Dipolar-mediated Double-quantum Spectroscopy

DQ spectroscopy is not only useful for homonuclear C— C correlation experiments which allow the identification of the backbone connectivity. In this section, the utility of other 2D DQ experiments which provide insight into, e. g., through-space proximities will be illustrated. As opposed to the solid-state INADEQUATE experiment introduced in the previous section, the experiments described in this section are based on the dipolar as opposed to the J coupling of spins. 

A H 2D DQ MAS spectrum [87] recorded in a rotor-synchronised fashion in (i. e., the ti increment is set equal to one rotor period such that all spinning sidebands in the DQ dimension (F ) fold back onto the centreband position) is shown in Fig. 9.20a. To create DQC as weU as to allow its conversion into observable SQC, the BABA [73] recoupling sequence (see Section 9.4.2) was used. This is a robust 

For this particular example, which corresponds to the aromatic protons of an aUcyl-substituted polycycHc aromatic system (HBC-C ) [48], three resonances (labelled A, B, and C) can be identified in the corresponding ID H (500 MHz) MAS spectrum, which is shown at the top of Fig. 9.20a. The six possible DQ peaks in this case are shown in Fig. 9.20b. Since the DQ frequency corresponding to a given DQC is simply the sum of the two SQ frequencies, DQCs between like (AA) and unlike (AB) spins can, in general, be distinguished in that, in the former 

Of the six possible DQ peaks, only two, namely AB and CC, are observed in the experimental spectrum in Fig. 9.20a. For this system, the aromatic protons are arranged into well-isolated pairs of bay protons the observed DQ peaks thus correspond to these bay protons pairs. As discussed in [48], the implied presence of only two types of pairs of aromatic protons, Ha—Hb and (see Fig. 9.20c) is a 

Such effects of ring currents on NMR chemical shifts are, of course, well established [90] however, it is only recently, with the development of solid-state NMR methods allowing the resolution of resonances, that the widespread importance of these effects in organic solids is gaining attention other clear examples of the phenomenon can be found in, e. g., [91, 92]. It is to be noted that, although the absolute shifts due to ring currents are similar for both H and C, the much smaller range of chemical shifts ( 20 ppm as opposed to 200 ppm) means that the influence is much more evident in H NMR. In addition, protons are normally located at the more exposed parts of the structure. 

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