Chapter 4 Multiple Choice Questions

. If atoms of the following pairs of elements need to be clearly distinguished in a crystal structure, for which pair does neutron diffraction offer the greatest advantage compared with X-ray diffraction?

C and O

C and N

N and Cl (natural isotope ratio)

W and Re

C and U

. A nuclear reactor generates neutrons continuously, but in a spallation source they appear in regular rapid short pulses, so measuring the time taken for neutrons to reach a detector provides a way of recording the velocity and hence the wavelength of every neutron. Why does this make a spallation source more suitable than a nuclear reactor for measuring neutron diffraction patterns with a broad range of wavelengths instead of using a monochromator to select a single wavelength?

Not selecting a single wavelength means the intensities are higher, so experiments can be performed more quickly.

Using all the wavelengths makes the diffraction pattern simpler.

The diffraction for one particular wavelength can be extracted from the total pattern, avoiding the cost of a monochromator.

. For which of the following materials would neutron diffraction offer the least advantage over X-ray diffraction?

An ice inclusion complex in which atoms of Xe are contained in pores in a framework of water molecules.

An iridium complex of a previously uncharacterized borane (boron hydride) cluster.

A 1:1 co-crystal of the fullerene C₆₀ with a fused polyaromatic hydrocarbon.

A mixed indium-tin iodide.

. If the individual microcrystal grains of a powder are very thin needles rather than approximately equidimensional, which of the following problems is likely to affect X-ray powder diffraction from this sample?

Absorption effects will be very different in different directions.

The needles will tend to be aligned with their long axes approximately parallel.

The needles will not pack together well.

No diffraction will be observed from some of the lattice planes.

. If it is not possible to assign indices to all the strong reflections found in a powder diffraction pattern of a compound for which chemical analysis gives a good fit to the expected chemical formula, and to obtain a unit cell, which of the following is the most likely explanation?

The structure is too large and complex.

The sample is not a single polymorph.

The sample is twinned.

The sample is not crystalline.

. Which of the following objectives cannot be achieved unless indices can be assigned to the lines in a powder diffraction pattern?

Identification of a single pure crystalline material.

Qualitative analysis of a mixture of crystalline materials (identification of the compounds present).

Quantitative analysis of a mixture of crystalline materials (relative amounts of the compounds present).

Determination of the crystal structure.

Measurement of the temperature at which a solid-state phase transition takes place.

. Which of the following is not a standard feature of protein crystals compared with 'small-molecule' crystals?

Less well-shaped crystals.

Larger unit cells.

Higher sensitivity to X-ray radiation damage.

Lower diffraction intensities.

More disorder.

. Which of the following features of synchrotron radiation has the least beneficial effect on macromolecular crystallography compared with a conventional laboratory X-ray source?

High X-ray intensity.

Finely focused incident X-ray beam.

Tuneable X-ray wavelengths.

Rapid pulse structure of the X-ray beam.

. Which of the following symmetry elements cannot occur in the space group of the crystal structure of a naturally occurring protein?

Unit cell centring.

A 4 axis (Schoenflies S₄).

A 4 axis (Schoenflies C₄).

A 4₁ axis.

A 4₂ axis.

. Which of the following presents the least obstacle to the development of reliable and general crystal structure prediction routines?

The conformational flexibility of many molecules.

The possibility of crystal structures with several molecules in the asymmetric unit.

Polymorphism.

Disorder in crystal structures.

Accurate calculation of lattice energies.

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