Non-extreme narrowing

A final point regarding the isotope studies If extreme narrowing may be safely assumed the isotope effects may equally well be studied at fixed frequency or fixed magnetic field. When non-extreme narrowing conditions are likely to occur it is preferable to make the studies at fixed frequency and adjust the field so as to reach the resonance conditions of the two isotopes. 

Derivation of the time dependence of the longitudinal and transverse magnetization for an ensemble of I = 3/2 nuclei in the presence of chemical exchange processes and under non-extreme narrowing conditions is by no means trivial. The general solution has been given by Bull [409], Here we will limit ourselves to a discussion of his results for a system of two sites. 

Carlstrom et al note that the conditions for motional narrowing are not met for the rod model of the helix at the higher molecular weights of the polypeptide. This has prompted us to investigate whether or not the results in Fig. 8.8 could be understood if the non-exponential decay of the magnetization for non-extreme narrowing conditions... 

Usually, nuclear relaxation data for the study of reorientational motions of molecules and molecular segments are obtained for non-viscous liquids in the extreme narrowing region where the product of the resonance frequency and the reorientational correlation time is much less than unity [1, 3, 5]. The dipolar spin-lattice relaxation rate of nucleus i is then directly proportional to the reorientational correlation time p... 

Synchrotrons produce photons with energies in the range of nuclear Mossbauer transitions and can, in principle, be used to excite these transitions. However, synchrotron radiation can be monochromatized to only about 1 meV with new monochromators. Because the accessible nuclear levels are extremely narrow (between 10 and 10 eV), it is only about 10 of the incident photons that can excite the nuclear levels (excitation cross-section could be as much as 10 Fq). This is far weaker than radiation that is non-resonantly scattered by the electronic processes in the solid arising from the scattering of the entire 1 meV width of the incident radiation. 

Living polymerizations. Of greater interest, however, is the fact that fluorophenyl-substituted bis(phenoxy-imine) Ti complexes easily show extremely narrow Mv/M ratios, around 1, indicative of living polymerization behavior.1132,1134,1140,1160 At 50 °C, the perfluorophenyl-substituted complex 137 is more active than the corresponding non-fluorinated complex 136 (4 x 104 vs. 3 x 103 gpK(mmolM) 1 h 1 bar-1 entries 67 and 4 in Table 18), and yields a PE with an extremely narrow molecular mass distribution 1.13).1134 Analogous perfluorophenyl Zr... 

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