Multiple-Quantum and Two-Dimensional NMR

In-depth treatments of the topic are available in several books [1-6] and in a large number of review articles. The monograph by Dong [6], for example, focuses on aspects like the dynamics of nuclear spins, orientational order, molecular field theories, nuclear spin relaxation theory, director fluctuation and spin relaxation, rotational and translational dynamics, internal dynamics of flexible mesogens, and multiple-quantum and two-dimensional NMR, topics that will be touched upon very briefly here. Re-... 

The arsenal of techniques used for solutions is being applied to solids or liquid crystal solutions as well. For example, multiple quantum spectroscopy and two-dimensional NMR have been combined with the techniques described above. Density matrix theory and the individual spin operator formalism have been used to understand the mechanisms of multiple pulse sequences and the design of new ones. Since FT spectrometers offer many of the techniques mentioned in this Section 6 as standard features, the chemist can utilize an array of multiple resonance multiple pulse techniques without benefit of theory, using Refs. 41-42 as guides. 

Two-dimensional NMR spectroscopy ((double quantum fdtering (DQF), correlation spectroscopy (COSY), hetero-nuclear multiple quantum correlation (HMQC), heteronuclear multiple bond correlation (HMBC)) as well as liquid secondary ionization mass spectrometry (LSI MS) and UV-Vis spectroscopies were used to establish crown structures of TTFs 33 ( =l-3). In the case of the macrocycle 33 ( = 1), two protons of each methylene group of the SCH2CH2O fragments were not identical and gave an AA BB system. This observation was in accordance with the expected low conformational mobility of the polyether bridge in ( )-33 ( = 1) as compared with (Z)-33 ( = 1). The macrocycle ( )-33 ( = 2) behaved similarly to ( )-33 ( =1), whereas the protons under discussion were equivalent in ( )-33 ( = 3) <2001CFJ447>. 

The inverse detection heteronuclear multiple quantum coherence (HMQC) experiment is another approach to two-dimensional NMR techniques, which consists of a transfer of chemical shift and coupling information from relatively insensitive nuclei such as and some metals, to more sensitive nuclei such as H. The advantage of this method is a substantial increase in the sensitivity obtained, due to the greater natural abundance of H (Kingery et al., 2001). 

While the main recent advance in NMR has been the development of multidimensional spectroscopy, novel catalytic applications include in situ studies and two-dimensional (2D) solid-state techniques such as correlation spectroscopy, spin diffusion, and quadrupole nutation. Completely new techniques have appeared, such as multiple-quantum spin counting, and old ones have developed in quite unexpected directions. For example, cross-polarization, a 20-year-old experiment, has recently been applied to quadmpolar nuclei to yield important new information on heterogeneous catalysts. Magic-angle spinning (MAS) of quadru-polar nuclei has been extended to methods in which the sample is spun about two different angles either simultaneously or sequentially (DOR and DAS). These experiments have been made possible by the significant advances in NMR instrumentation in the last decade. 

Some readers may be surprised to learn that inverse-detected two-dimensional NMR experiments are not a recent addition to the pulse sequence libraries of the NMR spectroscopist. The HMQC (Heteronuclear Multiple-Quantum Correlation) experiment of Bax and Subramanian (1986), now in widespread use, was preceded by the pioneering work of MiilleF (1979) 7 years earlier. Muller described a pulse sequence that is not substantially different from the HMQC experiment of Bax and Subramanian (1986). 

Live DH, Armitage IM, Dalgarno DC, Cowburn D (1985) Two-dimensional H- Cd chemical-shift correlation maps by H-detected multiple-quantum NMR in metal complexes and metalloproteins. J Am Chem Soc 107 1775-1777 Martin GE, Crouch RC (1991) Inverse-detected two-dimensional NMR methods applications in natural products chemistry. J Nat Prod 54 1-70 Martin GE, Crouch RC (1994) Two-dimensional NMR experiments in natural and unnatural products chemistry. In Croasmun WR, Carlson RMK (eds) Two-dimensional NMR spectroscopy - applications for chemists and biochemists, chap 11, 2nd edn. VCH, New York, pp 873-914... 

S.E. Ashbrook, S. Wimperis, Novel two-dimensional NMR methods that combine single-quantum cross-polarization and multiple-quantum Mas of quadrupolar nuclei,... 

Pulse and Fourier Transform NMR Two-Dimensional NMR Multiple Quantum NMR Medical Imaging... 

S. P. Brown and S. Wimperis, Two-dimensional multiple-quantum MAS NMR of quadrupolar nuclei a comparison of methods. /. Magn. Reson., 1997,128,42-61. 

The structural assignment of both 29 and 30 was accomplished through extensive two-dimensional (2-D) NMR heteronuclear multiple quantum correlation (HMQC) and heteronuclear multiple bond correlation (HMBC) spectroscopic studies <2004T8189>. In the HMBC spectrum of 29, the proton at 8.64p.p.m. shows a strong correlation Jq-h with the carbonyl carbon (C-10) at 180.9 ppm and the proton at 8.82p.p.m. with the carbonyl carbon (C-5) at 181.7 ppm. The HMBC spectrum of 30 shows a significant strong correlation Vq h of the C-5 carbonyl carbon with the H-6 proton at 8.52 ppm and the H-4 proton at 8.52p.p.m. 

It is interesting to note that several of the concepts for improving NMR technology, as listed by Levy and Craik, in 1988, already have been partially or fully achieved (1) two-dimensional Fourier transform (FT NMR) (2) high-resolution NMR in solids (3) new types of pulse sequences (4) chemically induced dynamic nuclear polarization (5) multiple quantum NMR and (6) NMR imaging (MRI). 

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