Chapter 1 Multiple Choice Questions

. Which of the following cannot be obtained from an X-ray crystallography study?

A bond angle Si-O-Si in a mineral.

The absolute configuration of a chiral natural product.

The degree of folding of a Zn₂Cl₂ four-membered ring.

The vibration frequency of a carbonyl group.

The spacing between two parallel aromatic rings.

. What is meant by the 'phase problem' in X-ray crystallography?

The sample must be in the crystalline solid phase.

The phase of an X-ray wave changes when it is scattered by an atom.

The relative phases of diffracted X-ray beams are lost when the diffraction pattern is recorded.

Non-centrosymmetric crystal structures always give centrosymmetric diffraction patterns.

. Which of the following statements about lattices and unit cells is correct?

Lattice points are chosen to lie on atoms.

Lattice points all have identical surroundings.

Lattices can be primitive or centred.

Unit cells are constructed by connecting adjacent lattice points to give the smallest possible repeat unit.

Different possible unit cell shapes define the seven crystal systems.

A unit cell contains a whole number of complete molecules.

. Consider the Bragg equation (1.3, page 20). If the value of the wavelength is doubled, which of the following is NOT true?

Bragg angles of reflections increase.

The d spacings become smaller.

The diffraction pattern expands.

Some previously accessible reflections can no longer be measured.

. Calculation of Z (the number of molecules in the unit cell of a crystal structure) gives a value of 5. Which of the following cannot be an explanation of this unusual result?

An incorrect chemical formula for the compound.

The presence of solvent of crystallization.

An error in determining the unit cell parameters.

An incorrect space group.

A wrongly measured crystal density.

. The calculated value of Z is half what is expected for the probable space group of a crystal structure. On which of the following symmetry elements could the molecules lie to satisfy this result?

Inversion centres.

Twofold screw axes.

Glide planes.

Fourfold rotation axes.

. Calculation of Z from a crystal density measurement gives a value of 4.41 for an expected molecular mass of 451.1 daltons. What is the likely solvent of crystallization?

Water H₂O

Methanol CH₃OH

Ethanol CH₃CH₂OH

Tetrahydrofuran C₄H₈O

Dichloromethane CH₂Cl₂

. The scattering contribution of one individual atom or ion to the total X-ray diffraction pattern of a crystal structure depends on all but one of the following properties of the atom/ion. Which is the property that has no effect?

The identity of the element.

The oxidation state.

The isotope.

The position of the atom/ion in the unit cell.

The vibrational motion of the atom/ion.

. For a centrosymmetric crystal structure, all diffracted X-ray beams have phases of either 0 or 180°; other phase values do not occur. What effect does this have on reverse Fourier transform calculations?

None.

Half the reflections make no contribution.

The different phase contributions can be ignored.

The sine terms multiplied by the imaginary number i are all zero, so complex exponentials become cosines.

The terms containing (hx + ky + lz) are all equal.

. Which of the following is not usually an advantage of synchrotron radiation compared with laboratory X-ray sources?

Intensity.

Speed of data collection.

Wavelength selection.

Finely focused X-ray beam.

Cost.

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