The Investigation of Slow Dynamics 2D Exchange

The basic principle of 2D exchange NMR involves the measurement of the frequency of the same molecular segment at two different times. A slow dynamic process is detected on account of the change, during a mixing time between the two evolution periods, in the NMR frequency caused by a reorientation of the molecular segment In this section, we describe static and MAS 2D exchange experiments [4]. 

In Section 9.5.2, a rotor-synchronised H- H DQ MAS spectrum was presented (Fig. 9.20). The 2D DQ MAS can be performed in an alternative fashion if the ti increment is reduced, which corresponds to an increase in the DQ spectral width, a DQ MAS spinning-sideband pattern is observed [120, 121]. Such DQ MAS sideband patterns exhibit characteristic unusual features. In particular, the observed patterns are very sensitive to the product of the dipolar coupling constant, 

and the recoupling time, r pi, with an increase in this product leading to the appearance of higher-order spiiming sidebands. 

Importantly, since r pi is known, the absolute value of D can be extracted by an analysis of DQ MAS spinning-sideband patterns. As a specific example. Fig. 9.27 presents experimental DQ MAS spinning sideband patterns for the aro- 

Comparing the evaluated D values for the crystalline and LC phases, a reduction of D by a factor of 0.40 + 0.04 is observed, corresponding to an order parameter [122] of 0.80 + 0.08. This could be explained by postulating the presence of out-of-plane motion in addition to the axial rotation of the molecule about an axis perpendicular to the ring. The good agreement with the value of 0.84 obtained from an analysis of ID NMR lineshapes is to be noted [123]. It is to be emphasised, however, that the DQ MAS method is applicable to as-synthesised samples, i. e., there is no reliance upon isotopic labelling. 

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