Multi-quantum NMR

Multi-quantum transitions can only be observed indirectly by a modulation of the detected signal with the phase of the multi-quantum coherence. This modulation is achieved in an experiment by variation of an evolution time prior to detection. Repetitive detection of the signal for different evolution times provides the information about the evolution of the multi-quantum coherence. The indirect detection of spectroscopic information based on phase or amplitude modulation of the detected signal is the principle of multi-dimensional NMR spectroscopy [Eml]. Thus multi-quantum NMR is a special form of 2D NMR. Also, NMR imaging can be viewed as a special form of multi-dimensional NMR spectroscopy, where the frequency axes have been coded by the use of magnetic field gradients to provide spatial information. 

Chasse, W. Valentin, J. L. Genesky, G. D. Cohen, C. Saalwachter, K., Precise Dipolar Coupling Constant Distribution Analysis in Proton Multi-Quantum NMR of Elastomers. J. Chem. Phys. 2011,134, 044907. 

In the following, we will discuss heteronuclear polarization-transfer techniques in four different contexts. They can be used as a polarization-transfer method to increase the sensitivity of a nucleus and to shorten the recycle delay of an experiment as it is widely used in 1H-13C or 1H-15N cross polarization. Heteronuclear polarization-transfer methods can also be used as the correlation mechanism in a multi-dimensional NMR experiment where, for example, the chemical shifts of two different spins are correlated. The third application is in measuring dipolar coupling constants in order to obtain distance information between selected nuclei as is often done in the REDOR experiment. Finally, heteronuclear polarization transfer also plays a role in measuring dihedral angles by generating heteronuclear double-quantum coherences. 

The selectivity of multi-quantum filters is demonstrated in Fig. 7.2.28 by spectra recorded for poly(isoprene) (PI) with the multi-quantum filter from Fig. 7.2.26(d) [Schl 1, Schl2, Schl3]. The conventional single-quantum H NMR spectrum of PI shows a broad line with little to no chemical-shift resolution. But the double- and triple-quantum filtered spectrum of the material exhibit peaks at different chemical shifts which can be assigned primarily to the signals from CH2 and CH3 groups, respectively. At longer... 

In NMR, multi-quantum coherences can be excited by just two pulses [Eml, Muni] but for rigid samples multi-pulse sequences are more efficient (cf. Fig. 7.2.26) [Bau2, Muni]. Because the receiver coil in the NMR experiment corresponds to a magnetic dipolar detector, only dipolar single-quantum coherence can be detected directly and not multi-polar multi-quantum coherences. However, the latter can be detected indirectly by methods of 2D NMR spectroscopy [Eml]. 

The volume opens with a report by L.D. Field on Multiple Quantum NMR of Partially Aligned Molecules following this is an account on Solid-State NMR Studies of Molecular Motion by M.J. Duler C. Odin reviews NMR Studies of Phase Transitions Application of Multi-way Analysis to 2D NMR Data is covered by H.T. Pedersen, M. Dyrby, S.B. Engelsen and R. Bro the final contribution is on High Resolution Protein Structure Determination by NMR and it is provided by H. Takashima. My sincere thanks go to all of these reporters and to the production staff at Elsevier for their assistance in the creation of this volume. 

The main emphasis of current carbohydrate structural analysis is the applicability of modern multi-dimensional NMR for solving the two crucial problems in complex carbohydrate structural analysis, namely, the elucidation of the sequence of glycosyl residues and the solution conformation and dynamics of a carbohydrate (150). Techniques include 2D Total Correlation Spectroscopy (TOCSY), Nuclear Overhauser effect spectroscopy (NOESY), rotational nuclear Overhauser effect spectroscopy (ROES Y),hetero-nuclear single quantum coherence (HSQC), heteronuclear multiple quantum correlation (HMQC), heteronuclear multiple bond correlation (HMBC), and (pseudo) 3D and 4D extensions. 

Despite the extremely wide use of multi-dimensional NMR techniques for conformational studies of biological macromolecules, at present, only a small part of the information contained in the NMR spectrum can be used for structure determination [118,119]. The reason is that there are no well-established correlations between NMR chemical shifts and the structural parameters [118] and only a few useful correlations besides Karplus relations [120,121] for nuclear spin-spin coupling constants. As a consequence, a great deal of important information about the system is not available without turning to quantum chemical approaches for the theoretical interpretation. 

Multi-quantum spectroscopy is important in solid-state NMR because it has several valuable advantages (1) it can be used to simplify a crowded spectrum because the higher the order of the transitions, the fewer in number they are (2) it can directly reflect structural and dynamic information because the creation of a high-order quantum transition requires many spins to evolve cooperatively (3) in some cases, multi-quantum decoupling is less demanding (4) the efiiect of a gradient field on an n-quantum transition (in spin-1/2 systems) is n times that of a single quantum transition. 

---