We systematically studied the zero-field splitting (ZFS) parameters of Fe(III) protoporphyrin IX chloride, or hemin, using the terahertz electron paramagnetic resonance (EPR) spectroscopy technique at ambient and high pressures. Although hemin is known as a model substance of hemoproteins, the pressure effect on the electronic structure has not yet been explored owing to the large ZFS. In this study, high-field and high-frequency EPR measurements were carried out in the frequency range up to 700 GHz and at hydrostatic pressures up to 2 GPa. At ambient pressure, multiple EPR branches were clearly observed, and the axial and rhombic components of ZFS were determined as $$D = 6.90\pm 0.01\hbox { cm}^{-1}$$ and $$E = 0.055 \pm 0.005\hbox { cm}^{-1}$$ , respectively. Upon pressure application, we observed a systematic shift of the resonance field, indicating a monotonous increase of the axial component from D = 6.9 to 7.9 $$\hbox {cm}^{-1}$$ at 2 GPa. The origin of this unusually large shift was discussed from a microscopic viewpoint of the electronic structure of iron under pressure.

中文翻译：

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高频高压电子顺磁共振谱研究氯化血红素的零场分裂参数

我们在环境和高压下使用太赫兹电子顺磁共振 (EPR) 光谱技术系统地研究了氯化铁 (III) 原卟啉 IX 或氯化血红素的零场分裂 (ZFS) 参数。尽管血红素被称为血红素蛋白的模型物质，但由于 ZFS 较大，尚未探索对电子结构的压力效应。在这项研究中，在高达 700 GHz 的频率范围和高达 2 GPa 的静水压力下进行了高场和高频 EPR 测量。在常压下，清晰地观察到多个 EPR 分支，ZFS 的轴向和菱形分量确定为 $$D = 6.90\pm 0.01\hbox { cm}^{-1}$$ 和 $$E = 0.055 \pm 0.005\hbox { cm}^{-1}$$ ，分别。在施加压力时，我们观察到共振场的系统偏移，表明轴向分量从 D = 6.9 单调增加到 7.9 $$\hbox {cm}^{-1}$$ 在 2 GPa。从压力下铁的电子结构的微观角度讨论了这种异常大转变的起源。