Electron Spin Resonance Studies of Internal Rotation of Nitrosyl Axial Ligand in Heme Complexes.

制备了许多5-配位亚硝基血红素配合物，并用ESR光谱对其进行了研究。其中Fe (TMP)^<15>NO在甲苯中已在10至380 K的温度范围内进行了最详细的测量，其中TMP为四聚基卟啉。在较低温度下（g_1*g_2g_3,10- 120k），谱图呈现出从菱形谱图到轴向谱图（g_1=g_2g_3,120-200K）以及在较高温度下（g_1=g_2=g_3,200-380K）的各向同性谱图的剧烈变化。在40 K以上，菱形对称光谱在(g_1+g_2)/2附近出现了额外的吸收峰，随着温度的升高，吸收峰的强度增加，在中温时成为轴对称光谱的垂直峰。利用改进的Bloch方程和随机Liouville方法对观测到的光谱进行了分析。用轴配位亚硝基血红素配合物的90阶跳跃内旋（R_2）和布朗旋（R_1）来解释温度依赖谱，两者的运动都随着温度的升高而变快。具体来说，在较高温度下，R_1和R_2的速率都足够快，足以平均磁各向异性。R_1的速率随着温度的降低而变慢，在200 K附近基本被冻结，而R_2的速率非常快，可以在120 K以上平均g_1-g_2的各向异性，并且在40 K以上仍然可以快速产生额外的峰。

A number of 5-coordinate nitrosyl heme complexes have been prepared and studied by ESR spectroscopy. Of these Fe (TMP)^<15>NO in toluene has been meaured in most detail in the temperature range between 10 and 380 K,where TMP is tetramesitylporphyrin. The spectral pattern showed a drastic change from rhombic one at lower temperatures (g_1*g_2g_3,10-120K) to axial one at intermediate temperatures (g_1=g_2g_3,120-200K) and to isotropic one at higher temperatures (g_1=g_2=g_3,200-380K). Furthermore, the spectra of rhombic symmetry showed extra absorption peaks near (g_1+g_2)/2 above 40 K,which, gaining intensities with raise in temperature, became the perpendicular peaks of axially symmetric spectra at intermediate temperatures. The observed spectra are analyzed using modified Bloch equations and the stochastic Liouville method. The temperature dependent spectra have been interpreted in terms of the 90ﾟ jump-internal-rotation (R_2) of axially coordinated nitrosyl ligand in addition to the Brownian rotational motion (R_1) of nitrosyl heme complexes, both motions become fast with increasing temperature. Specifically, both the rates of R_1 and R_2 are fast enough at higher temperatures to average out the magnetic anisotropies. R_1 become slower in rate with decreasing temperature and is essentially frozen out near 200 K,while the rate of R_2 is so fast to average out the g_1-g_2 anisotropy above 120 K and is still fast to cause the extra peaks above 40 K.