Neutron diffraction and synchrotron radiation experiments are carried out at Central Facilities and are used in combination with at least some preliminary laboratory studies. Powder diffraction techniques using high intensity neutron or X-ray beams are becoming increasingly powerful, but high resolution molecular structure determinations still rely on single crystal intensity data. Neutron diffraction experiments on single crystals are performed in the special cases where the results cannot be derived by any other technique, for example the location of critical hydrogen atoms. e.g. implicated in important mechanisms; phase changes dependent on hydrogen atom movements, etc.
The ability of neutron diffraction to locate hydrogen atoms precisely arises from the differences in the scattering processes for X-rays and neutrons. In the former, this is by the extra-nuclear electrons and in the latter by the nuclei. The consequences of this are manifold, but for crystallographers this means we can exploit these differences to chemical advantage, e.g. differentiating isotopes of the same elements, e.g. H/D, or differentiating between closely 'adjacent' elements in the periodic table. X-rays are less well adapted to the latter since the difference will be only a matter of 1 or 2 electrons of scattering density, e.g. C, N and O, since the X-ray scattering power of an atom relates directly to its atomic number.
Very high resolution intensity data recorded at low temperatures (2 < T < 100 K) can be used, in conjunction with neutron diffraction data, to determine the distribution of charge density in the molecule as well as the electrostatic potentials at molecular surfaces. These are unique experiments and require long, careful data collections and in Durham we have special cryogenic facilities for such studies.
The majority of our experimental research projects involve recording and analysing variable temperature X-ray data in the laboratory and complementary neutron diffraction data collected on ISIS or at the ILL in Grenoble. We also use the synchrotron sources at diamond, ESRF in Grenoble and PSI in Switzerland, when the problems require very high intensity, finely collimated beams and rapid data collections. In addition, we use the Cambridge Structural Database extensively as a research tool in combination with our own experimental data.
There are also computational projects in molecular modelling and simulations, molecular mechanics and conformational analyses, using existing commercial packages and development software in-house. Statistical studies using the wealth of stored structural information in the various crystallographic databases are routine in the group, from simple searches to sophisticated analyses of reaction pathways and structure/activity correlations. The rapid retrieval and manipulation of large quantities of structural data forms the basis of so-called 'drug design' by the pharmaceutical companies and there is a constant demand for improved parameters for input to 'modelling' software. It is the responsibility (and pleasure) of the crystallographer to furnish accurate data to this community.
The crystallography laboratory is equipped with six CCD and one Image Plate X-ray diffractometers for single crystal analysis. These are manufactured by Bruker, Rigaku and Oxford Diffraction and are used for different types of diffraction experiments. We have, in Durham, unique research facilities for very low temperature X-ray diffraction, the first of these was designed and built in-house a number of years ago, this was the Fddd diffractometer, comprising a high intensity, rotating Mo anode generator and a very substantial Huber 4-circle goniostat, which carries a helium Displex cryorefrigerator and enables us to reach temperatures of 10K controllably. This diffractometer has been upgraded (2009) with modern electronics and a new CCD detector and is used currently for high resolution, high pressure data collections. We have also very recently built and commissioned (2010) a new State-of-the-Art Ultra-low temperature diffractometer that operates from 2K to above RT. The new Huber Eulerian cradle is very robust and carries the 3-stage (2K) cryorefrigerator and the diffractometer is equipped with a high intensity Bruker rotating anode Mo X-ray TXS source and an Apex II detector. The detector and the X-ray source distances to the sample can be varied to suit the experiment. Our other routine low temperature device was designed originally for the SMART diffractometer, but is adaptable to other instruments. This is an Oxford Cryosystems' HELIX, which enables us to reach 30K in open stream mode. These unique capability instruments offer enormous possibilities for new studies in chemical, physical and biochemical crystallography. We have also developed a device for diffraction studies at moderate pressure and low temperature and we use Diamond Anvil Cells for higher pressure studies.
X-ray diffraction zone images showing a solid-state phase transition at 41 K involving a cooperative ordering of a fluxional pseudo-Jahn-Teller Cu(II) system. Note the tripling of one cell axis. (hk0 zone at 42 K and hk0 zone at 40 K) Howard et al (1999), Chem. Commun. , 2245-2246.
"Olex2: A Complete Structure Solution, Refinement and Analysis Program." Dolomanov, O. V., L. J. Bourhis, R. J. Gildea, J. A. K. Howard and H. Puschmann (2009). J. Appl. Cryst. 42: 339-341.
"Structural, Spectroscopic, Electrochemical and Computational Studies of C,C'-diaryl-ortho-carboranes, 1-(4-XC6H4)-2-Ph-1,2-C2B10H10(X = H, F, OMe, NMe2, NH2, OH and O-) . Fox, M. A., C. Nervi, A. Crivello, A. S. Batsanov, J. A. K. Howard, K. Wade and P. J. Low (2009).." J. Solid State Electrochemistry 13: 1483-1495.
"Refractive indices for molecular crystals from the response of X-ray constrained Hartree-Fock wavefunctions." Jayatilaka, D., P. Munshi, M. J. Turner, J. A. K. Howard and M. A. Spackman (2009). Phys. Chem. Chem. Phys. 11: 7209-7218.
"Pressure-induced Cooperative Bond Rearrangement in a Zinc Imidazolate Framework: A High-Pressure Single-Crystal X-Ray Diffraction Study." Spencer, E. C., R. J. Angel, N. R. Ross, B. E. Hanson and J. A. K. Howard (2009) J. Am. Chem. Soc. 131: 4022-4026.
"Molecular van der Waals symmetry affecting bulk properties of condensed phases: melting and boiling points.'' Y. L. Slovokhotov and J. A. K. Howard (2007). Struct.Chem. 18: 477-491.
."New Prototype of Light-Driven Molecular Machine on the Basis of Pseudorotaxane Complex of Dibetaine of Unsaturated Viologen Analogue with Cucurbituril." Vedernikov, A. I., N. A. Lobova, L. G. Kuzmina, Y. A. Stzelenko, J. A. K. Howard, N. M. Alfimov and S. P. Gromov (2007). Russian Nanotechnolgies 2(5-6): 56-60.
"Determination of the hydrogen absorption sites in Zn4O(1,4-benzenedicarboxylate) by single crystal neutron diffraction." Spencer, E. C., J. A. K. Howard, G. J. McIntyre, J. L. C. Rowsell and O. M. Yaghi (2006). Chem.Commun: 278-280.
Simple pressure cell for single-crystal X-ray crystallography, D.S. Yufit and J.A.K. Howard, J. Appl. Cryst. (2005) 38, 583-586.
"Gas Adsorption sites in a Large Pore Metal Organic Framework". J.L.C.Rowsell, E C Spencer, J Eckert, J.A.K.Howard and O.M Yaghi (2005) Science 309, 1350-4
A photomagnetic study of three iron (II) compounds containing ligands from the 2,6-di(pyrazol-l-yl)pyridine series, V.A. Money, J.S. Costa, S. Marcen, G. Chastanet, J. Elhaik, M.A. Halcrow, J.A.K. Howard and J.-F. Letard, Chem. Phys. Lett. (2004) 391, 273-277.