X-Ray Diffraction [Shankland]

Question 1

List 4 bits of information which can be gained from X-ray diffraction

Answer 1

Where atoms are
How they are connected to form molecules
How they pack to form crystals
To what extent they are vibrating

Question 2

How does x-ray diffraction work?

Answer 2

Involves shining x-rays onto crystalline materials and observing scattered radiation

Question 3

Name 3 pieces of information which are required to describe a crystal

Answer 3

Unit cell type and dimensions
Atomic co-ordinates
Space group symmetry

Question 4

What are the dimensions of a unit cell?

Answer 4

Three lengths (a, b, c)
Three angles (A, B, Y)
Angles are in degrees

Question 5

Name the 2 most common unit cell types

Answer 5

Orthorhombic = different lengths but all angles are 90c
Monoclinic = different lengths, A&Y are 90c, B is not 90c

Question 6

How are the lengths of a unit cell given?

Answer 6

Fractional co-ordinates as given in fractions of a cell edge

e.g. 0.33, 0.5, 0.9

Question 7

What is in a unit cell?

Answer 7

Often more than 1 molecule
The molecules are not independent but are related by symmetry elements
So the position of every molecule in the unit cell does not need to be known - only 1 molecule needs to be known as long as the symmetry relationships are known

Question 8

What does Z stand for?

Answer 8

The total number of molecules in the unit cells

Question 9

What does Z’ stand for?

Answer 9

The number of unique molecules in the unit cell

Z/(number of symmetry elements + 1)

Question 10

What is the space group used to describe?

Answer 10

The symmetry in a crystal

Question 11

Why do atoms differ in the amount of x-rays they scatter?

Answer 11

X-rays scatter off of electrons
Atoms with lots of electrons scatter strongly and so can be seen easily
Atoms with few electrons scatter little (e.g. H) and so are much more difficult to see

Question 12

How can diffraction be explained?

Answer 12

X-ray diffraction is most easily visualised as reflection from a plane in a unit cell

Question 13

What is the formula of Bragg’s Law?

Answer 13

FORMULA

Question 14

What is the link between Bragg’s law and diffraction/reflection?

Answer 14

A reflection is only seen when Bragg’s law is satisfied

Therefore, if a reflection is seen - the angle can be measured - so the unit cell dimensions can be calculated

Question 15

Define the terms of Bragg’s law

Answer 15

y is the wavelength of the radiation
d is the interplanar spacing
theta is half the observed diffraction angle (c)

Question 16

What does the strength of scattered x-ray suggest?

Answer 16

Strength of x-ray will reflect the distribution of atoms in the cell and their types
e.g. if a plane has lots of atoms with lots of electrons, it is likely to scatter strongly

Question 17

How can single crystal x-ray diffraction be used to identify a racemate or single enantiomer?

Answer 17

If the x-rays show that a drug crystallises in a space group with a centre of symmetry or a mirror plane
This means it must be racemic as the mirror or centre relates the left handed molecules to the right handed ones

Question 18

When is powder x-ray diffraction used?

Answer 18

When a single crystal of API can’t be grown

Bulk powders are often dealt with and single crystals are not always representative of the bulk

Question 19

List 2 applications of powder x-ray diffraction (PXRD)

Answer 19

Polymorph identification - comparing fingerprints (spectra)

Phase transformation - polymorphs frequently interconvert (e.g. to a more stable form, temperature)