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Dear EasySpin Community,

I am new to the simulation of EPR spectra using EasySpin. While I have understood the basics of simulation and fitting, I am facing challenges with fitting the EPR spectra of metal-radical complexes. Specifically, I encounter difficulties due to limited resources for measuring EPR spectra at low temperatures. For fast-motion EPR spectra, I use garlic, but the fewer peaks make it hard to understand the hyperfine coupling of the radical electron with nitrogen atoms in the ligand and the metal.

I've tried using pepper and chili, but I am unsure about the correctness of my code. For example, here is my code for simulating the EPR spectra of Mo(V) bonded with two chloride ligands and a bidentate radical (with two nitrogen atoms).

clear, clc, clf; % Clear workspace, command window, and figures

% Load experimental data
[B, spc] = textread('Mo_2.txt', '%f %f'); % Read magnetic field (B) and intensity (spc) data
%figure; plot(B, spc); % Plot the experimental EPR spectrum
xlabel('Magnetic Field (mT)');
ylabel('Intensity');
title('Experimental EPR Spectrum');

% Define the EPR system parameters
Sys.S = [1/2 1/2];        % Two electron spins: Mo(V) and radical
Sys.g = [1.96, 1.965, 1.973; 1.963 1.964 1.98]; % Slightly rhombic g-values for both spins
Sys.Nucs = 'Mo, 14N,14N';  % Nuclei with hyperfine interactions
Opt.PostConvNucs = [1 2]   % Natural abundances of isotopes
Sys.A = [120, 120;2,20;2,20]; % Initial hyperfine couplings (MHz)
Sys.lwpp = [0.35, 0.08];   % Linewidth (mT) - [Gaussian, Lorentzian]
Sys.dip = 0.00025 * 30000;    % Exchange coupling (MHz) between Mo(V) and radical
Sys.tcorr = 1e-09;

% Define spectrometer settings
Exp.mwFreq = 9.448;      % Microwave frequency (GHz)
Exp.nPoints = length(B);  % Reduced number of data points in the spectrum
Exp.Range = [328, 360]; % Narrowed magnetic field range (mT)

% Simulation using chili (for slow-motion systems)
[B_sim, spc_sim] = chili(Sys, Exp, Opt); % Simulate the EPR spectrum

% Plot simulated vs. experimental
figure; 
plot(B, spc/max(spc), 'b', B_sim, spc_sim/max(spc_sim), 'r'); % Blue for experimental, red for simulated
legend('Experimental', 'Simulated');
xlabel('Magnetic Field (mT)');
ylabel('Intensity');
title('EPR Spectrum Comparison');

I have attached the experimental spectra for reference.

Experimental EPR Spectrum

I would appreciate any guidance on how to accurately define the spin system and improve the simulation and fitting process. Thank you in advance for your help.

Best regards,
Sunil Kumar

Dear community,

Kindly suggest me about this epr simulation.

Best Regards,
Sunil Kumar

If you have a structure of your complex, it might be worthwhile to perform a DFT calculation to obtain reasonable g and A values that you can use as starting values for a fit.

Are you sure the g values for your radical are <2? Typically, they are above 2.0023.

Your dipolar coupling appears quite small - it might be completely averaged out at the t ns correlation time scale you are using.

If you are looking at a ligand radical directly bound to Mo(V), consider the possible presence of an exchange coupling.

Thank you Prof. for your suggestions.

I will work on your suggestions.

Best Regards,
Sunil Kumar