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    Growing Crystals That Will Make Your Crystallographer Happy  Paul D. Boyle  Department of Chemistry  North Carolina State University  Box 8204  Raleigh, NC, 27695 (NOTE: This article is also accessible on the World Wide Web at http://www.xray.ncsu.edu/GrowXtal.html ) with contributions from: Clarence Pfluger (cpfluger@keene.edu); Anthony Linden (alinden@oci.unizh.ch); Chuck Barnes (chemclb@mizzou1.missouri.edu); Andrea Sella (a.sella@ucl.ac.uk) Contents ã The Goal ã General Considerations ❏  Solvent ❏  Nucleation ❏  Mechanics ❏  Time ã Techniques ❏  Slow Evaporation ❏  Slow Cooling ❏  Variations on Slow Evaporation and Slow Cooling ❏  Vapor Diffusion ❏  Solvent Diffusion ❏  Reactant Diffusion ❏  Sublimation ❏  Convection ❏  Co-crystallants ❏  Counterions ❏  Ionization of Neutral Compounds ã References   Growing Crystals That Will Make Your Crystallographer Happy 2Practically every chemist knows the necessity of recrystallization as a method of purification. What is less well known, however, is methods by which “X-ray quality” crystals may be obtained. The purpose of this monograph is to briefly and informally outline some of the methods which can be used to obtain single crystals suitable for X-ray diffraction studies. This monograph is meant neither to be rigorous nor exhaustive, but rather, a sort of practical “how to” cookbook. A good general reference which covers similar material can be found in “Crystal Growing”, Chemistry in Britain   1981 , 17  , 222–225 by Peter G. Jones. The Goal The goal in growing crystals for a single crystal X-ray diffraction experiment is to grow single crystals (obviously) of suitable size. The optimum size for a crystal is one which has dimensions of 0.2–0.4 mm in at least two of the three dimensions. Most potential structure determinations are thwarted by a lack of suitable crystals. General Considerations The factors during crystal growth which affect the size of the crystals are, solubility of compound in the solvent chosen for recrystallization, the number of nucleation sites, mechanical agitation to the system, and time. Solvent. Choose a solvent in which your compound is moderately soluble. If the solute is too soluble, this will result in small crystal size. Avoid solvents in which your compound forms supersaturated solutions. Supersaturated solutions tend to give crystals which are too small in size. Nucleation. The fewer sites at which crystals begin to grow will result in fewer crystals each of larger size. This is desirable. Conversely, many nucleation sites result in a smaller average crystal size, and are not desirable. In many recrystallizations ambient dust in the laboratory provides sites of nucleation. It is important to minimize dust or other extraneous particulate matter in the crystal growing vessel. Mechanics. Mechanical disturbance of the crystal growing vessel results in smaller crystals. Let the crystals grow with a minimum of disturbance. This means: don’t try to grow crystals next to your vacuum pump, and don’t pick up the vessel everyday to check on how your crystals are growing. Set up the crystal growing attempt, in a quiet, out-of-the-way place and forget about it (if possible!) for a week. Time. This is related to mechanics. Crystals fully recognize that patience is a virtue and will reward those who practise it.   Growing Crystals That Will Make Your Crystallographer Happy 3 Techniques Crystal growing is an art, and there are as many variations to the basic crystal growing recipes as there are crystallographers. The recipes given below are ones which I have either tried or I have read about and sound reasonable. The techniques chosen will largely depend on the chemical properties of the compound of interest: is the compound air sensitive, moisture sensitive? is it hygroscopic? etc. etc. Slow Evaporation. This is the simplest way to grow crystals and works best for compounds which are not sensitive to ambient conditions in the laboratory. Prepare a solution of the compound in a suitable solvent. The solution should be saturated or nearly saturated. Transfer the solution to a CLEAN  crystal growing dish and cover. The covering for the container should not be air tight. Aluminium foil with some holes poked in it works well, or a flat piece of glass with microscope slides used as a spacer also will do the trick. Place the container in a quiet out of the way place and let it evaporate. This method works best where there is enough material to saturate at least a few milliliters of solvent. Slow Cooling. This is good for solute-solvent systems which are less than moderately soluble and the solvent’s boiling point is less than 100 °C. Prepare a saturated solution of the compound where is the solvent is heated to just its boiling point or a just below it. Transfer the solution to a CLEAN  large test tube and stopper. Transfer the test tube to a Dewar flask in which hot water (heated to a temperature of a couple of degrees below the solvent boiling point). The water level should exceed the solvent level in the test tube, but should not exceed the height of the test tube. Stopper the Dewar flask with a cork stopper and let the vessel sit for a week. A more elaborate version of this involves a thermostated oven rather than a Dewar flask. Variations on Slow Evaporation and Slow Cooling. If the above two techniques do yield suitable crystals from single solvent systems, one may expand these techniques to binary or tertiary solvent systems. The basic rationale for this is by varying the solvent composition one may inhibit growth of certain crystal faces and promote the growth of other faces, yielding crystals of suitable morphology and size. If you choose this route for growing crystals, it absolutely necessary to record the solvent composition you use! If crystal growing is an art, growing crystals from binary or tertiary solvent mixtures is that much more imprecise. Remember reproducibility is paramount in science. Vapor Diffusion. (excerpted and paraphrased from Stout and Jensen p. 65). This method is good for milligram amounts of material. A solution of the substance is prepared using solvent S1 and placed in test tube. A second solvent, S2, is placed in a closed beaker. S2 is chosen such that when mixed with S1 the solute will become less soluble. The test tube containing S1 is then placed in the beaker and the  beaker is sealed. Slow diffusion of S2 into the test tube and S1 out of the test tube will cause crystals to   Growing Crystals That Will Make Your Crystallographer Happy 4form. If S2 is more volatile than S1 the solvent level will increase and prevent microcrystalline crusts from forming on the sides of the test tube. Solvent Diffusion (Layering Technique). This method also is good for milligram amounts of materials which are sensitive to ambient laboratory conditions (air, moisture). Dissolve the solute in S1 and place in a test tube. Slowly  dribble S2 into the tube so that S1 and S2 form discrete layers. This will only be successful if (1) the density of S2 < S1 and (2) care is exercised in creating the solvent layer. I have found that a syringe is the best way to add the second solvent. The narrower the tube, the easier it is to  build up the layer. Five millimeter NMR tubes are excellent vessels to use for this crystal growing technique. CH 2 Cl 2 /Et 2 O is a good solvent combination to try this method (if your compound is insoluble in ether). Reactant diffusion. This similar to the other diffusion methods except that solutions of the reactants are allowed to diffuse into one another. If the product of the reaction is insoluble, crystals of the product will form where the reactants mix. There is mention in the literature of this technique being used with diffusion in silica gels (see  Acta Cryst. 1965 , 19 , 483). Sublimation. Andrea Sella ( a.sella@ucl.ac.uk  ) suggests a couple of ways to grow crystals of somewhat volatile air sensitive crystals. The first way is to simply seal a sample under vacuum into a glass tube and placing the tube into an oven for a few days or weeks. Larger crystals tend to grow at the expense of smaller ones. If it doesn’t work raise the temperature of the oven or move to another hotter one. In some cases a tube furnace can be used. Andrea uses an all-glass furnace which was srcinally designed  by Prof. Geoff Cloke of Sussex University (email her for the details if you want to try this method). The tube can be placed close to one end of the furnace so that there is a mild temperature gradient. The second method is to seal off a long piece of glass tubing at one end and put a joint with a vacuum adapter (or a Young’s/Ace/Kontes valve) at the other. The sample is placed at the sealed end of the tube and is followed by a glass wool plug which cleans the tube as it is pushed along and serves to prevent  bulk/crude material blowing along the tube itself. The tube is then evacuated and inserted into a length of copper pipe which will serve to produce a temperature gradient. The sealed end of the tube (where the sample is) and the copper jacket are then heated in an oil bath (vertical set-up) or in a tube furnace (either horizontal or vertical). The tube can be left under either dynamic or static vacuum (it’s worth trying both). Andrea learned this method from his Ph.D. supervisor M. L. H. Green of Oxford. Convection. One may attempt to grow crystals by convection by creating a thermal gradient in the crystal growing vessel. The idea behind this method is that the solution becomes more saturated in the warm part of the vessel and is transferred to the cooler region where nucleation and crystal growth occur.
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