Homonuclear chemical-shift correlated spectra

Section 11, A demonstrates how homonuclear chemical-shift correlated spectra provide additional information for NMR studies of phosphorus cell metabolites. Because the homonuclear method requires the presence of 3ip 3ip spin-spin coupling, no data are obtained for monophosphorylated... 

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. 

Filiurr 5. Homonuclear Chemical Shift Correlated (Cosy <5i ZD-NMR Spectrum of Amodlaquine H>drochloride... 

Ulrich E L, Westler W M, Markley J L 1983 Reassignments in the H NMR spectrum of flavin adenine dinucleotide by two-dimensional homonuclear chemical shift correlation. Tetrahedron Lett 24 473-476... 

Fig. 2. A 146-MHz, > P homonuclear chemical-shift correlated two-dimensional spectrum of an equimolar (50 mM) mixture of ADP and ATP. The data set (128 X 256) is disfjayed in the stacked-plot mode. Each trace corresponds to an increment in the evolution time. The data were acquired in 45 min. The stacked plot took 15 min to complete. The diagonal, which makes a 45 angle with the F, (ordinate) and Fj (abscissa) axes, reproduces the one-dimensional spectrum. The off-diagonal peaks correlate peaks on the diagonal that are spin-spin coupled to each other. Peak identifications can be found in Fig. 3.

Some of the most important 2D experiments involve chemical shift correlations between either the same type of nuclei (e.g., H/ H homonu-clear shift correlation) or between nuclei of different types (e.g., H/ C heteronuclear shift correlation). Such experiments depend on the modulation of the nucleus under observation by the chemical shift frequency of other nuclei. Thus, if H nuclei are being observed and they are being modulated by the chemical shifts of other H nuclei in the molecule, then homonuclear shift correlation spectra are obtained. In contrast, if C nuclei are being modulated by H chemical shift frequencies, then heteronuclear shift correlation spectra result. One way to accomplish such modulation is by transfer of polarization from one nucleus to the other nucleus. Thus the magnitude and sign of the polarization of one nucleus are modulated at its chemical shift frequency, and its polarization transferred to another nucleus, before being recorded in the form of a 2D spectrum. Such polarization between nuclei can be accomplished by the simultaneous appli-... 

The resulting spectrum displays crosspeaks correlating carbon chemical shifts in f2 and proton shifts in f] which are further spread by homonuclear proton couplings in fi. Fig. 6.32 displays a part of the carbon-proton shift correlation spectrum of the palladium complex 6.11. Despite the extensive crowding in the aromatic region, the carbon shifts are sufficiently dispersed to resolve all correlations (note some resonances are broadened by restricted dynamic processes within the molecule and some are split by coupling to phosphorus). 

The LR COSY-45° spectrum and H-NMR chemical shifts of an isoprenyl coumarin are given below. Determine the long-range H/ H homonuclear correlations based on the long-range COSV45° spectrum. Demonstrate with reference to problem -5.15 how they can be helpful in interconnecting different spin systems ... 

The SECSY spectrum of an isoprenyl coumarin along with the H-NMR chemical shifts are shown. Determine the homonuclear shift correlations between various protons based on the SECSY spectrum. 

The HMQC spectrum, H-NMR chemical shift assignments, and C-NMR data of vasicinone are shown. Consider the homonuclear correlations obtained from the COSY spectrum in Problem 5.14, and then determine the carbon framework of the spin systems. 

The appearance of a homonuclear 2D spectrum is different from what you have seen for HETCOR, the H-13C correlation. Because both frequency scales, F2 and F, are lH chemical-shift scales, we can observe the transfer of coherence from Ha to Hb as a crosspeak at F = 2a, F2 = b (Fig. 9.18, lower right), as well as the opposite sense of transfer from Hb to Ha, a symmetrically disposed crosspeak at F = 2b, F2 = 2a (upper left crosspeak). In HETCOR we observe 13C coherence (in F2) that was transferred from the attached proton, whose chemical shift appears in F. Transfer in the opposite sense (13C to H) is not possible... 

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