Chapter 2 Multiple Choice Questions

. Which of the following is least often true for a good-quality single crystal of a material with a fully ordered structure?

All unit cells have the same contents.

All unit cells have approximately the same orientation.

Crystal faces are parallel to unit cell faces.

Sharp optical extinction is observed every 90 with a crossed-polars microscope.

. Which of the following is least likely to improve the precision of a set of diffraction data and the resulting crystal structure determination?

Measuring symmetry-equivalent reflections rather than only the unique set of data.

Increasing the maximum Bragg angle for data collection.

Collecting data at low temperature.

Reducing the total X-ray exposure time.

. Absorption reduces the intensity of diffracted X-rays by a factor e^-^m^t^, where m is the absorption coefficient for the crystal, measured in mm^-¹, and t is the path length of X-rays through the crystal in mm. While an accurate calculation for an absorption correction involves integrating over the crystal shape, an estimate of the impact of absorption can be obtained by finding the ratio of the exponential factors for the smallest and largest crystal dimensions. What is this ratio for a crystal with minimum and maximum dimensions of 0.05 and 0.5 mm and an absorption coefficient of 2 mm^-¹?

1.0

1.6

2.5

10.0

. For which of the following compounds would a Patterson synthesis probably be least useful in attempts to solve the crystal structure?

A substituted coronene (six benzene rings fused together about a seventh central ring).

A mixed indium-tin iodide.

The barium salt of a carboxylic acid.

A brominated natural product.

. Which is the correct method to use to solve the structure of a high-symmetry carbonyl cluster complex of osmium for which preliminary unit cell calculations indicate that molecules lie on special positions at the intersection of various symmetry elements?

Patterson synthesis.

Direct methods.

Symmetry arguments.

Charge flipping.

None of these.

. Which of the following is not a standard use of difference Fourier maps in crystal structure determination?

Indicating wrongly assigned atom types.

Indicating atoms in slightly wrong positions.

Indicating wrongly assigned bond types.

Finding solvent molecules.

Finding hydrogen atoms.

. Why do all the F, N, and C atoms not show up clearly in the first electron density map calculated from the model structure containing just As atoms in Fig. 2.17(a)?

The As atoms are not in exactly the correct positions.

F_c from the As atoms do not agree well with F_o from the experiment.

The R factor is too high at this stage.

The phase angles calculated from the As atoms are not close enough to their correct values.

Too many of the calculated phases are exactly 180 wrong.

. Refinement of this ionic nitrogen-sulfur chain structure with anisotropic displacement parameters for all non-H atoms, and ignoring H atoms, involves about 150 refined parameters. Why is this considerably lower than 9 times the number of atoms (24) in the asymmetric unit?

The cation has two perpendicular mirror planes and a twofold rotation axis, reducing the number of independent parameters.

Some atoms have the same parameters as others.

The anion has octahedral symmetry.

Some parameters have fixed values because of crystallographic mirror symmetry and so are not refined.

. For three crystal structures of chiral compounds (A, B, and C), the refined absolute structure parameters are 0.02, 0.2, and 0.47, with standard uncertainties of 0.03, 0.5, and 0.05, respectively. What can be deduced about the absolute configuration in each case?

A and B are determined, C is unknown.

A is determined, B and C are unknown.

A is determined, B is unknown, C is a racemate.

A and C are determined, B is unknown.

All are determined.

All are unknown.

. Which of the following information is least likely to be available for a crystal structure from its entry in the relevant database?

The absolute configuration.

The pattern of hydrogen bonding.

The distance between two parallel aromatic rings in different molecules.

The identity and amount of solvent present.

Alternative orientations of a disordered substituent.

The starting materials from which the compound was made.

The journal in which the structure has been published.

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