Electron Spin Resonance Spectroscopy

Abstract: in this post we are going to describe the measurements made with our “home-built” ESR apparatus on some paramagnetic organic compounds (in particular stable free radicals) known for their strong response to electronic spin resonance.

Introduction

In the previous posts (Electron Spin Resonance, Electron Spin Resonance – HW Part, Electron Spin Resonance – SW Part) we described a simple demonstration apparatus for the phenomenon of electronic spin resonance. Electronic resonance strongly depends on many factors, which affect the g factor, including: interactions of the unpaired electron with its “molecular” environment, interaction of the electron with the nuclear spin, temperature, anisotropy of the electronic structure of the molecule. This multiplicity of factors gives rise to an ESR spectrum which is related to all the factors described above.

Our apparatus does not have a very high sensitivity. Sensitivity increases as the frequency increases, in fact professional equipment works with frequencies in the range of 10GHz and magnetic fields of about 0.3T, while our equipment works at 40MHz with magnetic fields of 2mT. This causes a notable decrease in sensitivity so that our apparatus can detect the phenomenon of electronic resonance only in substances characterized by a very strong signal. Despite these limitations we have tried to make measurements with some compounds.

TTF-TCNQ

The TTF-TCNQ is a charge transfer complex. A charge transfer complex (CT) or electron acceptor-donor complex is an association of two or more molecules, in which a fraction of the electron charge is transferred between molecular entities. The resulting electrostatic attraction provides a stabilizing force for the molecular complex. The source molecule from which the charge is transferred is called the electron donor and the receiving species is called the electron acceptor. The following images show the structural formulas of the compounds.

The organic compound TTF-TCNQ can be easily obtained from some electrolytic capacitors such as those of the Sanyo of the OS CON series, shown in the following image.

The TTF/TCNQ complex has unpaired electronic charges and therefore presents paramagnetic characteristics including a strong response to spin electronic resonance, as shown in the following graph where the trend in correspondence to the resonance conditions can be seen.

From the measurements carried out on the TCNQ compound we obtained:
f₀ = 41.3MHz
B₀ = 1.16mT measured externally to the coils. Valued 1.276mT in the center of the coils
ΔB ≅ 0.1mT

From these values we can calculate the factor g:
g = hf₀/μB₀ = 2.31

From the ESR measurements it appears that the TCNQ is characterized by strong electronic resonance response with a narrow line width, these characteristics make it ideal for being an ESR standard and for our demonstration applications.

TEMPOL

TEMPOL (aka 4-Hydroxy-TEMPO) is a heterocyclic compound. Like the similar compound TEMPO, it is used as a catalyst and as a chemical oxidant, by virtue of its property of being a stable radical. These compounds belong to the category of nitroxides, where the unpaired electron belongs to the oxygen atom linked to the nitrogen atom present on the ring. The following images show the structural formula of the two compounds.

The TEMPOL compound can also be found quite easily online, for example at this ebay link: TEMPOL.

The TEMPOL compound, having an unpaired electron, is a paramagnetic salt that produces a good response to spin electron resonance. The following graph shows the trend of the magnetic field and the ESR signal recorded by our apparatus.

As for the TCNQ also for the TEMPOL the signal was post-processed to highlight the trend of the signal in correspondence with the resonance conditions. The following graphs show the trend of the normalized signal.

From the measurements carried out on the TEMPOL compound we obtained:
f₀ = 40MHz
B₀ = 1.2mT measured externally to the coils. Valued 1.32mT in the center of the coils
ΔB ≅ 0.6mT

From these values we can calculate the factor g:
g = hf₀/μB₀ = 2.16

From the ESR measurements it appears that TEMPOL is characterized by a good electron resonance response (lower than TCNQ), the line width is also considerably enlarged due to the interactions of the unpaired electron with the other molecules of its crystal lattice.

Conclusions

Our “home-built” demonstrative ESR apparatus, despite its relatively low sensitivity, proved adequate for the detection/measurement of spin electron resonance in some paramagnetic organic compounds such as TCNQ and TEMPOL, known for their strong ESR signal. The apparatus could be improved by increasing the frequency range (and the intensity of the magnetic field) in order to make it more sensitive and thus increase the possibilities of application.

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