Heteronuclear multiple principles

NMR has become a standard tool for structure determination and, in particular, for these of Strychnos alkaloids. The last general article in this field was authored by J. Sapi and G. Massiot in 1994 [65] and described the advances in spectroscopic methods applied to these molecules. More recently, strychnine (1) has even been used to illustrate newly introduced experiments [66]. We comment, here, on their advantages and sum up the principles of usual 2D experiments in Fig. (1) and Fig. (2) (COSY Correlation SpectroscopY, TOCSY TOtal Correlation SpectroscopY, NOESY Nuclear Overhauser Enhancement SpectroscopY, ROESY Rotating frame Overhauser Enhancement SpectroscopY, HMQC Heteronuclear Multiple Quantum Coherrence, HMBC Heteronuclear Multiple Bond Correlation). This section updates two areas of research in the field new H and 13C NMR experiments with gradient selection or/and selective pulses, 15N NMR, and microspectroscopy. To take these data into account, another section comments on the structure elucidation of new compounds isolated from Strychnos. It covers the literature from 1994 to early 2000. 

Korb, J.-R, Monteilhet, L., McDonald, P.J., and Mitchell, J. 2007. Microstructure and texture of hydrated cement-based materials A proton field cycling relaxometry approach. Cem. Conor. Res. 37 295-302. Koskela, H., Kilpelainen, L, Heikkinen, S., and Cahsqc, L.R. 2003. An application of a Carr-PurceU-Meiboom-Gill-type sequence to heteronuclear multiple bond correlation spectroscopy. J. Magn. Reson. 164 228-232. Kozlov, RV. and Burdygina, G.I. 1983. The structure and properties of solid gelatin and principles of their modification. Polymers 24 651-666. 

We have implemented the principle of multiple selective excitation (pulse sequence II in fig. 1) thereby replacing the low-power CW irradiation in the preparation period of the basic ID experiment by a series of selective 180° pulses. The whole series of selective pulses at frequencies /i, /2, , / is applied for several times in the NOE build-up period to achieve sequential saturation of the selected protons. Compared with the basic heteronuclear ID experiment, in this new variant the sensitivity is improved by the combined application of sequential, selective pulses and the more efficient data accumulation scheme. Quantitation of NOEs is no longer straightforward since neither pure steady-state nor pure transient effects are measured and since cross-relaxation in a multi-spin system after perturbation of a single proton (as in the basic experiment) or of several protons (as in the proposed variant) differs. These attributes make this modified experiment most suitable for the qualitative recognition of heteronuclear dipole-dipole interactions rather than for a quantitative evaluation of the corresponding effects. 

Homonuclear or heteronuclear Hartmann-Hahn mixing periods are versatile experimental building blocks that form the basis of a large number of combination experiments (see Section XIII). In practice, the actual multiple-pulse sequence that creates Hartmann-Hahn mixing conditions can usually be treated as a black box with characteristic properties. In this section, design principles and practical approaches for the development of Hartmann-Hahn mixing sequences are discussed. 

This pulse generates a mixture of heteronuclear double- and zero-quantum coherence, which then evolves during tv In principle this term evolves under the influence of the offsets of spins 1 and 2 and the coupling between them. However, it has already been noted that the offset of spin 1 is refocused by the centrally placed 180° pulse, so it is not necessary to consider evolution due to this term. In addition, it can be shown that multiple-quantum coherence involving spins i and j does not evolve under the influence of the coupling, Jip between these two spins. As a result of these two simplifications, the only evolution that needs to be considered is that due to the offset of spin 2 (the carbon-13). 

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