Simulation of EPR spectra of Mo(V)-radical system.

[Sunil Kumar]

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. Additionally, my complex has three radical-ligands attached to an Fe(II) metal center. The spectral peaks seem to correspond to a single radical ligand and the metal, which complicates defining the correct spin system.

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):

Code: Select all

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, 1.961, 1.962; 1.961 1.962 1.963]; % Slightly rhombic g-values for both spins
Sys.Nucs = '95Mo,97Mo,14N,14N,35Cl,37Cl';  % Nuclei with hyperfine interactions
Sys.n = [0.26, 0.26, 1, 1, 0.7577, 0.2423];   % Natural abundances of isotopes
Sys.A = [30, 30, 30; 2, 2, 2; 2, 2, 2; 5, 5, 5; 5, 5, 5]; % Initial hyperfine couplings (MHz)
Sys.lwpp = [0.01, 0.1];   % Linewidth (mT) - [Gaussian, Lorentzian]
Sys.J = 0.5 * 30000;    % Exchange coupling (MHz) between Mo(V) and radical
Sys.tcorr = 1e-9;       % Rotational correlation time (s) for slow-motion simulation

% Define spectrometer settings
Exp.mwFreq = 9.448;      % Microwave frequency (GHz)
Exp.nPoints = length(B);  % Number of data points in the spectrum
Exp.Range = [min(B), max(B)]; % Magnetic field range (mT)

% Simulation using chili (for slow-motion systems)
[B_sim, spc_sim] = chili(Sys, Exp)[img]; % Simulate the EPR spectrum
[/img]
% Plot simulated vs. experimental
figure; 
plot(B, spc, 'b', B_sim, spc_sim, 'r--'); % Blue for experimental, red for simulated
legend('Experimental', 'Simulated');
xlabel('Magnetic Field (mT)');
ylabel('Intensity');
title('EPR Spectrum Comparison');

% Fitting setup - parameters to vary
SysVary.g = [0.03, 0.03, 0.03; 0.03, 0.03, 0.03]; % Allow g-values to vary
SysVary.lwpp = [0.01, 0.1];  % Allow linewidths to vary
SysVary.A = [20, 20, 20; 2, 2, 2; 2, 2, 2; 5, 5, 5; 5, 5, 5]; % Allow hyperfine couplings to vary
SysVary.J = 0.5 * 30000;     % Allow exchange coupling to vary

% Perform least-squares fitting
esfit(spc, @chili, {Sys, Exp}, {SysVary}); % Optimize parameters to fit the data

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