x-ray crystallography - diffraction Flashcards
(28 cards)
x-ray crystallography definition + function
a technique used to produce crystal structures of a molecule
useful for analysing structure + connectivity in the solid state
give 3 advantages of x-ray crystallography as an analytical technique
- non destructive method, structures are usually unchanged/undamaged by the x-ray, this means it is also forensically useful
- unambiguously determines complete structural connectivity, locating positions of atoms/ions and providing bond lengths + angles
- minimal sample prep required
give one requirement for x-ray crystallography to be used
sample must be crystalline - it is not appropriate for amorphous solids, gases, liquids or solution, or anything without a clearly defined shape/form
crystalline definition
ordered and repetitive structure in the solid state
how long does x-ray crystallography take + how does this limit the substances that can be analysed?
can take up to 8 hours
this means the sample must be stable - crystal structures normally are stable anyway, this technique cannot be used for short lived intermediates
what is the wavelength of an x-ray + why is this important?
short wavelengths, 100-300pm (or 1-3Å)
in between UV and gamma
this is important because the wavelength needs to be as small as the atoms/molecules being analysed, so when it interacts with them it is diffracted to the greatest effect
how was diffraction discovered + what did it prove?
max von laue directed x-rays at crystals and recorded the diffraction pattern
this proved that x-rays had wave properties and that the crystalline material was ordered
diffraction definition
the spreading of waves around obstacles when an EM wave hits an obstacle
how are diffraction patterns created?
when waves are diffracted, they are bent around corners/obstacles and build up - if they interact in-phase they combine to produce a large maxima, if they interact out-of-phase the waves will cancel out, if its in the middle they’ll produce a smaller maxima
these interactions create a series of light and dark regions, this is the diffraction pattern
what are dark regions caused by?
destructive interactions of waves
what are light regions caused by?
constructive interactions of waves
what particular kind of radiation source is needed for x-ray crystallography?
a monochromatic x-ray beam - meaning all the waves are the same wavelength, this can be obtained in the lab by filtering the rays
how much of the crystal sample is necessary to produce the diffraction pattern?
only a single crystal is needed, but crystal size needs to be approximately 0.1 x 0.1 x 0.1 mm
what is SCXRD?
single crystal x-ray diffraction - when just 1 crystal is used
how does an x-ray crystallographer work?
the crystal is mounted on a glass needle and held in place with oil
a single beam of x-ray is directed at the crystal
the x-rays are scattered by the atoms in the crystal structure, depending on the positions of the atoms and create a series of spots on the image plate = diffraction pattern
what data can be collected?
the unit cell can be collected in about 10 minutes, then the full data set can be collected, this will take a further 3-8 hours
why are glass and oil used to mount the crystal?
glass is a transparent amorphous solid, meaning it doesn’t disrupt the diffraction pattern
oil also doesn’t disrupt the diffraction pattern
what causes different diffraction intensities?
the electron clouds present in material - these are what diffract the x-rays
this means substances with the same/similar structures/formulae will still have different diffraction patterns if they contain different elements as their electron clouds are different
under what conditions are the diffractions collected and why?
under/in a cryostream using liquid nitrogen - this ensures the crystal is cool + doesn’t heat up too much with x-ray radiation, as well as ensuring atoms are cool and don’t vibrate as much
- this means the positions of electron density is sharper and diffraction data is better
how does degree of scattering depend on the atom?
degree of scattering in proportional to atomic number Z
how are crystal structures obtained from a diffraction pattern?
from the diffraction pattern the location of electron density in a crystal is reconstructed computationally
what does it mean if the reconstructed structure has round spherical atoms, vs longer not-spherical atoms?
round spherical atoms is a good sign that the assigned atom/element number is correct
if they are not spherical this suggests that something is not correct
give 4 limitations of x-ray crystallography
- since diffraction scattering is proportional to electron density concentration, it can be hard to locate H atoms as electron density = 1
- growing crystals for the experiment is difficult, should be grown slowly to get a regular arrangement
- if crystals are too small it needs to be analysed via PXRD, this limits information that can be extracted
what is PXRD?
powder x-ray diffraction
