Coupling, in NMR spectroscopy

J. H. Prestegard X. B. Yi, Structure and Dynamics of Carbohydrates Using Residual Dipolar Couplings. In NMR Spectroscopy and Computer Modeling of Carbohydrates Recent Advances] J. F. G. Vliegenthart, R. J. Woods, Eds. John Wiley Sons Chichester, 2006 Vol. 930, pp 40-59. 

You may wonder why it is that we observe hydrogen couplings in NMR spectroscopy, yet we do not notice the converse, namely, carbon couplings, in H NMR. The answer lies in the low natural abundance of the NMR-active isotope and the high natural abundance of the nucleus. Thus, we could not detect coupling in our proton spectra. 

Decoupling (Section 13 17) In NMR spectroscopy any process that destroys the coupling of nuclear spins between two nuclei Two types of decoupling are employed in NMR spectroscopy Broadband decoupling removes all the H- C couplings ojf resonance decoupling removes all H- C couplings except those between directly bonded atoms... 

Phosphorus has only one stable isotope, J P, and accordingly (p. 17) its atomic weight is known with extreme accuracy, 30.973 762(4). Sixteen radioactive isotopes are known, of which P is by far the most important il is made on the multikilogram scale by the neutron irradiation of S(n,p) or P(n,y) in a nuclear reactor, and is a pure -emitter of half life 14.26 days, 1.7()9MeV, rntan 0.69MeV. It finds extensive use in tracer and mechanistic studies. The stable isotope has a nuclear spin quantum number of and this is much used in nmr spectroscopy. Chemical shifts and coupling constants can both be used diagnostically to determine structural information. 

All have zero nuclear spin except (33.8% abundance) which has a nuclear spin quantum number this isotope finds much use in nmr spectroscopy both via direct observation of the Pt resonance and even more by the observation of Pt satellites . Thus, a given nucleus coupled to Pt will be split into a doublet symmetrically placed about the central unsplit resonance arising from those species containing any of the other 5 isotopes of Pt. The relative intensity of the three resonances will be (i X 33.8) 66.2 ( x 33.8), i.e. 1 4 1. 

Contact shifts give information on the electronic structure of the iron atoms, particularly on the valence distribution and on the magnetic coupling within polymetallic systems. The magnetic coupling scheme, which is considered later, fully accounts for the variety of observed hyperfine shifts and the temperature dependence. Thus, through the analysis of the hyperfine shifts, NMR provides detailed information on the metal site(s) of iron-sulfur proteins, and, thanks to the progress in NMR spectroscopy, also the solution structure 23, 24 ). 

On-line coupling of separation techniques to NMR has recently been reviewed [459,651-653], and solvent suppression methods in NMR spectroscopy in particular [654],... 

It is clear that NMR spectroscopy could be used to detect certain nuclei (e.g. H, i9p, 3ip) and, also to estimate them quantitatively. The real usefulness of NMR spectroscopy in chemistry is based on secondary phenomena, the chemical shift and spin-spin coupling and, to a lesser extent, on effects related to the time-scale of the NMR experiment. Both the chemical shift and spin-spin coupling reflect the chemical environment of the nuclear spins whose spin-flips are observed in the NMR experiment and these can be considered as chemical effects in NMR spectroscopy. 

As in many other aspects of polymer stereochemistry, polypropylene also plays a central role in NMR spectroscopy. Since 1962 numerous articles have dealt with the interpretation of its proton spectrum (125-128) the state of knowledge at the end of that decade has been well described by Woodbrey (117). The difficulty in this study stems fiom two factors The narrow frequency range comprising the entire spectmm and the large homonuclear coupling between CH2, CH, and CH3 protons. The whole spectrum is within a range of <1.5... 

The hyperfine interactions of proton nuclei with the unpaired electron in a radical splits the proton signals into doublets with large coupling constants (up to 100 MHz) in comparison to the J values of 10 Hz observed in NMR spectroscopy. For the small magnetic field employed in an ESR spectrometer, all uncoupled protons absorb at effectively the same rf. The ENDOR spectmm appears as a series of doublets with lines spaced equally upheld and downfield from the expected absorbance of an uncoupled proton. 

In the future, we can expect the development of novel experimental techniques in solid-state NMR spectroscopy for investigation of functioning catalysts. Important goals are (i) the enhancement of the sensitivity of solid-state NMR spectroscopy, for example, by a selective enhancement of the nuclear polarization taking advantage of laser-polarized xenon, (ii) increases in the temperature range accessible for the characterization of solid-catalyzed reactions, and (iii) the coupling of NMR spectroscopy with other techniques such as mass spectrometry. Furthermore, modern two-dimensional techniques of solid-state NMR spectroscopy such as MQMAS NMR spectroscopy will be applied to improve the resolution of the spectra. 

Recourse to innovations in NMR spectroscopy is essential for further advances in the investigation of complex saponins. The use of a 600 MHz spectrometer gives the required sensitivity, consistent with the low amounts available by modern isolation techniques, while the use of sequential ID and inverse-detected 2D NMR techniques, couples the short time necessary to perform the experiments with the selfconsistency of the obtained results and, thus, the unambiguous structure assignment [128]. 

While the coupling of NMR spectroscopy and MS in the same system is undoubtedly the most productive of the possible multiple hyphenations, others have been performed which deserve consideration. Probably the easiest to implement is the inclusion of a UV diode-array detector (DAD) to enable UV... 

Directly coupled HPLC-NMR spectroscopy has been used in a number of other studies of chemical impurities. An impurity in a bulk drug sample of the glycinamide ribonucleotide transformylase inhibitor AG2034, shown below ... 

Figure 3.10 Comparison of lipoprotein standards and HPLC-resolved lipoproteins, (a) Partial 600 MHz HPLC-1 H NMR spectra (80.5-2.0) of the HDL, LDL and VLDL fractions separated by using directly coupled HPLC-NMR spectroscopy at 15.5, 58.9 and 86.6 min, respectively, (b) Partial 600 MHz 1H NMR spectra (8 0.5-2.0) of standard (std) HDL, LDL and VLDL. Abbreviations VLDL1, LDL1 and HDL1 indicate the resonances from the terminal methyl groups of the mobile fatty acid chains bound in the various lipoproteins VLDL2, LDL2 and HDL2 indicate resonances from the methylene groups of the mobile fatty acid chains...

The work horses of separation techniques are, and maybe always will be, high performance liquid chromatography (HPLC) and capillary high performance liquid chromatography (CHPLC), respectively, followed by gas chromatography (GC), which is applicable for volatile and temperature-resistant compounds. Thus, it is only logical, that HPLC was the first separation technique which was coupled to NMR spectroscopy, thus leading nowadays to a powerful tool in structure elucidation. 

Axenrod, T., Structural effects on the one-bond 15N-H coupling constant, in NMR Spectroscopy of Nuclei Other Than Protons, Axenrod, T. and Webb, G.A., Eds., Wiley Interscience, New York, 1974. 

A resonance in NMR spectroscopy is generally characterized by five parameters, the chemical shift (8), the intensity or the area of the resonance (proportional to concentration), the multiplet structure [related to the spin-spin coupling constant (J)], and two relaxation... 

In NMR spectroscopy, double resonance refers to experiments or techniques involving irradiation of two different isotopes, for example, 13C and 1H. Such experiments are used to manipulate spin couplings or to enhance the SNR of a lower gyromagnetic... 

Similar procedures might turn out to be necessary in the case of 2D vibrational spectroscopy. The spectroscopy demonstrated so far essentially reflects the first step of the procedure in NMR spectroscopy (COSY). So far, we have investigated only the amide I subspace. However, all amide vibrations (N-H, amide II, etc.) might turn out to be equally important in revealing the required information, each of them giving different and hopefully complementary pieces of information. Couplings between different amide subspaces need to be explored. Incoherent population transfer out of the amide I transition appears to be very efficient (Ti = 1.2 ps), and the mechanism and the course of this transfer is completely unknown. 

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