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How do nuclear magnetic resonance and electron-spin resonance differ?

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magnetic resonance Figure 1: Precession of a magnetic dipole moment μ in the presence of a constant field H and a rotating field H′ (see text)

Nuclear magnetic resonance (NMR) and electron-spin resonance (ESR) are both forms of magnetic resonance that involve the interaction of magnetic moments with external magnetic fields, but they differ in several fundamental ways:

Target Particles: NMR involves the magnetic moments of atomic nuclei, whereas ESR (also known as electron paramagnetic resonance) involves the magnetic moments of unpaired electrons.12
Substance Restrictions: NMR can be observed in nearly all stable nuclei with non-zero magnetic moments. In contrast, ESR is restricted to substances containing unpaired electrons, such as transition elements, free radicals, metals, and specific paramagnetic defects.32
Frequency Ranges: Due to the differences in magnetic moments, the resonance frequencies differ significantly. In a magnetic field of 10 kilogauss, NMR frequencies typically fall in the radio-frequency range (e.g., 42.58 MHz for protons), while ESR frequencies occur in the microwave or radar range (e.g., 28,000 MHz).2
Sensitivity: ESR is generally far more sensitive to the immediate environment than NMR. Because electronic subshells are easily distorted by interactions in bulk matter, ESR frequencies can differ greatly from those of free spins, whereas NMR shifts are typically much smaller.3
Detection Limits: Under optimal conditions, ESR is significantly more sensitive in terms of the number of spins required for detection, often capable of detecting as few as $10^{10}$ spins, compared to approximately $10^{18}$ spins for NMR.2