Question 1

What makes a crystal suitable for X-ray diffraction analysis?

Accepted Answer

X-ray-quality crystals form symmetric shapes with smooth, light-reflecting faces. Under a polarizing microscope, they appear transparent but turn dark when rotated 90 degrees, indicating highly ordered molecular arrangement. This ordered structure is essential because crystal quality directly influences the accuracy of structural determination in X-ray crystallography.

Question 2

Why does slow crystal growth produce better results for X-ray analysis?

Accepted Answer

Rapid crystal formation causes molecular disorder, incorporates impurities, or creates fused crystals instead of single crystals. Slow growth allows molecules to arrange orderly and prevents contamination. A smaller solvent interface during liquid-liquid diffusion reduces the diffusion rate, yielding larger, purer crystals suitable for X-ray crystallography.

Question 3

How does liquid-liquid diffusion work in crystal growth?

Accepted Answer

Liquid-liquid diffusion uses two miscible solvents: a low-density precipitant where the compound is insoluble, and a high-density solvent where it dissolves. The precipitant is layered onto the concentrated compound solution. As the precipitant gradually mixes with the solution, the compound becomes less soluble, causing slow, controlled crystallization that produces high-quality crystals.

Question 4

What is the role of the pipette filter in the crystal growth procedure?

Accepted Answer

The pipette filter removes solid impurities from the compound solution before crystallization begins. A small piece of Kimwipe is placed in the glass pipette and pressed to the bottom. The sample solution passes through the filter by gentle bulb pressure, ensuring only pure dissolved compound enters the NMR tube for clean crystal formation.

Question 5

How does X-ray crystallography determine molecular structure?

Accepted Answer

When X-rays hit a crystal with ordered atoms, electrons scatter the rays, producing constructive interference and a characteristic diffraction pattern. The crystal is rotated to collect patterns from multiple angles. With sufficient diffraction data, the three-dimensional spatial arrangement of atoms can be derived, revealing the molecule's complete structure and composition.

Question 6

What alternative methods can be used to grow X-ray-quality crystals?

Accepted Answer

Besides liquid-liquid diffusion, recrystallization methods include vapor diffusion, slow evaporation, and heating-cooling cycles. In slow evaporation, the compound dissolves in solvent in a container with a small hole; as solvent evaporates, concentration increases until crystallization occurs. Protein crystallization often requires specialized techniques like purifying compounds by recrystallization under mineral oil.

Question 7

Why is single-crystal X-ray diffraction preferred over powder diffraction?

Accepted Answer

Single-crystal X-ray diffraction preserves three-dimensional structural information because each crystal orientation is distinct. Powder diffraction loses this spatial data since all possible orientations exist simultaneously in the sample. While powder diffraction excels at analyzing crystalline mixtures and assessing amorphous structure crystallinity, single-crystal methods provide superior structural detail for determining molecular shape.

Préparation de cristaux pour analyse par diffraction des rayons X - Quiz d'évaluation