Department of Chemistry - University of Zurich


Crystallography Services for External Clients

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Services offered for academic and industrial clients

Please note: Academic clients are normally expected to pay the academic services fees. Do not send samples without first contacting and obtaining the approval of the Manager of the X-ray Crystallography Facility. Unsolicited samples that arrive, for which no prior contact and arrangement has been made, will not be accepted.

The X-ray Crystallography Facility offers the following range of services associated with single crystal structure analyses of small molecules:

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Priority

Internal work from our institute takes priority over samples from external clients. Analyses submitted with Normal Priority will be conducted usually within 1-2 weeks.

Industrial clients only may request Express Service, in which case the analysis will be conducted within 2-3 days (excepting equipment failures, maintenance requirements or an already running experiment requires several days for completion). A 50% surcharge on the normal fees will be applied.


Guidelines for Sample Preparation

Clients are responsible for producing diffraction quality single crystals. Advice will be given on suitable crystal growth techniques and a visual evaluation of the suitability of a sample for analysis can be made at any time for no charge. If the sample does not appear to contain any suitable crystals, it will be necessary for the client to arrange for a recrystallisation in their own laboratory.

Where possible, samples should be provided in a dry and loose form in a small vial. Drying crystals under vacuum should be avoided. If solvent loss or crystal decomposition appears to be a problem, the sample may be provided damp or in mother liquor. The ideal crystal is a cube. The maximum length of any side should not normally exceed about 0.25 mm and plates or needles of thickness down to about 0.03 mm may be satisfactory provided the remaining dimension(s) approach the maximum allowed. However, the quality of very small crystals is often inferior to that of larger crystals, so the use of very small crystals should be avoided whenever possible. Larger crystals can usually be cut, but some crystals cannot be cut without shattering.

More information is available on crystal growth techniques and sample preparation and sample submission.

If a visual inspection of a sample indicates that it is probably suitable for analysis, the client must fill in a Request Form with the information requested thereon. The client will then be liable for charges as outlined in the Schedule of Fees.


Explanation of Measurements and Calculations

Diffractometer time. A data collection usually requires from 4 to 24 hours, depending on the crystal quality and its diffracting power. Good, strongly diffracting crystals will permit data collection to be completed within just a few hours. Crystal quality can have a significant influence on data collection speed and the quality of the overall results.

Refinement. The computing time increases with the number of atoms in the molecule. Therefore, refinements are charged on a per atom basis calculated according to the total number of atoms required to be defined in order to build the entire structure (i.e. one asymmetric unit). This is usually equivalent to just one molecule or set of ions, but occasionally there can be solvent molecules or more than one chemically equivalent molecule in the asymmetric unit. This cannot usually be predicted beforehand, but these additional atoms will be included in the charges.

Disorder. The treatment of complex disorder or other complicated structural difficulties can involve a large amount of additional work. In such cases, the model will be developed to a reasonable approximation of the effect. If the model must be improved further (e.g. for publication purposes), industrial clients will be charged for the additional time required.

Absolute configuration. Clients should indicate if they require the absolute configuration of an enantiomerically pure chiral crystal to be determined. If the chirality of one stereogenic centre in the molecule is already known unequivocally from other experiments or chemical knowledge it should be stated; the configurations of the other stereogenic centres can then easily be deduced. If the configurations of all stereogenic centres in a molecule are unknown, the success of the determination of the absolute configuration by X-ray diffraction depends on the heaviest element present in the structure. With high-quality crystals, an indication of absolute configuration might be possible with the heaviest element in the structure just being oxygen, but preferably a heavier element (silicon or heavier) is needed. The heavy atom does not have to be in the molecule of interest; it could be a counter-ion, or in a co-crystallised species such as a solvent molecule.

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last update: Friday, 09-Dec-2016 13:25:36 CET
Department of Chemistry, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland