MQNMR

Fig. 2. Basic 3-pulse sequence for generating MQNMR spectra.

Fig. 3. Two-dimensional MQNMR spectrum of 3-chloroiodobenzene (2) oriented in liquid crystalline solution with non-selective excitation and detection (Fig. 2). 1,024 increments in fu 12 scans per increment, 48 kHz in f2, 160 kHz in j, 400 MHz at 295 K, transmitter offset 18 kHz from the centre of the 1H spectrum. The transmitter offset was set to provide dispersion between the various MQ spectra.

Fig. 4. MQNMR spectra of 3-chloroiodobenzene (2) aligned in liquid crystalline solution, extracted as the f projection from the 2D spectrum in Fig. 3. (Adapted with permission from ref. 40.)...

Fully protonated hexane (C6H14) has 14 protons in the spin system and Drobny9 reported the MQ spectra of //-hexane-terminal methyl groups) dissolved in a nematic solvent. Analysis of the 6Q and 7Q spectra of hexane-c/6, (6) was used to establish the relative signs of the dipolar couplings. The 6-quantum proton spectrum contains 36 transitions and the 7 quantum spectrum contains 4 transitions, as expected for an 8-proton spin system with C2h symmetry. The best fit of theoretically generated MQNMR spectra to the experimental spectra were obtained with a model where only the all-trans conformations of the alkane chain were appreciably populated. 

An automated protocol for sequentially analysing MQ spectra to obtain reasonable estimates of spectral parameters then analysing IQ spectra has been developed.61 The analysis of MQNMR spectra has frequently been used to provide good initial estimates of parameters to permit the analysis of IQ spectra where these are well resolved. 

In MQNMR, individual spins become correlated with each other over time by interaction through their dipolar couplings. The rate at which MQ coherences develop is a complex function of the coupling constants present in the spin system. In solids, where the network of dipole-coupled spins is... 

Table 3 summarises the partially aligned molecules and proton spin systems that have been studied and analysed by MQNMR techniques. 

Thomann and co-workers have reported that all of the spins are localized in 99% C enriched trans- and cis-(accurate composition is cisIQ. -tmnslO.l) polyacetylene, studied by the multiple quantum NMR (MQNMR) detected by the ESE-technique [165,186]. This is, however, clear experimental evidence demonstrating the validity of the difFuse/trap model [6,82]. 

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