J-coupling multiplets

Similar to lifetime effects based on chemical shift differences, the J coupling can be exploited to study changes in reaction rates as well (49). Using this approach, the collapse of the J-coupling multiplets in ethanol (from the hydroxyl proton coupling with the CH2 protons) was monitored as a function of pressure... 

One of the most attractive features of the CIDNP multiplet effect is that it allows detennination of the sign of the J coupling, which is often difficult to do by other methods. 

The individual J,Mj) multiplets cannot be, in principle, described by a single Slater determinant. This is a consequence of the general rules of coupling of two angular momenta eigenfunctions of all projections of the total angular momentum which is lower than the maximal possible (L + S), are represented as linear combinations of several determinants [36]. 

The spacing between lines within a multiplet is typically constant furthermore, the spacing in each coupled multiplet is constant. This constant distance, which is independent of // , is called the coupling constant, J. and is expressed in Hz. The value of the coupling constant depends on the structural relationship of the coupled H s, and becomes a valuable tool for structure proof. Some typical values are given in Table 12-4. 

The ratio A vIJ must be larger than about 8 Av is the distance in Hz between the midpoints of the coupled multiplets. J is the coupling constant. 

Figure 2.3 shows the spectra lined up on a ppm scale rather than a hertz scale (the vertical scale is increased and the tall methyl and OH peaks are clipped off). This is the universally accepted format for presenting NMR data. Although all the peaks (resonances) appear at the same place in the spectrum (same chemical shift in ppm), the multiplet patterns appear to shrink horizontally as we go to higher field strength because the J couplings in hertz get smaller and smaller on the ppm scale. For example, on a 200-MHz spectrometer, a typical... 

If the separation between two resonances is A Hz and their coupling is J Hz, then the rales above hold if A J. The multiplet is said to be first-order. When AfJ <5, distortions and extra lines appear, and the multiplet is then second-order. See Abraham et a/3 for a good description of how to analyse such spectra. A first-order analysis is adequate for solving all the problems in this workbook, although many of the multiplets are distinctly second-order. 

As the ratio of A 8v (the difference in chemical shift between two coupled nuclei) to J decreases, the relative intensities of the lines in a multiplet deviate further from first-order (e.g., Pascal triangle) ratios. Inner lines (those facing the coupled multiplet) increase in intensity, while outer lines lose intensity. This slanting of the multiplets is one type of second-order effect. At very small values of A Sv/J, not only may extra lines appear in the multiplets but also apparent line positions and spacings may not equate with true chemical shifts and coupling constants (e.g., deceptive simplicity and virtual coupling). 

Figure 2.6. Coupling multiplets of a spin which is coupled to A. one and B. two spin- /2 nuclei, and C. example of Sn in BaSnOj showing the J-coupling with crystallographically equivalent but magnetically inequivalent Sn, from Clayden, Dobson and Fern (1989) with permission of the Royal Society of Chemistry.

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