Growing Crystals for X-ray Diffraction Analysis

Overview

Source: Laboratory of Dr. Jimmy Franco - Merrimack College

X-ray crystallography is a method commonly used to determine the spatial arrangement of atoms in a crystalline solid, which allows for the determination of the three-dimensional shape of a molecule or complex. Determining the three-dimensional structure of a compound is of particular importance, since a compound's structure and function are intimately related. Information about a compound's structure is often used to explain its behavior or reactivity. This is one of the most useful techniques for solving the three-dimensional structure of a compound or complex, and in some cases it may be the only viable method for determining the structure. Growing X-ray quality crystals is the key component of X-ray crystallography. The size and quality of the crystal is often highly dependent on the composition of the compound being examined by X-ray crystallography. Typically compounds containing heavier atoms produce a greater diffraction pattern, thus require smaller crystals. Generally, single crystals with well-defined faces are optimal, and typically for organic compounds, the crystals need to be larger than those containing heavy atoms. Without viable crystals, X-ray crystallography is not feasible. Some molecules are inherently more crystalline than others, thus the difficulty of obtaining X-ray quality crystals can vary between compounds. The growth of X-ray crystals is similar to the process of recrystallization that is commonly used for purifying compounds, but with an emphasis on producing higher quality crystals. Often, higher quality crystals can be obtained by allowing the crystallization process to proceed slowly, which may occur over the course of day or months.

Procedure

1. Preparation of the Crystal Tube and Filter

Place an NMR tube in an Erlenmeyer flask.
Prepare a pipette filter.
1. Construct the filter by placing a piece of lint-free wipe (1 in. by 1 in.) in the pipette, then use a rod to firmly wedge the wipe into the bottleneck portion of the pipette (Figure 1).
2. Make two pipette filters for every crystal tube needed.

2. Adding the Sample to the Crystal Tube

Dissol

Results

The technique of liquid-liquid diffusion was used to create X-ray quality crystals of tetraphenylporphyrin. Using dichloromethane as the solvent and methanol as the anti-solvent, the liquids were allowed to slowly diffuse over the course of a week without being disturbed. Large, well-defined, dark purple-reddish crystals formed at the interface of the two solvents (Figure 3). The growth of the crystals can be visually observed. The crystals grew with very well defined fac

Application and Summary

X-ray quality crystals can be grown by liquid-liquid diffusion. The slow diffusion of the binary solvent system allows for the creation of crystals suitable for X-ray diffraction.This method allows the crystal lattice to form slowly, often leading to larger and more well defined crystals. The use of NMR tubes facilitates the slow diffusion of the solvents, allowing for optimal crystal growth. This process can take anywhere from a few days to several months. Often during the crystallization process solvent molecules are i

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