EPR simulation with two spins (radical pair)

[Abbey Philip]

Dear all,
I'm trying to simulate the EPR spectrum for a radical pair system using garlic. I have tried to match the simulated spectrum as close to the experimental data but the result still show deviations from the experimental one. How can I improve the simulation furthermore, so that it exactly matches the experimental spectrum? The code is as below and the experimental data in ascii as well as raw data (JEOL format) is attached with the query.

Code: Select all

clear

SysA.S = 1/2;
SysA.g = 2.012;
SysA.Nucs = '1H,1H,1H,1H,1H';
SysA.A = mt2mhz([15.95,15.95,14.35,14.35,14.1]/10);
SysA.lwpp = 1.33;

SysB.S = 1/2;
SysB.g = 2.012;
SysB.Nucs = '1H,1H,1H,1H,1H';
SysB.A = mt2mhz([15.95,15.95,14.35,14.35,14.1]/10);
SysB.lwpp = 1.33;
  
%Experimental details

Exp.mwFreq = 9.137;
Exp.Range = [310.569 339.9945];
Exp.nPoints = 64293;

garlic({SysA SysB},Exp);
hold on;

%Loading experimental spectrum.
data=textread('PA_exp_norm B.dat');
x=data(:,1);
y=data(:,2);
plot(x,y,'k');

hold off;

Please help.

Thanks & regards,
Abbey

[Matt Krzyaniak]

I'll do my best to help, though I'll need a bit more information.
You've got a radical pair, I assume it's either photo-induced or chemically generated. In which case do you expect it to have J coupling? What about spin polarization?
In both of these cases garlic isn't quite the program to be using as it doesn't handle spin-spin coupling, and in general ES doesn't handle polarization such as you might have in a singlet born SCRP very easily.

Did you collect your spectrum with field modulation or are we looking at the absorption spectrum? did you collect the spectrum in quadrature?

And finally, you have a pair radicals, what do expect they are?
As you have it written right both radicals are the same.

[Abbey Philip]

Dear Matt,
Thanks for the reply.

Yes the radical are generated by photoexcitation. Well the CW-EPR was recorded on a basic EPR setup from JEOL (JES - FA200 ESR Spectrometer) at X-band (9.137 MHz) with illumination from a xenon lamp source (not a selective excitation or a time-resolved experiment).

The radical are from a donor-acceptor pair, so it is a cation and anion radical pair (as characterized by transient absorption studies).

I guess the experiment was run with field modulation. The experimental parameters during acquisition are there in the attached image file. I'm not sure whether quadrature was used during the acquisition.

Thanks & regards,
Abbey

[Matt Krzyaniak]

Ok you're working under experimental conditions where I'm having a hard time thinking about the best way to simulate the spectrum.

I mostly deal with transiently generated radical-pairs that start in a spin polarized state. We direct detect in quadrature without field modulation and obtain an absorption/dispersion spectrum.

Based on your charge separation lifetimes, do you think your system has had a chance to reach thermal equilibrium(1/T1>>kcr) or are you observing a spin polarized system?
In the former case your spectrum should look "normal" in that you have a pair of coupled radicals, and you should be using pepper with some spin-spin(j) coupling. In the latter case, the spectral simulation isn't as straightforward and I'm not sure what the best way to simulate it would be.