Mechanisms for the longitudinal and transverse relaxation of iron-57

Abstract

For a separation of the contributions of different relaxation mechanisms to the longitudinal relaxation rate of 57Fe, the spin-lattice relaxation times T1 of this nucleus in various iron complexes [Fe(CO)5, ferrocene, (butadiene)Fe(CO)3, (isoprene)Fe(CO)3, (Z-) and (Epentadiene)Fe(CO)3] were determined in two widely different fields (B0 = 2.114 T and 9.395 T corresponding to 90 and 400 MHz proton Larmor frequency, respectively) at temperatures in the range 275 to 321 K. The spin-lattice relaxation rates are analyzed for a field-independent contribution, which is largely due to spin rotation, and a quadratically field-dependent one, which is due to chemical shift anisotropy combined with molecular reorientation. For the nearly spherically symmetric Fe(CO)5 molecule, spin rotation is the only spin-lattice relaxation mechanism, whereas for all other Fe compounds under investigation, chemical shift anisotropy is the dominant process at B0 = 9.4 T, while spin rotation dominates at 2.1 T. In addition to these mechanisms, the transverse relaxation, determined at 2.1 T, is influenced by a small amount of visible or colloidal particles, resulting from decomposition of the compounds under investigation. Adsorption of the iron complex on such a suspended particle results in a change of the 57Fe Larmor frequency by chemical exchange and/or scalar interaction. Such a change, for a time interval longer than the 57Fe Larmor period, influences only the transverse relaxation rate. © 1987.