X Ray Crystallography Flashcards
What do crystalline materials possess
Long range order - atoms of molecules arranged in the same way
If unit cell has all lengths the same and all angles the same (90degree) what type is it?
Orthorhombic
Define Z and Z’
Z- total number of molecules in unit cell
Z’ is the total number of unique molecules int he unit cell (as a result of symmetry)
What is space group
Used to describe symmetry in a crystal
Atoms with lots of electrons wil scatter strong or weak?
Strong
Define Bragg’s law
A reflection from a crystal is only seen if Bragg’s law is satisfied - wavelength = 2d sin (0)
Where wavelength is equal to twice the distance between the planes it’s reflecting off
When a crystal scatters x rays, the patterns we see are the result of what?
Atoms in the crystal (not the planes) - diffraction occurs
What does the strength of the scattered x Ray reflect?
The strength of a scattered x Ray reflects the distribution of atoms and the TYPES OF ATOMS
What is single crystal diffraction used for
For full definitive crystal structure determination
Must grow crystal
Why is XRD used to determine chirality?
We often require drugs to be formulated as 1 enantiomer so it tells you whether a drug is racemic or not eg
If it crystallises in a space group with a centre of symmetry of mirror plane we know its a race mate as symmetry accounts for > 1 molecule
When is powder XRD used ?
When you can’t grow a single crystal of API - use powders which hare millions of tiny crystals
And single crystals aren’t always representative of the bulk
How can PXRD identify polymorph - which one occurs?
Because crystals of the same chemical compound they will have different crystal structures and properties - they differ in their unit cell arrangement of atoms so:
Peak positions will be different due to different unit cells
Peak intensities will be different due to different atomic positions
What change in PXRD is detected when there is a crystalline-crystalline phase transformation?
Change in reflection positions and intensities due to inter conversion
What would an amorphous material show on PXRD
Only short range order - no sharp peaks
No clear Bragg’s diffraction
Amorphous hump
What do peak positions tell us in PXRD
Unit cell dimensions
