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

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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.