Nitrosyl Heme Complexes. ESR Spectral Simulation of Internal Rotation of Axial Ligand.

在甲苯中，在10 ~ 380 K的温度范围内测量了5配位亚硝基血红素配合物Fe (TMP) NO的ESR光谱，其中TMP为四聚酰基卟啉。光谱显示出在较低温度（10 ~ 120k）下从斜方对称（g_1≠g_2≠g_3）到中温（120 ~ 200k）下的轴向对称（g_3≠g_1=g_2≡g_⊥）以及在较高温度（220 ~ 380k）下的各向同性（g_1=g_2=g_3≡g_0）的剧烈变化。由于^<57>Fe核（I=1/2）的超细分裂常数以10^<-4> cm^<-1>为单位确定为|A_1|=11.3, |A_2|=9.5, |A_3|=6.6, |A_1|= 9.5, |a_0|=5.2，其中A_3常数为正号，其他为负号。用随机Liouville法对观测到的光谱进行了模拟。除了亚硝基血红素配合物的布朗旋转运动（R_1）外，还用轴配位亚硝基配体的90°跳-内旋（R_2）来解释温度相关光谱。在较高的温度下，R_1和R_2的速率都足够快，足以平均磁各向异性。R_1随着温度的降低而变慢，在200 K附近基本冻结，而R_2在120 K以上的速率非常快，使g_1、g_2各向异性平均，在40 K以上的速率仍然很快，在(g_1+g_2)/2附近产生额外的峰，这是轴配位亚硝基配体90°跳变-内旋的特征。

ESR spectra of 5-coordinate nitrosyl heme complex, Fe (TMP) NO, have been meaured in toluene in the temperature range between 10 and 380 K, where TMP is tetramesitylporphyrin. The spectra showed a drastic change from rhombic symmetry (g_1≠g_2≠g_3) at lower temperatures (10〜120 K) to axial one (g_3≠g_1=g_2≡g_⊥) at intermediate temperatures (120〜200 K) and to isotropic one (g_1=g_2=g_3≡g_0) at higher temperatures (220-380 K). Hyperfine splitting constants due to the ^<57>Fe nucleus (I=1/2) were determined in units of 10^<-4> cm^<-1> as |A_1|=11.3, |A_2|=9.5, |A_3|=6.6, |A_⊥|=9.5, |a_0|=5.2, with the positive sign for the A_3 constant and the negative one for the others. The observed spectra are simulated using the stochastic Liouville method. The temperature dependent spectra have been interpreted in terms of the 90° jump-internal-rotation (R_2) of axially coordinated nitrosyl ligand in addition to the Brownian rotational motion (R_1) of the nitrosyl heme complex. Both the rates of R_1 and R_2 are fast enough at higher temperatures to average out the magnetic anisotropies. R_1 becomes slower with decreasing temperature and is essentially frozen near 200 K, while the rate of R_2 is so fast above 120 K to average out the g_1, g_2 anisotropy and is still fast above 40 K to cause the extra peaks near(g_1+g_2)/2, which characterizes the 90° jump-internal-rotation of axially coordinated nitrosyl ligand.