Chapter 3 Multiple Choice Questions

. Why is a difference electron density peak found at the position of the Hg atom of case study 1 in Fig. 3.2?

The Hg atom is not in exactly the correct position.

The Hg atom does not have the correct displacement parameters.

The Hg atom has the wrong number of electrons.

The bonding electron density has not been included in the model.

. H atoms are particularly difficult to find, usually, in the presence of such a heavy atom as Hg. Nevertheless, inclusion of all the H atoms in the refinement for case study 1 is straightforward and unambiguous. Why is this?

The H atoms are bonded to C atoms, not to heavier atoms.

There are almost twice as many H atoms as all the other atoms put together.

The positions of the H atoms are completely predictable from the stereochemistry of the hydrocarbon substituents.

. If a different column (or one of the rows) of Table 3.2 were used to calculate the position of the Rh atom of case study 2, what would be the result?

Exactly the same position for Rh.

The same coordinates for Rh, but with one or more signs changed.

A symmetry-equivalent position for Rh.

A position for Rh corresponding to the choice of a different unit cell origin.

One of the above, depending on the choice of row or column.

. On what basis are H and D isotopes assigned in case study 2?

Information from the chemical synthesis and crystallization.

Crystal density.

Relative heights of difference electron density peaks.

C-H/D bond lengths.

H/D displacement parameters.

. Larger crystals of tetracycline hydrochloride (case study 3) might have been obtained by recrystallization, making the use of synchrotron radiation unnecessary. Why was this not done?

No suitable solvent was available.

The compound could have reacted with the solvent.

A different polymorph or solvate might have been formed.

The sample was too valuable to lose any in recrystallization.

. What is the main reason for freely refining H atoms located in a difference map for case study 3 rather than calculating their positions and refining with geometrical constraints?

There are plenty of data available for a large excess over the number of parameters even with free refinement.

Calculation and constrained refinement assumes particular geometry that may not be correct.

It is not possible to calculate positions for some of the H atoms because OH groups can rotate freely about the O-C bonds.

. How would you expect the structure of case study 4 to be solved?

Patterson map to find the unique iodine atom.

Direct methods.

Symmetry arguments.

. If the disordered DMF solvent is ignored in the structure of case study 4, instead of being dealt with by Fourier transform contribution method described, what effect will this have on the rest of the final structure?

No effect.

The molecular geometry will be incorrect (worse accuracy).

The standard uncertainties of geometrical parameters will be higher (worse precision).

. Which of the following are not usually different for two polymorphs of a compound?

The chemical formulae.

Physical properties such as melting point.

Torsion angles.

Intermolecular interactions.

. One way in which two polymorphs of a compound may differ is in the number of molecules in the asymmetric unit (Z). If one polymorph has Z = 1 and the other has Z = 2, which of the following will not be (exactly or approximately) twice as large for the second polymorph?

The number of refined parameters.

The volume of the unit cell.

The number of unique reflections measured to the same maximum Bragg angle.

The number of crystallographically independent bond lengths.

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