Two dimensional, carbon-13 NMR

J-resolved two-dimensional carbon-13 NMR spectra [60, 61] separate chemical shifts and coupling constants in the two-dimensional JCH, <5C plane. Their practical value can be estimated on looking at Fig. 2.52. In the one-dimensional 13C NMR spectrum of... 

Muller L, Kumar A and Ernst RR (1975) Two-dimensional carbon-13 NMR spectroscopy. Journal of Chemical Physics 63 5490. 

L.D. Hall and G.A. Morris, Measurement of carbon-13 proton coupling constants in oligosaccharides by two-dimensional carbon-13 NMR spectroscopy, Carbohyd. Res. 82, 175 (1980). 

The data flux of a two-dimensional carbon-proton shift correlation is similiar to that described in Fig. 2.50(a) for a. /-resolved 2D CMR experiment, with one difference Instead of carbon-proton couplings JCH, proton chemical shifts (iH are stored in the evolution time tl. Fourier transformation in the (2 domain thus yields a series of NMR spectra with carbon-13 signals modulated by the attached proton Larmor frequencies. A second Fourier transformation in the domain generates the dH, Sc matrix of a two-dimensional carbon-proton correlation. 

The development of carbon-13 NMR during the last eight years has been characterized by a continual increase in the sensitivity and quality of spectra. A reduction in measuring time - equivalent to an enhancement in sensitivity has been achieved mainly by cryomagnet technology. The efficiency with which NMR information can be obtained has been substantially improved by new computer-controllable pulse sequences for one-and two-dimensional NMR experiments. A selection of these new methods, in particular, those used for multiplicity analysis and homo- or heteronuclear shift correlations, is presented in chapter 2 of this edition. 

Asakura, T. Nakayama, N. Demura, M. Asano, A. Carbon-13 NMR spectral assignments of regioirregular polypropylene determined from two-dimensional INADEQUATE spectra and chemical shift calculations. Macromolecules 1992, 25, 4876-4881. 

Spin—lattice relaxation is the time constant for the recovery of magnetiTation along the z-axis in a NMR experiment. Various methods are available for the measurement of spin lattice relaxation times. The interested reader is referred to the series of monographs echted by Levy on Carbon-13 NMR spectroscopy [44, 45] for more details. The energy transfer between nuclear moments and the lattice , the three-dimensional system containing the nuclei, provides the mechanism to study molecular motion, e.g. rotations and translations, with correlation times of the order of the nuclear Larmour frequencies, tens to hundreds of MHz. We will limit our chscussion here to the simple inversion-recovery Tj relaxation time measurement experiment, which, in addition to providing a convenient means for the quick estimation of Tj to establish the necessary interpulse delay in two-dimensional NMR experiments, also provides a useful entry point into the discussion of multi-dimensional NMR experiments. 

One- and two-dimensional NMR experiments were performed to examine the interactions between a hydrophobic pollutant, pyrene and cuticular material. Carbon-13 NMR experiments on labelled pyrene were performed. MAS... 

Robinson J A, Turner D L 1982 Total assignment of the carbon-13 NMR spectrum of monensin by two-dimensional correlation spectroscopy. J Chem Soc Chem Commun 148-151... 

Tbrpstra D 1979 Two-dimensional Fourier transform carbon-13 NMR. In Levy G C (ed) Topics in carbon-13 NMR spectroscopy, vol 3. John Wiley New York, 62-78... 

G. Bodenhausen, R. Freeman, and D.L. Turner, Two-dimensional J spectroscopy Proton-coupled carbon-13 NMR, J. Chem. Phys. 65, 839 (1976). 

G. Bodenhausen and R. Freeman, Correlation of proton and carbon-13 NMR spectra by heteronuclear two-dimensional spectroscopy, J. Magn. Resonance 28, 471 (1977). 

If the amount of the sample is sufficient, then the carbon skeleton is best traced out from the two-dimensional INADEQUATE experiment. If the absolute configuration of particular C atoms is needed, the empirical applications of diastereotopism and chiral shift reagents are useful (Section 2.4). Anisotropic and ring current effects supply information about conformation and aromaticity (Section 2.5), and pH effects can indicate the site of protonation (problem 24). Temperature-dependent NMR spectra and C spin-lattice relaxation times (Section 2.6) provide insight into molecular dynamics (problems 13 and 14). 

Figures 13.7 and 13.8 are two examples of two-dimensional NMR spectroscopy applied to polymers. Figure 13.7 is the proton homonuclear correlated spectroscopy (COSY) contour plot of Allied 8207A poly(amide) 6 [29]. In this experiment, the normal NMR spectrum is along the diagonal. Whenever a cross peak occurs, it is indicative of protons that are three bonds apart. Consequently, the backbone methylenes of this particular polymer can be traced through their J-coupling. Figure 13.8 is the proton-carbon correlated (HETCOR) contour plot of Nylon 6 [29]. This experiment permits the mapping of the proton resonances into the carbon-13 resonances.

In future, the methods of two-dimensional NMR spectroscopy (Section 2.10) will be the ones used for the unequivocal signal identifications of longer chain peptides. Complete 13C shift assignments are achieved from carbon-proton shift correlations via two- and three-bond couplings, as is demonstrated for the orange-red chromopeptide antibiotic actinomycin D (Fig. 5.13, Table 5.28, [603, 812]). 

Two-Dimensional Experiments A full NMR analysis of a carbohydrate, in which each lH and 13C peak in the spectrum is assigned to a particular position in the molecule, requires the use of two-dimensional (2D) NMR. In a 2D spectrum, there are two chemical shift scales (horizontal and vertical) and a spot appears in the graph at the intersection of two chemical shifts when two nuclei ( H or 13C) in the molecule are close to each other in the structure. For example, one type of 2D spectrum called an HSQC spectrum presents the chemical shift scale on the horizontal (v) axis and the 13 C chemical shift scale on the vertical (y) axis. If proton Ha is directly bonded to carbon Ca, there will be a spot at the intersection of the H chemical shift of Ha (horizontal axis) and the 13 C chemical shift of Ca (vertical axis). Because the peaks are spread out into two dimensions, the chances of overlap of peaks are much less and we can count up the number of anomeric and... 

However, in contrast to these sequences, in HETCOR the proton shift is in Fx and the carbon-13 shift is in F2. In the early days of two-dimensional NMR this was a popular sequence for shift correlation as it is less demanding of the spectrometer there are no strong signals from protons not coupled to carbon-13 to suppress. 

Summers MF, Marzilli LG, Bax A (1986) Complete Proton and Carbon-13 Assignments of Coenzyme B12 Through the Use of New Two-Dimensional NMR Experiments. J Amer Chem Soc 108 4285... 

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