电子顺磁共振Electron paramagnetic resonance

共振条件 hν = g·μ_B·B₀，μ_B 为玻尔磁子，自由电子 g = 2.0023。常用 X 波段（约 9.5 GHz，共振磁场约 0.34 T），一般固定微波频率扫描磁场，记录吸收的一阶导数谱。g 值反映电子所处环境（自旋-轨道耦合使过渡金属的 g 偏离 2）；未成对电子与附近磁性核（¹H、¹⁴N 等）的超精细耦合使谱线裂分，裂分数为 2nI + 1，可确定自由基结构和电子离域情况。

The resonance condition is hν = g·μ_B·B₀, with Bohr magneton μ_B and g = 2.0023 for a free electron. X-band (about 9.5 GHz, resonance near 0.34 T) is common; the microwave frequency is usually fixed and the field swept, recording the first derivative of absorption. The g value reflects the electron's environment (spin–orbit coupling shifts transition-metal g values from 2), and hyperfine coupling to nearby magnetic nuclei such as ¹H and ¹⁴N splits lines into 2nI + 1 components, revealing radical structure and delocalisation.

1944 年苏联喀山大学的物理学家叶夫根尼·扎沃伊斯基首次观测到电子顺磁共振信号（在 CuCl₂·2H₂O 等顺磁盐中）。二战后雷达技术发展出的微波器件推动了 EPR 迅速普及，牛津大学的布利尼等系统研究了过渡金属离子的 EPR。

In 1944 Yevgeny Zavoisky at Kazan University in the Soviet Union first observed EPR signals, in paramagnetic salts such as CuCl₂·2H₂O. Microwave components developed for wartime radar then made EPR spread rapidly, and Bleaney and colleagues at Oxford studied transition-metal ions systematically.

EPR 用于检测和鉴定短寿命自由基（配合 DMPO 等自旋捕获剂）、研究自由基聚合和光化学反应机理、表征金属酶活性中心（如细胞色素、铁硫蛋白）、催化剂中的金属价态、半导体和金刚石中的缺陷（氮-空位中心）、食品辐照检测、考古与地质的 EPR 测年，以及用自旋标记研究蛋白质构象。

EPR detects and identifies short-lived radicals (with spin traps such as DMPO), probes mechanisms of radical polymerisation and photochemistry, characterises metalloenzyme active sites such as cytochromes and iron–sulfur proteins, metal oxidation states in catalysts and defects in semiconductors and diamond (nitrogen–vacancy centres), detects irradiated food, dates archaeological and geological samples, and studies protein conformation with spin labels.