What is spin exchange and why is it interesting?
When we talk about spin exchange, we mean dilute paramagnets: atoms in gases, solutions of free radicals and paramagnetic complexes in liquids, triplet excitons in molecular crystals, etc.
Spin exchange is a change in the state of the spins of paramagnetic particles during their bimolecular collision, induced by the Heisenberg exchange interaction of two colliding particles
Here, the arrows indicate different spin states of two colliding particles. During a collision, an exchange interaction is activated, which changes the state of the electron spins
The rate of this process for solutions is determined by the rate constant of effective bimolecular collisions known from Smoluchowski’s theory.
Here, rex is the effective radius of spin exchange during a collision, D is the coefficient of mutual diffusion of two colliding particles.
When two particles collide, the exchange interaction changes their spin state only when the collision partners belong to different subensembles, i.e. to particles with different EPR resonance frequencies of individual particles. The rate of change of the spin state of a specific selected spin, for example from the k-th subensemble of spins, due to collisions with spins from the n-th subensemble is equal to ϕnKexC, where ϕn is the statistical weight of spins in the n-th subensemble, C is the concentration of paramagnetic particles in the solution.
Spin exchange is of great interest as a tool for detecting the fact of bimolecular collisions and measuring the rate of this process for a wide variety of molecules and in a wide variety of environments, including biological systems, polymer solutions, and liquids in nanoporous systems.
Wide application of spin exchange for studying collisions of molecules of interest to the researcher is achieved by chemically attaching a paramagnetic particle (usually a stable nitroxyl radical) to these molecules or close analogues.
As a result, we obtain a spin probe and it is already possible to apply electron paramagnetic resonance (EPR) methods to monitor bimolecular collisions of these spin probes.
What can be measured using spin exchange?
The frequency of bimolecular collisions even in very complex systems (in chemistry, molecular biology, physical chemistry).
Molecular mobility, the diffusion coefficient of molecules even in very complex systems.
The magnitude of the exchange integral, the degree of overlap of the wave functions of two paramagnetic particles during their random collisions.
What can spin exchange be used for?
To develop technology for targeted delivery of medicinal molecules to the body of humans and animals.
To improve the spatial resolution of magnetic resonance imaging.
For the development of various filters, such as water purification.
Who is interested in spin exchange?
Specialists in the field of electron paramagnetic resonance (EPR) spectroscopy.
Developers of EPR spectrometers, since it is necessary to keep in mind that due to the “recoil” effect during spin exchange, the shape of individual EPR resonance lines has a mixed shape of the sum of a Lorentz absorption line and a Lorentz dispersion line with the same values of frequency and resonance width. And it is important that the proportion of the dispersion contribution is different for different individual lines of the EPR spectrum.
For those who have to interpret the shape of EPR spectra recorded under stationary conditions and the time dependence of the signal response in pulsed EPR experiments.
For those who use the spin probe method to determine the frequency of bimolecular collisions in gases, liquids, polymer solutions, biological environments, filters, etc.
For those who use the effect of dynamic nuclear polarization (DNP) to increase the sensitivity of nuclear magnetic resonance (NMR) spectroscopy, including to increase the resolution in medical magnetic resonance imaging scanners.