Correlated spectroscopy coupled proton correlation

One kind of 2D NMR is called COSY, which stands for correlated spectroscopy With a COSY spectrum you can determine by inspection which signals correspond to spin coupled protons Identifying coupling relationships is a valuable aid to establishing a molecule s connectivity... 

In the case of an unknown chemical, or where resonance overlap occurs, it may be necessary to call upon the full arsenal of NMR methods. To confirm a heteronuclear coupling, the normal H NMR spectrum is compared with 1H 19F and/or XH 31 P NMR spectra. After this, and, in particular, where a strong background is present, the various 2-D NMR spectra are recorded. Homonuclear chemical shift correlation experiments such as COSY and TOCSY (or some of their variants) provide information on coupled protons, even networks of protons (1), while the inverse detected heteronuclear correlation experiments such as HMQC and HMQC/TOCSY provide similar information but only for protons coupling to heteronuclei, for example, the pairs 1H-31P and - C. Although interpretation of these data provides abundant information on the molecular structure, the results obtained with other analytical or spectrometric techniques must be taken into account as well. The various methods of MS and gas chromatography/Fourier transform infrared (GC/FTIR) spectroscopy supply complementary information to fully resolve or confirm the structure. Unambiguous identification of an unknown chemical requires consistent results from all spectrometric techniques employed. 

TOCSY (total correlation spectroscopy) is an extension of the COSY experiment, in which the coherence transfer is not limited to a single jump from one proton to another via a J coupling. Instead, coherence is spread out over an entire spin system of coupled protons via multiple /-coupled jumps. For example, in a string of carbons CHa-CHb-CHc-CHd, coherence can be transferred by the TOCSY mixing sequence from Ha to Hc or from Ha to Hj. Thus, crosspeaks will be observed at F% = va and I = i b, vc or (Fig. B.5). 

I) Correlated Spectroscopy (COSY) or Homonutlear Correlation Spectro-scopy HOMCOR-2D) provides proton-proton coupling information ( H-lH connectivities). This information can also be obtained from a series of I-D spin decoupled spectra but difficulties arise with strongly coupled protons, overlapping multiplets and in optimizing conditions. 

The advantages of 2D NMR spectroscopy in monomer sequence analysis have been demonstrated for several copolymers. The copolymer of vinyl-idene chloride (V) and isobutylene (I) shows no correlation in the H COSY spectrum, since it has no /-coupled protons. However, 2D NOE spectroscopy... 

Correlations anticipated in various homonuclear ( H- H) and heteronuclear ( C- H) 2D NMR experiments are conceptualized in Eigure 5.1. A hypothetical model compound (the chemical shifts are not accurate and are for illustrative purposes only) with three aromatic protons and four side-chain protons on its three side-chain carbons is used to illustrate the information available from each experiment. A set of five experiments, in addition to the standard ID proton and carbon spectra, are useful for characterizing any model compound or lignin. The correlation spectroscopy (COSY) experiment correlates directly coupled protons (Figure 5.1a). 

The correlation spectroscopy (COSY) experiment provides a means of identifying mutually coupled protons and is the most widely used 2D experiment. It finds use when the homonuclear decoupling experiment is unsuitable, for example in complex spectra where selective decoupling is not possible because of resonance overlap. The COSY e qieri-ment is a very efficient way of establishing links when a large number of coupling networks need to be identified as it maps all correlations... 

The NMR and NMR spectra discussed in the preceding sections have one frequency axis and one intensity axis 2-D NMR spectra have two frequency axes and one intensity axis. The most common 2-D spectra involve H- H shift correlations, which identify coupled protons (that is, protons that split each other s signal). This is called H- H shift-COrrelated SpectroscopY, known by the acronym COSY. 

The significance of n.m.r. spectroscopy for structural elucidation of carbohydrates can scarcely be underestimated, and the field has become vast with ramifications of specialized techniques. Although chemical shifts and spin couplings of individual nuclei constitute the primary data for most n.m.r.-spectral analyses, other n.m.r. parameters may provide important additional data. P. Dais and A. S. Perlin (Montreal) here discuss the measurement of proton spin-lattice relaxation rates. The authors present the basic theory concerning spin-lattice relaxation, explain how reliable data may be determined, and demonstrate how these rates can be correlated with stereospecific dependencies, especially regarding the estimation of interproton distances and the implications of these values in the interpretation of sugar conformations. 

A more useful type of 2D NMR spectroscopy is shift-correlated spectroscopy (COSY), in which both axes describe the chemical shifts of the coupled nuclei, and the cross-peaks obtained tell us which nuclei are coupled to which other nuclei. The coupled nuclei may be of the same type—e.g., protons coupled to protons, as in homonuclear 2D shift-correlated experiments—or of different types—e.g., protons coupled to C nuclei, as in heteronuclear 2D shift-correlated spectroscopy. Thus, in contrast to /-resolved spectroscopy, in which the nuclei were being modulated (i.e., undergoing... 

The HOHAHA spectrum (100 ms) of podophyllotoxin is presented. The HOHAHA, or TOCSY (total correlation spectroscopy), spectrum (100 ms) shows coupling interactions of all protons within a spin network, irrespective of whether they are directly coupled to one another or not. As in COSY spectra, peaks on the diagonal are ignored as they arise due to magnetization that is not modulated by coupling interactions. Podophyllotoxin has only one large spin system, extending from the C-1 proton to the C4 and 015 protons. Identify all homonuclear correlations of protons within this spin system based on the crosspeaks in the spectrum. 

Homonuclear shift-correlation spectroscopy (COSY) is a standard method for establishing proton coupling networks. Diagonal and off-diagonal peaks appear with respect to the two frequency dimensions. 

---