Jan 31 Flashcards
What does mosaicity do do peak collected for the intensity of a diffraction pattern point/
More mosaicity is slight variations in the unit cell angles, they aren’t all perfectly aligned, so get wider distribution of angles, so wider peak. If it is low mosaicity, get tall and narrow peak.
What is electron density a function of for FT? How do we modify electron density eqn after?
It is a function of x,y,z, and does integrals for the structure factor (vector) of h,k,l. We then modify it as F (h,k,l) is not a continous function, so only has values when h,k,l are integers like 0,1,2,… So we use summations of all the Fhkl values
What do we measure for each hkl value? How related to structure factor?
We get the intensities directly, then the square root of intensity is proportional to the abs value of the amplitude of the Fhkl structure factor term for that DP point.
How is the frequency of Fhkl related to the interplanar spacing?
It is inverse, so Fhkl=1/dhkl
On XRD, how larger angle of reflection correlated to frequency of structure factor and interplanar spacing?
A higher frequency fourier term will give higher angle of reflection so smaller interplanar spacing
What would happen if add F00 term to summation of structure factors to give electron density?
F00 had intensity of 1, so everything shifted by 1, so would look like everything is electron dense, so would need to shift parameters so focus on values 1 unit higher
What is fj?
Each DP point from structure factor that has contributions from each atom. Electrons in molecules are centered around atoms, so look at electron density in terms of the atoms. fj is the atomic scattering factor for atom j. For atom j, it would have fractional coordinates xj,yj, and zj
What is the atomic scattering factor, what are fractional coordinates?
Atomic scattering factor is fj. The fractional coordinates for atom j are xj, yj, zj
What are rings on XRD pattern?
They are from ice. Ice has smaller unit cells, so higher angle of reflection. As ice crystals are in all orientations, you get many points of diffraction that make a ring on XRD pattern.
As sin theta/wavelength increases on x axis, what happens to atomic scattering factors on y axis? Why?
Atoms have electrons centered around them but are spread out, not in a certain point. So as resolution increases scattering from electrons in same atom cancel out, so get smaller scattering factor. So at sin theat/lambda =0. get atomic scattering factor (f0) at the number for those atoms (oxygen is 8 for 8 electrons), then as sin theta/lambda increases, f0 decreases.
What are temperature factors?
They are because atoms not stationary, they move around a point. So electrons are moving around atom that is moving, so lots of electron spread reduces scattering even more. Assume motion is isotropic (same in all directions), so need only 1 variable for temperature factor.
