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

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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.