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How can electron-spin resonance identify chemical bonds and unpaired electrons in molecules?
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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)
Electron-spin resonance (ESR), also known as electron paramagnetic resonance (EPR), identifies unpaired electrons by measuring the absorption of microwave radiation as these electrons precess within a strong, external magnetic field. Because unpaired electrons possess spin, they act as tiny magnets that interact with their immediate environment, creating a unique resonance spectrum.123
This technique reveals details about chemical bonds and molecular structure through several key features:
- Fine Structure: The interaction between electronic shells and the surrounding ligand field creates distinct energy levels, resulting in a spectrum of resonance frequencies.3
- Hyperfine and Superhyperfine Structure: The electronic magnetization interacts with nearby nuclear moments, causing the resonance lines to split. This splitting allows researchers to map the distribution of electronic magnetization across multiple atoms.3
- Covalent Bonding: The presence of superhyperfine structure provides conclusive evidence of covalent bonding between ions, as it demonstrates that the magnetic electron is not localized on a single atom but interacts with neighboring nuclei.3
By analyzing these spectral patterns, scientists can test microscopic models of matter, identify free radicals, and study paramagnetic defects in crystals.3