cw EPR for system with high number of nucleus

[rebeda]

I want to simulate cw EPR spectrum for my molecule which is ferromagnetic in nature and it consist of 27 H-atoms, 19 C-atoms, 2 Cl atoms, 2 N-atoms, 1 Fe atom and 1 P atom (i.e, totally 52 nucleus). I am trying with the core function pepper since it is a powder sample. I am finding difficulties setting tensor values for Sys.A and Sys.AFrame . Please help me

regards,
Rebeda

[Matt Krzyaniak]

52 nuclei... that's quite a bit, though you are unlikely to actually resolve every nuclei present, especially in a powde rdue to inhomogeneous broadening (you'd have a chance in a single crystal). In addition you can generally disregard some species due to their natural abundances being low, such as carbon and the iron, the H and Cl are another two that you can generally ignore, often their hyperfine interactions are small enough that you don't actually resolve them in a powder. To get at their couplings you can use high resolution techniques such as ENDOR or ESEEM.
So now we are down to probably just 3 nuclei, the nitrogen and phosphorus.

Often when simulating cwEPR spectra of powder samples its easiest to start simply and build the system up from there. Identify what features you can and include them in your simulation, things such as zerofield splitting(if S>1/2), g-anisotropy, the strongly coupled nuclei.

[rebeda]

Dear Matt,

Thank you very much for your kind reply. It will help me a lot in my simulation. But my sample is a single molecular magnet and I cannot neglect Fe since it is the main component in my simulation. So can I take 1 Fe, 1 N and 1 P (i.e, totally three nuclei) instead? will you please tell me if my code has any errors??

clear all;
Sys.g = [2];
Sys.S = 1/2;
Sys.A = [[5 0 0; 0 5 0; 0 0 5]; [10 0 0; 0 10 0; 0 0 10];[15 0 0;0 15 0;0 0 15]];
Sys.AFrame = [0 0 0; 0 10 90; 12 -30 34]*pi/180; % one electron spin, three nuclei
Sys.Nucs ='1H,56Fe,31P';
Sys.lwpp = 110; % peak-to-peak linewidth, in MHz
Exp.Temperature=298;
Exp.CrystalOrientations=[]; % powder
Exp.mwFreq = 9.5;
Exp.CenterSweep = [330 700]; % in mT
Exp.Range = [0 800]; % in mT
pepper(Sys,Exp);
Opt.Verbosity=2;
[Field,Spec] = pepper(Sys,Exp,Opt); % additional simulation options in Opt
axis([0,800,-12048,2047]);
plot(Field,Spec);

Regards,
Rebeda

[Sam Greer]

Of the nuclei in your system the only one you really have any chance of seeing is phosphorous. The vast majority of iron has a nuclear spin of zero meaning it will have no hyperfine. The best thing for you try is to first simulate the spectrum using only electronic components of the hamiltonian i.e. g and D. It is very unlikely that you will observe anything coming from the nuclei. So I suggest using the following code:

Code: Select all

clear all;
Sys.g = [2];
Sys.S = 1/2;
Sys.lwpp = 10; % peak-to-peak linewidth, in mT!
Exp.Temperature=298;
Exp.mwFreq = 9.5;
Exp.Range = [0 800]; % in mT
pepper(Sys,Exp);
[Field,Spec] = pepper(Sys,Exp); % additional simulation options in Opt
plot(Field,Spec);

Notice the units of lwpp are mT not MHz

Since you are looking at a single molecule magnet I assume your spin is actually larger than 1/2 in which case you may want to include:

Code: Select all

Sys.D=[D E]; %in MHz

where D is the axial component of zero field splitting and E is the rhombic component. However it is entirely possible that your spectrum will be simulated very well with an effective S=1/2 model.

[Matt Krzyaniak]

Well when I said to ignore the Fe, what I meant was ignore the Hyperfine interaction unless you're enriched for 57Fe, that doesn't mean to ignore unpaired spin density on the iron which will likely result in g-anisotropy and depending on crystal field of the iron and its redox state you could have S>1/2 and zerofield splitting.

With respect to what you included in Sys.A and Sys.AFrame, you provided a purely isotropic interaction for the nuclei, in that case AFrame is unnecessary, you'd need hyperfine anisotropy for Aframe to mean anything.

[rebeda]

Thank you very much for your reply. It will be of great help for me. I have attached png file of the EPR spectra I got for my system experimentally. Will you suggest any other parameters for tuning the amplitude and width so that it will match with the spectra obtained experimentally??