Second-Order Spectra—Strong Coupling

Hertz is too small in magnitude for the NMR instrument to be able to distinguish the separate peak components.) Nonequivalence of Jab and 7bc of course, happens most often when the chemical shifts of the protons on carbons A, B, and C are all quite different. 

Chains of any length can exhibit this phenomenon, whether or not they consist solely of methylene groups. For instance, the spectrum of the protons in the second methylene group of propylben-zene is simulated as follows. 

The splitting pattern gives a crude sextet, but the second line has a shoulder on the left, and the fourth line shows an unresolved splitting. The other peaks are somewhat broadened. 

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Strongly overlapping multiplets may be resolved by two-dimensional J,<5-spectros-copy2" 11G 118, where the first frequency domain (F,) contains coupling and the second frequency domain (F2) chemical shift information. The spectrum in Figure 2 (homonuclear [JH H, 6 ( H)]) demonstrates the use of this technique by showing unperturbed multiplets for ll signals. Second-order effects are principally not eliminated. Heteronuclear experiments [7uc,<5(13C)] are also common. 

The data in Fig. 5 illustrate very nicely a less well known feature of 2D C/H shift correlation. The cross-sections of C-2 and C-3 (Fig. 6) show first order multiplets for protons which are strongly coupled in the ID spectrum. This is because the 2D spectrum is dealing with the satellites in the spectrum (one carbon at a time) and whereas H-2 is strongly coupled to H-3, the satellites of H-2 are not strongly coupled to H-3 and vice yersa. As a corollary of this one must also be aware that it is possible for the 2D experiment to give second order multiplets when the ID spectrum is first order. 

Including all 15 vibrational modes, however, was not possible within the linear model in a satisfactory manner. In particular, inclusion of the i/3 (Ai) mode provided problems, as it is also significantly coupled to the excitation. The problem was traced to the lack of second-order coupling. On calculating these parameters, it was found that the 1 2 and r-s modes were strongly coupled, and undergo Duschinsky rotation. No correction of the first order terms was then required and the resulting spectrum from the 15-mode model is shown in Fig. 5(c). ... 

Several other examples of veiy strong intramolecular H bonding are known. For example, in tropolone the carbonyl band is essentially missing from the spectrum. The absorption that occurs near 1605 cm may involve some C—O displacement (Figure 7.9). f. Second-order coupling (Fermi resonance)... 

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