Coupling-resolved spectra

Heteronuclear two-dimensional /-resolved spectra contain the chemical shift information of one nuclear species (e.g., C) along one axis, and its coupling information with another type of nucleus (say, H) along the other axis. 2D /-resolved spectra are therefore often referred to as /,8-spectra. The heteronuclear 2D /-resolved spectrum of stricticine, a new alkaloid isolated by one of the authors from Rhazya stricta, is shown in Fig. 5.1. On the extreme left is the broadband H-decoupled C-NMR spectrum, in the center is the 2D /-resolved spectrum recorded as a stacked plot, and on the right is the con tour plot, the most common way to present such spectra. The multiplicity of each carbon can be seen clearly in the contour plot. 

Figure 5.2 Presentation of 2D /-resolved spectra. In the ID plot (i), both 8 and / appeared along the same axis, but in the 2D /-resolved spectrum (ii), the multiplets are rotated by 90° at their respective chemical shifts to generate a 2D plot with the chemical shifts (8) and coupling constants (/) lying along two different axes, (iii) The 2D /-resolved spectrum as a contour plot.

Figure 5.5 shows the heteronuclear 2Dy-resolved spectrum of camphor. The broad-band decoupled C-NMR spectrum is plotted alongside it. This allows the multiplicity of each carbon to be read without difficulty, the F dimension containing only the coupling information and the dimension only the chemical shift information. If, however, proton broad-band decoupling is applied in the evolution period tx, then the 2D spectrum obtained again contains only the coupling information in the F domain, but the F domain now contains both the chemical shift and the coupling information (Fig. 5.6). Projection of the peaks onto the Fx axis therefore gives the Id-decoupled C spectrum projection onto the F axis produces the fully proton-coupled C spectrum.

In homonuclear 2D /-resolved spectra, couplings are present during <2 in heteronuclear 2D /-resolved spectra, they are removed by broad-band decoupling. This has the multiplets in homonuclear 2D /-resolved spectra appearing on the diagonal, and not parallel with F. If the spectra are plotted with the same Hz/cm scale in both dimensions, then the multiplets will be tilted by 45° (Fig. 5.20). So if the data are presented in the absolute-value mode and projected on the chemical shift (F2) axis, the normal, fully coupled ID spectrum will be obtained. To make the spectra more readable, a tilt correction is carried out with the computer (Fig. 5.21) so that Fi contains only /information and F contains only 8 information. Projection... 

One of the most commonly studied systems involves the adsorption of polynuclear aromatic compounds on amorphous or certain crystalline silica-alumina catalysts. The aromatic compounds such as anthracene, perylene, and naphthalene are characterized by low ionization potentials, and upon adsorption they form paramagnetic species which are generally attributed to the appropriate cation radical (69, 70). An analysis of the well-resolved spectrum of perylene on silica-alumina shows that the proton hyperfine coupling constants are shifted by about four percent from the corresponding values obtained when the radical cation is prepared in H2SO4 (71). The linewidth and symmetry require that the motion is appreciable and that the correlation times are comparable to those found in solution. 

The frequency-resolved spectrum, at the right of Figure 2.2, shows that the single ZOBS transition is split into a multiplet, due to the coupling with the ZODS, which is lifetime broadened, exhibiting a linewidth F due to interaction with the bath states. The observation of such a spectrum provides evidence for IVR. The lifetime-imposed linewidth is proportional to the product of the mean-squared... 

Fig. 13. (a) 1H/(31P)/15N correlation of a mixture of Mes P( = NH) = NMes (compd. 2, Mes = 2,4,6-tri-t-butylphenyl) and Mes P(NHMes )-N1 = N2 = N3 (compd. 3) with correlations involving the iVH and aromatic protons in the P-Mes substituents. The spectrum was obtained with the pulse sequence shown in Fig. 11a. The tx noise around S1H = 5.1 is due to a solvent signal (CH2C12) which is 4 105 times more intense than that of the 15N-satellites of the iVH-resonance of 3. (b) Expansion of a -detected 2D-/P N-resolved spectrum of the same mixture with correlations of the aromatic protons in the P-Mes -substituents as obtained with the pulse sequence shown in Fig. 12. 2q cross-sections of the 2D-spectrum at the chemical shifts of the aromatic protons of 2 and 3 are given in (c) and (d), respectively, and reveal the presence of one (2) and three (3) resolved JP N couplings. Reproduced from Ref. 43 by permission of John Wiley Sons.

Morton et al.135,141) were the first to study the poly(butadienyl)lithium anionic chain end using (b). They found no evidence of 1,2-chain ends and concluded that only 1,4-structures having the lithium cr-bonded to the terminal carbon were present. A later study by Bywater et al.196), employing 1,1,3,4-tetradeuterobutadiene to minimize the complexity of the spectrum that arises from proton-proton coupling, found that the 1 1 adduct with d-9 fert-butyllithium in benzene exists as a mixture of the cis and trans conformers in the ratio 2.6 1. Glaze et al. 36) obtained a highly resolved spectrum of neopentylallyllithium in toluene and found a cis trans ratio of about 3 1. 

A well-resolved spectrum of the /2-chloroethyl radical was obtained by Kawamura and coworkers in solution at various temperatures194. The negative g-shift indicated that the unpaired electron was delocalized on orbitals around the chlorine nucleus. Moreover, the two /2-protons were equivalent and exhibited a coupling (10.25 G) which was inferior to the minimum value expected by the (A + B cos26) law generally used for protons in the /2-... 

How do heteronuclear couplings behave in a homonuclear /-resolved spectrum See 8.2.3. 

D shift-correlated spectrum. When the Ft axis corresponds to a coupling constant scale (e.g., H- H or H- - C), we generate a 2D 7-resolved spectrum. 

The spin-echo pulse sequence can also be applied to produce homonuclear (e.g., H) J-resolved spectra. For such a system it is not possible to apply broadband decoupling, so the F2 dimension might be expected to display the ordinary coupled H spectrum. However, because the coupling information is independently available, it is not difficult to process the data in such a way that only the chemical shifts are displayed in the F2 dimension, as illustrated in Fig. 10.7. 

In the J-resolved projection, the aromatic rings of the SCAL are clearly present where as the polymer resonances have dropped out of the spectrum. In addition, the proton coupling constants arising from alloc group are readily measured as seen in the expanded spectrum (Fig. 13). If a more detailed measure of the couplings is desired, then the full 2D J-resolved spectrum can be evaluated in the normal manner as exemplified in Fig. 14. A similar method to obtain accurate proton-proton coupling constants based on E.COSY spectra has also appeared recently (43). 

Figure 7.6. The proton-detected selective heteronuclear J-resolved spectrum of adenosine 73. The HI proton has been selectively inverted and the doublet splittings in f record its long-range couplings to C8 and C2 (reproduced with permission from reference [6]).

Figure 7.14. Measurement of proton heteronuclear couplings from the tilted homonuclear J-resolved spectrum (a). Spectrum (b) is the f2 projection of (a) and displays only proton shifts and coupling. Spectrum (c) is the conventional ID spectrum displaying shifts and both H- H and couplings.

Figure 7.16. Fig. 16.The direct homonuclear J-resolved spectrum and the indirect homonuclear J-resolved spectrum of 7.1. Both experiments present homonuclear couplings in fi dispersed by either the proton or the carbon chemical shift respectively.

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