Figure 9.35

large coupling (>8 Hz). If both couplings (Jap and Jap') are small, we have conformer (a). If one is large and one is small, we have either (b) or (c). Since we do not know which p proton resonance is the pro-R and which is the pro-S (i.e., the p and p labels could be swapped in Fig. 9.36), we cannot distinguish between these two possibilities. If the couplings are intermediate (6-8 Hz) we have a flexible molecule with averaging over more than one conformation. The example in Figure 9.35 is averaged J couplings (Japr = 8.65, Jap = 6.7 Hz) with a slight preference for conformations (b) and (c).

When measuring J couplings, one should keep in mind that the simple distance between peak maxima for a doublet is not always an accurate measure of the J coupling, especially if the peak width is similar to the J coupling. In-phase doublets "creep" together as peak

Figure 9.37
Figure 9.38

width increases, leading to an underestimate of the J value, whereas antiphase doublets move apart, leading to an overestimate. Figure 9.37 shows two calculated Lorenztian peaks with linewidth of 10 Hz, separated by a coupling of 7.5 Hz with one peak inverted. The sum is an antiphase doublet with a separation of 8.63 Hz, which would be mistakenly interpreted as an anti relationship (>8 Hz) instead of an average of conformers (actual J = 7.5 Hz). An in-phase doublet with the same 10 Hz linewidth and 7.5 Hz J value would give a separation of 5.63 Hz (Fig. 9.38).

0 0

Post a comment