Introduction

Like UV and IR, nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) are also absorption spectra.

NMR and EPR involve placing the sample in a strong magnetic field and then irradiating the sample with a radio frequency source. NMR is a resonant transition that causes the atomic nucleus with a magnetic moment to undergo a magnetic energy level; and ESR is a resonant transition that causes unpaired electrons to generate a spin energy level.

Main differences between NMR and EPR

• EPR is to study the interaction between electron magnetic moment and external magnetic field, which is generally considered to be caused by electronic Zeeman effect, while NMR is to study the transition between nuclear Zeeman energy levels of atomic nuclei in external magnetic field. In other words, EPR and NMR study the energy required to reorient electron magnetic moments and nuclear magnetic moments in an external magnetic field, respectively.

• The resonant frequency of EPR is in the microwave band, and the resonant frequency of NMR is in the radio frequency band. Note: The external static magnetic field of electron paramagnetic EPR (ESR) is small, but the resonance frequency is large.

• Differences in the structure of EPR and NMR instruments: the former is a constant frequency and adopts the sweeping field method; the latter is a constant magnetic field and adopts the sweeping frequency method.

• The sensitivity of EPR is higher than that of NMR, and the absolute concentration of free radicals required for EPR detection is on the order of M. This is because the electronic magnetic moment is much larger than the nuclear magnetic moment, and the sensitivity is proportional to the cube of the magnetic moment.

Difference of Research Objects

• EPR is the only spectroscopic technique that can directly detect and study paramagnetic substances containing unpaired electrons. Its research objects include free radicals, triplet molecules, metal atoms or clusters, transition metals and rare earth ions in specific valence states, doping or defects, etc.

• NMR is a spectroscopy technique for the study of nuclei with spin quantum numbers equal to zero. Nuclei with a nuclear spin quantum number of 1/2 (such as 1H, 19F, 13C, 31P, etc.) are especially suitable for nuclear magnetic resonance spectrometer experiments, because it can be regarded as a sphere with uniform distribution of charge and spins like a top, there is a magnetic moment generated. The abundance of 1H, 19F, and 31P in nature is close to 100%, which can be easily determined by NMR spectrometer. In particular, hydrogen nuclei are not only easy to measure, but also the main elements of organic compounds, which are very important in nuclear magnetic analysis.

• The resonance frequency of electron paramagnetic EPR (ESR) is in the microwave band, such as 0.34T(9.5GHz) and 1.25T(35GHz). The resonance frequency of NMR is in the radio frequency band, such as 6.97T (300MHz), 18.6T (800MHz).

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