Para lineshapes

Temperature dependent lineshape variation of the Pake doublet represents then an experimental evidence of a motion occurring in the solid state. 2H NMR investigations on selectively deuterated compounds, therefore, are important tools for the recognition of molecular motions in solids. This method has been applied to the characterization of molecular motions in molecular solids [8] and inclusion compounds [9]. As an example in Fig. 3.2.4 is reported the effect of the motion about the molecular symmetry axis, either by two-fold or n-fold (n>3) flips, on the deuterium spectrum of a deuterated para-substituted benzene [10]. 

This model has been used for the description of Arrhenius curves of H-transfers as described in more detail in Chapter 6. The next two chapters show applications of these symmetry effects. First the para-hydrogen induced polarization (PHIP) experiments are discussed. There the symmetry induced nuclear spin polarization creates very unconventional NMR lineshape patterns, which are of high diagnostic value for catalytic studies. Then in Section 21.4 symmetry effects on NMR line-shapes and relaxation data of intramolecular hydrogen exchange reactions are discussed and examples from iH-liquid state and H-solid state NMR are presented and compared to INS spectra. The last section gives an outlook on possible future developments in the field. 

In the case of a simple reaction from the para-H2 state to the product state it is possible to derive analytical solution of the lineshape of PHIP spectra [65]. In the following an alternative derivation of the lineshape is given. For simplicity the reaction is assumed as a one-way reaction, i.e. no back reaction ky O, a, b denote the two different sites). 

Simulated NMR spectral lineshapes at the fast limit of motion for QD benzene (centre) undergoing motion around the molecule s Ce axis and (right) for H3C-C5D4-CH3 p-xylene undergoing n flips around the molecule s para axis. These are compared with (left) the spectrum of static... 

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