Electron paramagnetic resonance (EPR) or electron spin resonance (ESR) spectroscopy technology is a method of studying the structure, dynamics and spatial distribution of substances containing unpaired electrons. EPR/ESR spectroscopy can provide in-situ and micro-scale information such as electron spin, orbit, and atomic nucleus. When a substance containing unpaired electrons is placed in a static magnetic field, the emission or absorption of electromagnetic wave energy by the substance will be observed if an electromagnetic wave signal of a certain frequency is applied to the sample. The characteristics of electrons and their surrounding environment are able to be studied by monitoring and analyzing the electromagnetic wave signals. Therefore, the analysis of material structure and other applications can be carried out. In addition, substances containing unpaired electrons are widely distributed, such as isolated single atoms, conductors, magnetic molecules, transition metal ions, rare earth ions, ion clusters, doping materials, defective materials, biological free radicals, metal proteins, etc. Many substances themselves do not contain unpaired electrons, and will produce unpaired electrons after being excited by light.
Figure 1. Panel (a) shows experimental VT EPR spectra of HAT6 doped with discotic rigid core probe recorded every 3 K between 420 and 360 K on cooling. Panel (b) shows EPR spectra between 360 and 320 K on cooling. Panels (c) and (d) correspond to the simulated EPR spectra for selected temperatures representing most characteristic line shapes. (Oganesyan, V. S. 2018)
Application of EPR/ESR Spectroscopy
EPR spectroscopy technology has been widely used to analyze the metal centers, organic and inorganic free radicals involved in chemical processes. Alfa Chemistry supports its application in various field such as coordination chemistry, metal organic compounds, electrochemistry, redox processes, kinetics involving free radical intermediates, photochemistry and catalysis.
- Enzyme reaction
Many enzyme reactions involve a single-electron oxidation step, and the formation of a paramagnetic transient of the enzyme can be detected by EPR spectroscopy.
A diversity of chemical reactions involve electron transfer, and each electron transfer causes unpaired electrons to produce paramagnetic free radicals which can be clearly observed by EPR spectroscopy.
Most photochemical reactions also occur through the formation of free radicals.
- Catalytic reaction
EPR spectroscopy is used for detection of free radicals generated by catalytic reactions. It is also an important tool for understanding the reactivity of the catalyst and the reaction mechanism of the catalytic reaction, which is very essential for improving the performance of the catalyst.
Combined with the electrochemical generation method and EPR spectroscopy, free radicals from organic and inorganic compounds is able to be determined and studied.
EPR spectroscopy analysis can be used to determine the oxidation state of biological systems by using endogenous long-lived free radicals (ascorbic acid free radicals, tocopherol free radicals, melanin) as markers.
Dynamic Regimes of ESR spectra
- Isotropic limit
In the isotropic limit, the simulation of ESR spectra of paramagnetic centers that tumble fast enough to completely average out all anisotropic interactions is simple. Alfa Chemistry has developed a well-designed function to compute isotropic-limit spectra.
- Fast motion regime
If, starting from the very fast tumbling in the isotropic limit, the rotation of the paramagnetic molecules is slowed down either by cooling or by increasing the viscosity of the solvent, we enter the fast-motion regime. Our simulation algorithm for the fast-motion regime is identical to the one used for isotropic spectra. This method is not limited to nitroxide radicals, spectra of vanadyl or copper complexes can also be computed.
- Slow motion regime
When the rotational motion of the paramagnetic molecules slows down further, we leave the fast-motion regime and enter the slow-motion regime. A simple simulation of a slow-motional ESR spectrum is developed based on that assuming Brownian rotational diffusion with an isotropic diffusion tensor.
- Rigid limit
In the rigid limit, the paramagnetic molecules in the sample are fixed, and their orientations do not change with time. Alfa Chemistry supports a useful simulation that adaptively models the energy level diagram, from which the positions of resonance lines are then obtained.
At Alfa Chemistry, our groups are capable of conducting prediction of EPR spectra directly and completely from single dynamical trajectories generated from density functional theory and molecular dynamics simulations. Our fast and high-quality services include the following:
- Density functional theory (DFT) calculations
We provide a general protocol to interpret electron paramagnetic resonance (EPR) spectra using DFT calculation of spin Hamiltonian parameters gZ and AZ.
Our scientists select efficient functional which depend on the molecular/spectroscopic properties and on the metal under examination.
We choose a large basis set in which polarization, diffuse functions and relativistic effects are also included.
We also consider the role of the solvent through continuum solvation models such as polarizable continuum model (PCM) or solvation model based on density (SMD).
Moreover, the performance of multiple functionals (B3LYP, B3PW91, PBE0, CAM-B3LYP, BH, HLYP, B2PLYP, etc.) are tested based on the mean absolute percent deviation (MAPD) and standard deviation (SD).
- Molecular dynamics (MD) simulations
Alfa Chemistry supports large scale fully atomistic molecular dynamics simulation methods to perform prediction of variable temperature EPR spectra.
The atomic resolution of the simulations we apply allows the interpretation of the molecular motions and interactions in terms of their impact on the sensitive EPR line shapes.
Prediction of EPR line shapes directly from MD trajectories of actual structures allows unambiguous interpretation of EPR spectra in terms of complex motions.
Only a single truncated dynamical trajectory generated until the point when correlation functions of rotational dynamics are completely relaxed is required for a reliable simulation of motional EPR spectra.
Alfa Chemistry's Advantages
- We can simulate ESR spectra of most paramagnetic systems under a wide range of dynamic conditions.
- Our experts can handle complex paramagnetic molecules with several unpaired electrons and several magnetic nuclei.
- Our ESR spectra prediction is applicable to an arbitrary system of electron and nuclear spins described by a general form of the spin-Hamiltonian for the entire motional range.
EPR/ESR simulation prediction provides an effective way to detect free radicals. Our EPR/ESR simulation prediction services remarkably reduce the cost, promote further experiments, and enhance the understanding of chemical process for customers worldwide. Our personalized and all-around services will satisfy your innovative study demands. If you are interested in our services, please don't hesitate to contact us. We are glad to cooperate with you and witness your success!
- Oganesyan, V. S. EPR spectroscopy and molecular dynamics modelling: a combined approach to study liquid crystals. Liquid Crystals. 2018: 1-19.