Growing Quality Crystals

——献给志同道合的朋友们

资料一

According to the old rule Garbage In = Garbage Out, a crystal structure is only as good as
the crystal used for data collection. Therefore it is worthwhile spending time on improving the
quality of your crystals. Even though growing crystals is more of an art than a science and luck
is a major factor, there are some things to do and some other things not to do. The following
paragraphs mention some of the more important ones. First some theoretical background about
crystallization.

Saturation and Supersaturation

Theoretically, crystallization should start when the concentration of a compound in a solvent is

higher than the solubility product of this compound. Generally, however, crystallization is
kinetically hindered and crystals grow only from supersaturated solutions. There are several
ways to achieve this metastable state of supersaturation.

The easiest is increasing the concentration by evaporation of the solvent until crystallization
sets in. This can be achieved by not closing the lid of the tube or flask very tightly and simply
wait. Many crystals are obtained from NMR tubes. NMR tubes are usually sealed with that
little colorful baseball-cap shaped plug, which is not overly tight. When forgotten in the fridge
or on the lab bench for several months, the solvent slowly evaporates from the NMR tube, the
solution becomes first saturated, then supersaturated, and crystals grow.

Another way of obtaining a supersaturated solution is making use of the fact that many
compounds are better soluble in hot solvents than in cold ones. A hot solution that is almost
saturated is likely to yield crystals at room temperature or, if appropriate, below. However,
crystals that grow at higher temperature are frequently twinned or show static disorder.

Another way to supersaturation, frequently the best way to grow quality crystals, is the use of
binary solvent systems. You need two liquids that mix well, and your compound should be
soluble in only one of them. The liquid in which you compound is soluble is called the solvent,
the other liquid the precipitant. As your compound is less soluble in a mixture of the two
liquids, you can grow crystals by slowly mixing a (not too) concentrated solution of your
compound with the precipitant. This can happen as liquid-liquid diffusion, gas-phase diffusion
or via a membrane (dialysis).
Nucleation

Crystallization is proceeded by nucleation, which happens either spontaneously or is included

by vibration or particles. If nucleation sets in too quickly, too many too small crystals will
grow. The figure below shows an equilibrium diagram of a crystallization from a solution. For
a diffraction experiment you need no more than one good single crystal. The best way of
growing a few nice crystals, when opposed to a lot of bad crystals, is to change the
concentration slowly into the area of nucleation, without getting too deep into it. The formation
of nuclei (not too many) and the starting crystallization will reduce the concentration and bring
the solution back into the region of oversaturation. That is where existing crystals grow, but no
new nuclei form. You want to keep your system there. That means all changes of your system
need to be slow.

Size Matters

Diffraction quality crystals need to be relatively large. Maybe not quite on the engagement ring
scale, but 0.1 to 0.3 mm in each dimension is a good number. In order to grow large crystals, it
is important to avoid having to many nucleation sites (see above). Crystals that grow more
slowly, tend to be larger. For crystals that were grown by slow cooling of the solvent: it usually
improves the quality and size of the crystals, if the solution is slowly warmed up until almost
all crystals are dissolved again and than cooled down a second time very slowly. This can
reduce the number of crystals obtained and usually improves quality and size.

I wasted time, and now doth time waste me

A good crystal grows slowly. A good time frame for a crystallization experiment seems to be
some two to seven days. Crystals that grow within minutes usually don't diffract as well as they
could.

Crystallization Techniques

Slow Evaporation

As mentioned above this is the simplest method to grow

crystals. Prepare a nearly saturated solution of your
compound in a suitable solvent, transfer at least a couple of
milliliters into a clean container, ideally with a large
surface, and cover. Don't cover it too tightly, though
(aluminum foil with some punched holes seems to work
very well), as you want the solvent to evaporate over the
following days. Set the container aside and disturb the experiment as little as possible
(remember: vibration can cause nucleation).
Advantages: Easy.
Disadvantages: Needs a lot of material, starts with (almost) saturated solution, which can lead
to too much nucleation, not so good for air-sensitive compounds.

Slow Cooling

Prepare a nearly saturated solution of your compound at or

close to the boiling point of the solvent of your choice.
Transfer the solution into a clean container and cover.
Place the container into a heat bath at about the same
temperature and allow to cool slowly. A Dewar with hot
water frequently does the trick.
A variation of this method is to prepare a saturated
solution at room temperature and place the container in a
cold place. E.g. THF is still liquid at -80°C, which allows growing crystals in a dry-ice acetone
bath (or in the -80°C freezer).
Advantages: Easy, works best for only moderately soluble substances.
Disadvantages: Needs a lot of material, starts with saturated solution (too many small crystals),
usually takes place at high temperature, which can lead to disordered or twinned crystals.

Vapor Diffusion

For this method you need a binary solvent system. Choose

two liquids that mix well. Your compound should be
soluble relatively well in the liquid with the higher boiling
point -- we call this liquid the solvent --, and as good as
insoluble in the liquid with the lower boiling point, which
we call the precipitant. Prepare a solution of your
compound in a small open container. Place this receptacle
inside a larger one that contains some precipitant and seal the outer vessel well. Over time the
precipitant, which is more volatile, will diffuse over the gas phase into the solvent, leading to
oversaturation, nucleation and, if all goes well, finally crystallization. You can regulate the
diffusion speed by varying the temperature.
Advantages: Works well with small amounts, usually gives good crystals, parameters are easy
to control.
Disadvantages: Not quite as simple, finding two suitable solvents can be hard.

Liquid-Liquid Diffusion

As with the vapor diffusion method, you need a binary

solvent system. In this case the boiling points don't matter
very much but the specific densities of the two liquids need to be different. Prepare a
concentrated solution of you compound in the solvent and have the precipitant handy. Transfer
a small volume of whichever liquid has the higher specific density in a narrow receptacle and
carefully layer it with the other liquid. This works best with a syringe and hypodermic needle.
Over time the two solvents will mix and, if you are lucky, crystals will form.
A variation of this method is to freeze the lower layer before you add the second liquid. That
makes it much easier to get a clean separation between the two layers.
Advantages: Works well with small amounts, parameters easy to control.
Disadvantages: A little complicated, finding two suitable solvents can be hard.

Sublimation

Sublimation should not be the method of choice to grow diffraction quality crystals.
Sublimation usually takes place at relatively high temperatures, which means that there is to a
lot of energy in the system when the crystals form. At high temperature the differences
between two similar molecule orientations can become insignificant which results in a twinned
or statically disordered crystal. In addition, crystals are usually growing too fast when they are
obtained by sublimation, which can also facilitate twining or disorder

Convection

Albeit somewhat exotic convection can be a good method to grow high quality crystals.
Generating a temperature gradient in the crystallization vessel by either cooling or heating part
of it leads to a slow and steady flow within the liquid phase. The idea is that more substance
dissolves in the hotter part of the container, travels to the colder region where it starts to
crystallize. The crystals move with the stream, trvelling to the hooter zone, where they totlly or
partially dissolve. The ones dissolving only partially will grow larger on their next trip from
warm to cold and back to warm. Several hundred rounds can make for a very nice diffraction
quality crystal. The velocity in the vessel is proportional to the heat gradiend, which should not
be too large, as too rapid convection will not leave enough time for nucleation.
Imitating David Watkin's construction (Watkin, D. J., J. Appl. Cryst.
(1972), 5, 250.), Chuck Barnes came up with the following idea:
"Cut off the tip of a Pasteur pipet about one cm above the beginning of
the narrowing part, and then heat seal the small end, you get a nice cheap
thermal gradient vial. Place a slurry of your material in a relatively poor
solvent in the vial and centrifuge to pack the undissolved material in the
tip. After centrifuging, you have a pellet of solid (~5mm) in the tip,
covered by clean "poor" solvent. Seal the vial with teflon tape and
parafilm. I make the heater from the small cylinder shaped ceramic
resistors which are usually available from the electronics shop. I found
some green ones of 100 ohms which are a good size. Place just the tip of
the vial with the solid pellet in the heater, with the vial at ~45 degrees
from vertical. Apply voltage to get maybe 50 C at the resistor, and you
have a nice thermal gradient up the vial. With luck, you get crystals growing up the vial. It
seems important to make sure al and you have a nice thermal gradient up the vial. With luck,
you get crystals growing up the vial. It seems important to make sure all the amorphous
material is packed down...no powder where you want the crystals to grow." Chuck adds about
this method, " This has given me excellent crystals at times, and has on occassion given
superior crystals (smaller mosaic spread) even when crystals were available from evaporation."

How to treat your crystals once you have them

First and foremost: Never, I mean NEVER remove the solvent! Frequently, solvent
molecules cocrystallize with your compound, which makes them integral parts of the crystal
lattice. Removing the mother liquor from the crystals exposes the crystals to air (or whatever
gas you have in your glove box) and the volatile solvent molecules slowly evaporate from the
crystal lattice, leaving empty holes. Very small holes reduce the maximum resolution the
crystal diffracts to, larger holes destroy the crystal.

It is always a good idea to not change the environmental conditions for your crystals too often.
Leave them alone, when you can.
 资料二

Crystallization belongs to the oldest techniques used in chemical laboratories.

Nevertheless crystallization is referred to as more of an art than a science. In many cases
crystallization is more difficult and less successful than other techniques like distillation or
chromatography. Additionally there is often a lack in education about crystallization techniques.
Many textbooks don't treat this topic.
As stated in many introductory chemistry textbooks, crystallization should start, if the
concentration of a solution is higher than the corresponding solubility product. In fact,
crystallization is kinetically hindered. This leads to the metastable condition of supersaturation:
The concentration is higher than the solubility product, but crystallization has not yet started.
This condition of supersaturation is the optimal condition for crystal growing. In fact it is the
only condition for crystal growing: If the concentration is higher, uncontrolled spontaneous
crystallization sets in; if the concentration is lower, already formed crystals will dissolve.
An often used process in the laboratory is therefore the cooling of the reaction mixture
until the state of supersaturation is achieved. But as stated before, crystallization is kinetically
hindered and will not start, even if you wait for month. What you need are crystal seeds. The
generation of these crystal seeds (nucleation) is the most difficult part of the whole
crystallization process.
You can try out one of the following techniques for nucleation:
1. Nucleation is in most cases an endothermic process. Evaporation of the solvent at
increased temperatures is therefore a better choice for getting crystal seeds than just cooling
down. If the first crystals are visible, the growing of the crystals is thereafter controlled by
lowering the temperature (crystal growing is exothermic in most cases).

N.B. Crystallization at higher temperatures can lead to other crystal forms compared to
lower temperatures. Sodium sulfate crystallizes as decahydrate when crystallized below 305.53
K and without hydrate water when crystallized above 305.53 K. Calcium carbonate prefers the
aragonite and vaterite modifications at higher temperatures and the calcite modification at
lower temperatures.
2. The solution is cooled down until (uncontrolled) spontaneous crystallization sets in.
The solution is then warmed up until nearly all of the crystals are dissolved again. The
remaining crystals are allowed to grow by controlled cooling.
N.B.: Unfortunately many chemist are proud of having crystals at all. They don't have the
courage to re-dissolve the crystals. They just bring the results of the uncontrolled spontaneous
crystallization to the crystallographer. The quality of these crystals is very poor and in many
cases also the results of the structure determination.
3. Ultrasound often helps to get crystal seeds. Ultrasound is used in cleaning baths, but
can also be generated by scratching at the glass of the crystallization vessel.
4. Hetreogeneous nucleation. Nucleation can be affected considerably by the presence of
mere traces of impurities in the system. Impurities may act as inhibitors or accelerators.
Golden rules
1. "Don't crystallize in the dark."
Many chemist put their crystallization vessels to a dark place, leave it there for hours, days or
month, and wait somewhere else. That's the wrong way. You have to watch the process!!! If the
first crystal seeds are visible, you must control the growing process (temperature, cooling rate).
If no crystal seeds are visible you have to change something.

2. "Don't crystallize too concentrated."

Crystallization from too concentrated solutions often leads to glass, syrup or other unpleasant
results. A good example are sugars which are very soluble in water. They therefore form high
concentrations and nearly never crystallize from water. Better choose a solvent (or mixture of
solvents), in which the solute is less soluble.

3. "Don't hurry."
If you need month or years for the synthesis of a compound, why should the crystallization be
finished in a few hours? You don't stop the synthesis after the first disappointing attempts, why
do you stop crystallization so early.

4. "Don't forget about details."

You write an excellent, detailed laboratory notebook about your synthesis. You mention boiling
and melting points, concentrations, colors, reaction times and so on. But you don't mention
your crystallization conditions at all or only with one sentence. The remark "Suitable crystals
were obtained by slow evaporation of the solvent" is to no help to the reader.

5. "Don't use glass vessels."

With evaporation the solvent creeps up the walls of a glass vessel, because the hydroxyl-groups
of the silicate glass make it hydrophilic. Nucleation occurs therefore outside the solution. Much
better are vessels made of polypropylen or teflon. The solvent doesn't creep up these vessels.
You can also make glass vessels hydrophobic by silanation.

6. "Use acceptors."
Many substances are difficult to crystallize because the system don't contain acceptors for
hydrogen bonds. The Cambridge Structural Database contains more than 350 substances
crystallized as picrates. In other cases triphenylphosphine oxide can be used alternatively (M.C.
Etter, P.W. Baures, J. Am. Chem. Soc. (1988) 110, 639-640).

Recommended books
People who want to get a deeper understandig of the matter are referred to the following
three books:
A. Holden, P. Morrison: Crystals and Crystal Growing,
MIT Press, Cambridge, Massachusetts (1982)
ISBN 0-262-58050-0 (pbk.), 13.95 US$
(Very basic introduction to the subject. Because of the nice price also suitable as a gift for
children of 14 years onwards.)
J.W. Mullin: Crystallization,
Butterworth-Heinemann, Oxford, Great Britain (1993)
ISBN 0-7506-1129-4, 85.00 ?
(Good overview. Thermodynamics of the nucleation and crystal growth processes. Industrial
applications of crystallization.)
K.-Th. Wilke, J. Bohm: Kristallz 點 htung,
Verlag Harri Deutsch, Thun; Frankfurt/Main (1988)
ISBN 3-87144-971-7, 220.00 DM
(Comprehensive standard work. Only available in German. Nearly all techniques mentioned.
List of compounds contains approximately 2000 substances.)
Further reading
Further Reading about Crystal Growing
C. Orvig: A Simple Method to Perform a Liquid Diffusion Crystallization,
J. Chem. Educ. (1985) 62, 84.
P. van der Sluis, A.M.F. Hezemans, J. Kroon: Crystallization of Low-Molecular-Weight
Organic Compounds for X-ray Crystallography,
J. Appl. Cryst. (1989) 22, 340-344.
P. van der Sluis, J. Kroon: Solvents and X-ray Crystallography,
J. Cryst. Growth (1989) 97, 645-656.
M.C. Etter, D.A. Jahn, B.S. Donahue, R.B. Johnson, C. Ojala: Growth and Characterization of
Small Molecule Organic Crystals,
J. Cryst. Growth (1986) 76, 645-655.
资料三
外部物理化学条件对晶体生长的影响

1.结晶温度越高，晶体越完善。
2.结晶溶液越稀越好。
3.有晶种结晶速度会加快。
4.较慢的冷却速度。要想得到完善的晶体当然条件还有很多，得到完善的单晶是很不容易的
就我所知，晶体长得越快就越小，缺陷越多。那么为了让晶体生长的慢而完整，就应该从溶剂的纯
度，温度，周围环境的干扰（防尘）等方面多考虑.

从低温（室温到 70℃左右）溶液中生长晶体是一种最古老的方法。由于近代科学技术发展的需
要，要求从低温溶液中培育出各种高完整性的大晶体.而决定晶体生长外形的因素，除了内部结构因
素外，还存在外部物理化学条件的影响。它不仅影响到晶体生长外形，而且有时还能影响到晶体内部
结构的变化。

过饱和度：从溶液中生长晶体的最关键的因素是控制溶液的过饱和度，晶体只有在过饱和溶液中
生长才能确保其质量。过饱和度大，一次形成的晶核多，晶体颗粒小。晶体在低饱和度下生长时，速
度较慢，适当增加过饱和度，晶体生长速度增快有利于晶体的自提取作用和晶形简化。要使晶粒长大，
可用籽晶进行再结晶或长时间放置，或利用蒸汽扩散法，使过饱和度缓慢地变化。

溶剂：选择恰当的溶剂是培养晶体的重要因素。2-氨基嘧啶易溶于水，己二酸稍溶于冷水，易溶于
乙醇。由实验结果证明，单纯使用乙醇或蒸馏水均无法培养出合适的单晶，较好的溶剂是乙醇-水溶
剂，且水的量应大于乙醇。

温度： 生长温度对晶体的习性和质量的影响可以认为是改变晶体生长各个过程的激活能。2-氨基嘧
啶易升华，己二酸能升华，因此实验温度不能过高，采用恒温蒸发法培养晶体较好。由于温度恒定，
晶体的应力较小，但很难控制蒸发量，故用此法较难长成大块的单晶体。温度的变化导致晶体各个晶
面相对生长速率的变化，从而改变晶体的生长外形。

杂质：杂质的影响包括晶体学、动力学和热力学等方面的效应，它可以影响溶解度和溶液的性质，
会显著地改变晶体的结晶习性，对晶体的质量也有影响。如果把溶剂看成杂质，也可以归结于杂质变
化的影响。随着杂质的种类及其含量的不同，影响也有差异。合成中的加入极少量水以改变溶解度和
溶液的黏度，有利于晶体生长；而实验中絮状晶体就有可能是由杂质所引起的。所以，实验过程中所
使用的药品和仪器均要保持洁净，溶液也不要暴露在空气中。

PH 值: 溶液中氢离子浓度对晶体生长的影响是较为显著的。如果溶液 PH 值调节不适合，即使是其

它生长条件合适，也长不出所需尺寸的好晶体。PH 值的影响相当复杂，包括通过影响溶解度,改变杂
质活性等间接或直接影响晶体生长。本实验培养晶体较适合的 PH 值是 6.5-7.0。

此外，还有液流等影响因素。
资料四

晶体的生长是一个动力学过程，由化合物的内因（分子间色散力偶极力及氢键）与外因（溶剂极
性、挥发或扩散速度及温度）决定。晶体的培养实质是一个饱和溶液的重结晶过程，使溶液慢慢饱和
的方法（如溶液挥发、不良溶剂的扩散及温度的降低）都可。有些化合物易结晶，经常有人将无机盐
晶体去检测的例子（无机盐易结晶）。

有以下两种方法较常用：
1) 挥发溶剂法：
将纯的化合物溶于适当溶剂或混和溶剂。(理想的溶剂是一个易挥发的良溶剂
和一个不易挥发的不良溶剂的混和物。)此溶液最好稀一些。用氮/氩鼓泡
除氧。容器可用橡胶塞(可缓慢透过溶剂)。为了让晶体长得致密，要挥发得
慢一些，溶剂挥发性大的可置入冰箱。大约要长个几天到几星期吧。
2) 扩散法：
在一个大容器内置入易挥发的不良溶剂(如戊烷、已烷)，其中加一个内管，置入
化合物的良溶剂溶液。将大容器密闭，也可放入冰箱。经易挥发溶剂向内管扩散
可得较好的晶体。时间可能比挥发法要长。
另外如果这一化合物是室温反应得到，且产物比较单一，溶解度较小，可将反应物溶液分两层放
置，不加搅拌，令其缓慢反应沉淀出晶体。容易结晶的东西放在那里自己就出单晶，不容易结晶的怎
么弄也是不出。好象不是想做就能做出来的。首先看一下产物的溶解度，将产物抽干后用良性溶剂溶
解成饱和溶液 （如用二氯甲烷），然后加入相同体积的不良性溶剂，若产物不稳定应在惰性气体的
保护下进行操作，完成后置于冰箱中冷冻至单晶析出，或直接用惰性气体鼓泡直至单晶析出。（应缓
慢）。

资料五

培养单晶指南

综述：
你将会发现，培养单晶不仅需要耐心，而且还需要一双灵巧的双手。结晶过程对温度和其它轻微
的扰动都非常敏感。因此，你应该在相似的条件下多尝试几个不同的实验温度，并为单晶的生长寻找
一个没有干扰的安静环境。这里有一些经验性的贴士供你参考，以利于你的实验开展。

方案＃1
● 有时好的单晶仅需冷却溶液即可生长。你也可以尝试加热溶液至所有物质完全溶解，达到过饱
和，再慢慢地使其冷却。

方案＃2
1）选取一种可以溶解你的目标化合物的溶剂，制成饱和溶液。
2）如果有必要，可以通过过滤除去其中的不溶性杂质。对于少量溶液，可使用一种有效的过滤
器，其制备方法是：将玻璃毛（甚至可以用面巾纸）塞入一根一次性Pasture滴管中，然后填入一英寸
左右助滤物（如硅藻土Celite）。用新鲜溶剂湿润硅藻土，然后用球形压力器将溶液压过该管进行过
滤。
3）寻找另一种溶剂，使目标化合物在其中不溶解（或仅微量溶解），而且这种溶剂能够和前一
种溶剂混溶，并具有较低的密度。
4）将第二种溶剂小心地铺在小瓶中饱和溶液的上面。在两相界面上可看到一些混浊物。单晶将
会沿着这个界面生长。

方案＃3

● 将盛有饱和溶液的小瓶放置在另外一个较大的瓶中。在外面的大瓶中加入第二种溶剂并且盖紧盖
子。第二种溶剂将会慢慢地扩散到饱和溶液中，晶体就会出现了！为了进一步减慢这个过程，可将这
个扩散装置放在冰箱中。

可以尝试的溶剂系统：
CH Cl ／乙醚或戊烷 THF/乙醚或戊烷
2 2

甲苯/乙醚或戊烷 水/甲醇

CHCl3/正庚烷