Spin-lattice relaxation time, electron paramagnetic resonance

The NMR spectra of these complexes under ambient conditions exhibit sharp, paramagnetically shifted features that can span up to 400 ppm (90, 134). Electron exchange is fast on the NMR time scale, so there is an effective twofold symmetry, which approximately halves the number of distinct features observed. The sharpness of the resonances is due to the short electron spin-lattice relaxation time of the Fe(II) center, which allows even the CH2 protons adjacent to the coordinated nitrogen atoms to be observed. 

The conditions necessary for observation of proton magnetic resonance spectra in paramagnetic systems are well established (1). Either the electronic spin-lattice relaxation time, T, or a characteristic electronic exchange time, Te, must be short compared with the isotropic hyperfine contact interaction constant, in order for resonances to be observed. Proton resonances in paramagnetic systems are often shifted hundreds of cps from their values in the diamagnetic substances. These isotropic resonance shifts may arise from two causes, the hyperfine contact and pseudocontact interactions. The contact shift arises from the existence of unpaired spin-density at the resonating nucleus and is described by 1 (2) for systems obeying the Curie law. 

R. Lopez and S. K. Misra, Improvement in the Measurement of Spin-Lattice Relaxation Time in Electron Paramagnetic Resonance , in Biological Magnetic Resonance, eds. C. J. Bender and L. J. Berliner, Springer, 2006, vol. 25, Computational and Instrumental Methods in EPR, p. 31. 

The line broadening in spectra of paramagnetic compounds is caused by short electronic spin-lattice relaxation times and/or hyperfine electron-nuclear coupling. Consequently, it is usually the case that materials giving useful EPR spectra have nuclear magnetic resonances so broad as to be unobservable. The two methods are therefore to a large extent complementary. 

Lopez R. 1993. Improvement in measurement of spin-lattice relaxation time T in electron paramagnetic resonance application to diluted copper calcium acetate and a Fe203-doped borate glass. PhD dissertation, Universite Paul Sabatier, Toulouse, France [English translation by SK Misra]. In Biological magnetic resonance, vol. 25. Ed C Bender, L Berliner. New York Springer, 2006. 

IMPROVEMENT IN THE MEASUREMENT OF SPIN-LATTICE RELAXATION TIME IN ELECTRON PARAMAGNETIC RESONANCE... 

Improvement in tbe Measurement of Spin-Lattice Relaxation Time in Electron Paramagnetic Resonance... 

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