How to Grow X-Ray Quality Crystals

Written by Dr. Rene J. Lachicotte

There are numerous factors during crystal growth which affect both the size and quality of the crystal. The most important in terms of organic compounds are purity, the solvent chosen for recrystallization, the number of nucleation sites, mechanical agitation to the system, and time.

1. Solvent: some generalizations
   Typically, it is good to choose a solvent in which your compound is moderately soluble. If the solute is too soluble, crystal size will tend to be small. Avoid solvents in which your compounds form saturated solutions, because again, saturated solutions typically yield crystals which are too small.
   Exceptions: When a compound is soluble in most liquids, then slow cooling of saturated solutions can yield good single crystals. Be sure that the flask and solvent are dust free!
   
2. Nucleation
   The fewer "nucleation" sites at which crystals begin to grow results in fewer crystals, generally of large size, which is desirable! Conversely, many nucleation sites result in a smaller average crystal size, and it is not desirable. Care should be taken to avoid dust in the laboratory, and microscopic paper fillings (scraping cpd. from paper filters), which hinder the growth of high quality crystals.
   Minimize dust and extraneous particulate matter in the solution of the compund and the growing vessel.
   
3. Mechanics
   Mechanical disturbance of the crystal growing vessel can result in ruining all of the above efforts. Let the crystals grow with minimum disturbance. This means:
   - don't try to grow crystals next to your vacuum pump
   - don't check the progress of your crystals on a daily basis
   
4. TIME!!
   Crystals fully recognize that patience is a virtue and will reward those who practice it.

Techniques

"Crystal growing is an art." Not.

The techniques chosen depends less on "art" and "intuition" than on the chemical properties of the compound of interest:

• Is the compound hygroscopic?
• What are the compound's solubility properties?
• Is the compound air or light sensitive?
• What about decomposition?
• etc. etc. etc.

• Slow Evaporation

  Simplest way to grow compounds and works well for compounds that are not sensitive to ambient conditions in the laboratory
  • Prepare a solution of the compound in a suitable solvent (saturated or nearly saturated).
  • Filter the solution through a clean glass frit into a clean vessel* and cover, but not tightly.
  • Gently put the container in a quiet, out of the way place and allow the solvent to evaporate slowly.
  • This method works best when there is ample material to allow for at least a few milliters of solvent.

• Slow Cooling

  This method works well for solute-solvent systems which are less than moderately soluble and the solvent's boiling point is less than 100 ^oC.
  • Working with a saturated solution: No Dust/Hair etc., Heat the solvent to boiling point or slightly below it. Transfer by filtering through a warm frit into a vessel* (test tube, or scintillation vial) and stopper tightly. Transfer vessel to flask in which hot water* (oil may also be used) has been heated to a couple of degrees below the solvent boiling point.
  • The water level should exceed the solvent level in the vessel but should not exceed the height of the vessel (or you can use a thermostated oven).

• Variations on Slow Evaporation and Slow Cooling

  If these techniques do not yield suitable crystals from single solvent systems, these techniques can be expanded to binary, tertiary, and even quaternary solvent systems.
  • The basic rationale is that by varying the solvent composition, growth of certain crystal faces may be inhibited while the growth in some other direction is promoted, thus yielding crystals of suitable morphology and size.
  • Reproducibility is paramount in science! Growing crystals from multiple solvent systems will be imprecise unless solvent compositions are recorded.... hence "art."

• Liquid Diffusion

  very sucessful method for obtaining single crystals of organic compounds*
  • A small amount of solution is placed in a tube, and a suitable precipitant is layered carefully down the side of the tube onto the solution. It is very important that this second liquid be less dense than (or visa versa*), and miscible with, the solvent.
  • The volume ratio solvent:ppt will be variable, but a good place to start is 1:4 or 1:5.
  • Slightly turbidity should form at the interface.
  • If there is no crystal growth after 24 hr, try a more concentrated solution.

• Use of Seed Crystals

  This method is useful when other methods provide crystals which, although of reasonable quality, are too small. These small crystals can be used as "seed" crystals in a slow cooling saturated solution.
  • Draw the seed crystals in a pipette together with some mother liquor (do not allow the seed crystals to dry!)
  • Carefully deposit these "seed crystals" in the saturated solution. The fewer the "seeds" the bigger the resulting crystals

• Convection

  This is a less standard method, but one may attempt to grow crystals by convection by creating a thermal gradient in the crystal growing vessel. The theory 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 takes place. To create convection, one must use either local heating ot local cooling.
  • Trick to making this easy: MeCN/Et ₂O hot frit

• Counterions

  If your compound is ionic and is not giving suitable crystals with a given counterion, perform a metathesis reaction to change the counterion.
  • Ions of similar size tend to pack better and subsequently give better crystals

• Ionization of Neutral Compounds

  If the compound of interest is neutral and contains proton donor or acceptor groups, better crystals may be grown by first protonating or deprotonating the compound.
  • The ionic form of the compound could take advantage of factors such as hydrogen bonding to yield better crystals. Of course, this will alter the electronic properties of your compound, but if a general conformation is what is needed from the structrue determination, then this should not be a problem.

References

The Growth of Single Crystals, R. A. Laudise, Solid State Physical Electronic Series, Nick Holonyak, Jr. Editor, Prentice-Hall, Inc., 1970.

See also: Crystal Growing Tips (University of Florida)

See also: Crystallization Methods (U Penn)