Exchange lineshape analysis

D EXSY is also important for cases where one-dimensional experiments are feasible (i) more extensive data sets of rate constants as a function of temperature become available which ultimately improves the quality of data of activation energies determined from Arrhenius plots (ii) an independent check is possible if the assumed model of exchange chosen for lineshape analysis of variable-temperature one-dimensional CP/MAS spectra is indeed correct... 

Figure 5.6 Graph of the relation between exchange time estimated by lineshape analysis of IR-SEC data for 1-3 and solvent relaxation time, tie-...

Study of the effect of exchange on line-widths and frequencies constitutes a major branch of NMR spectroscopy, and is discussed in detail in a number of reviews (e.g.. Refs. 6,7). There are circumstances in which analysis of chemical exchange by NMR is relatively straightforward, as in slow exchange described by Eqn. 24 and fast exchange described by Eqn. 27. However, in many situations, notably at intermediate exchange rates, complex lineshape analysis is required for accurate quantitation, often necessitating assumptions or approximations that are difficult to verify. 

One-dimensional quadrupole echo NMR lineshape analysis of powder samples is particularly informative when fast, discrete jumps occur between sites of well-defined geometry as, for example, in a phenyl group undergoing two-site exchange. In this case, the characteristic Pake-pattern is transformed into an axially asymmetric lineshape with an apparent asymmetry parameter r] 9 0 (see Equation (6.2.3)) [1-8]. The asymmetric lineshapes, shown on the left in Fig. 6.2.2, can be derived by considering how the individual components of the principal EFG tensor become averaged by the discrete jumps. Within the molecular frame, and in units of as defined by Equation (6.2.2), the static axially symmetric tensor consists of the components = 1, = — 1/2, and V y = — 112. This traceless tensor satisfies the... 

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