XRD values Flashcards
Formula
Simple - the molecular formula of the unit cell
Dcalc/ g cm-3
calculated density of unit cell
mass/v
Mu/mm-1
The absorption coefficient
When X-rays with intensity I0 penetrate a uniform substance,
the intensity I after transmission through distance x is given by.
I = IO e^-xmu
mu = linear absorbtion coefficient
-increases with more atomic number, increases with wavelength
-dependant on wavelength and density of substance
Formula Weight
combined molecular weight of everything in unit cell
Size/mm3
size of crystal - 0.1-0.5 mm
T/K
temperature of crystal - acheived by liquid N2
Crystal System
“sest of point groups”
crystal system of the unit cell
there are 7
eg triclininc, monoclinic etc)
Space group
-A space group is the symmetry group of the unit cell.
=230 of them
-combination of the different symmetry operations results in a total of 230 different space groups describing all possible crystal symmetries.
a/b/c
size and shape of unit cell
length
alpha/beta/gamma
size and shape of unit cell
angles
V/ A3
volume of the unit cell
Z’
the number of molecules in the asymmetric unit
Wavelength/ A
Wavelength of X-ray radiation used given.
The wavelength refers to the radiation used
to measure intensities (i.e. mean K rather t
Radiation type
For Copper the characteristic
wavelengths (λ) are:
* Cu Kα1 = 1.540Å
* Cu Kα2 = 1.544Å
* Cu Kb = 1.392Å
- For Molybdenum they are:
- Mo Kα1 = 0.70932Å
- Mo Kα2 = 0.71354Å
- Mo Kb = 0.63225Å
- We use MoKα (avg.) radiation
- (λ) = 0.71073Å
- Or CuKα (avg.)
- (λ) = 1.54178Å
theta min
lowest angle reflections - high intensity
theta max
highest angle - reflections - low intensity copper source, better
Measured reflections
Total number of reflections measured
Indepentant reflections
Number of independent reflections measured (not repeated, not
related by symmetry)
Refl’s I≥2 sigma(I)
Intensity/error - signal to noise ration - big is better more signal to noise
Rint
Parameters
parameters (coordinates, displacement parameters, scale factor) in the model.
Restraints
constraints - softer
restraints - stricture
RIGU
SAME
DELU
ISOR
even HFIX is a restrainy
Largest Peak
feature of the difference map, and are useful for deciding how to further improve the model
Deepest Hole
feature of the difference map, and are useful for deciding how to further improve the model