Doublets of doublets

The alcohol methine proton in the ds isomer is equatorial and thus has a 60° dihedral angle to all four adjacent protons that give rise to a pentet (which is really a doublet of doublets of doublets of doublets with equal coupling constants) with a coupling constant / = —3 Hz, which is seen in Figure 8.25. 

Another way to describe the situation above is to consider the Hb methylene protons as a doublet of doublet of doublet of doublets (dddd) where Vab = Vab = Jbc = Jac- With four distinct couplings, the sum of the line intensities for the Hb multiplet will be 2 = 16. By constructing a splitting tree for Hb and distributing the intensities for each doublet, one arrives at the same conclusion Hb is an apparent quintet with line intensities 1 4 6 4 1 = 16 (Figure 7.35). 

Even experienced chemists, however, often struggle to extract all of the coupling constants from resonances that have four couplings in them (doublet of doublet of doublet of doublets, or dddd) and even more complex multiplets. A straightforward systematic method exists, however, that allows for complete analysis of any (even the most complex) first-order multiplet. Practicing this method on the more easily analyzed ddd multiplets discussed above will allow the student to gain confidence in its usefulness. This systematic multiplet analysis was most succinctly presented by Hoye and Zhao, and is presented below. 

Figure 1.3. Quartet, doublet of doublets, pseudotriplet and threefold doublet (doublet of doublets of doublets)...

Fluorine spectra of two fluoroethylenes, both from the Japan Halon compilation [19], are shown m Figures 4 and 5. The splitting pattern of fluoroethene (doublet of doublets of doublets) is clarified by using a branching display above the peaks, from which coupling constants can be measured easily. The AA XX spectrum of 1,1-difluoroethene is also shown. 

Without considering chirality, one might expect that the signal for the fluorine of 1,2-dibromofluoroethane would be comprised of a doublet of triplets (six peaks), but as you can see from the actual fluorine spectrum given in Figure 2.9, the signal is split into eight peaks (doublet of doublets of doublets). 

You can infer from Example 8.9 that a triplet is really a doublet of doublets with both coupling constants are equal. Similarly, a quartet can be seen to be a doublet of doublets of doublets, with all three coupling constants equal. 

When the alkylating agent is a substituted allylic bromide, reaction could occur at either the a- or the y-carbon. Only a-attack is observed and the phosphonium salt (8) from regiospecific alkylation at the less hindered carbon is produced. Again, the two possible regioisomers are distinguishable by virtue of the difference in the multiplicity of their fluorine signals in the 19F NMR spectrum. Salt (8) appears as a doublet of doublet of doublets, whereas (9) would be expected to be a doublet of doublets. 

Figure 5.15 shows a pattern that may be called a doublet of doublets of doublets. After constructing a tree diagram, it is relatively easy to select appropriate peaks to use for the determination of the three coupling constants (solid arrows). 

The 400.13 MHz H-NMR spectrum in SO2CIF/SO2F2 at -105°C (Fig. 1) shows the signal of the proton H attached to the positively charged carbon atom at 9.10 ppm as a doublet of doublets of doublets. The coupling pattern results from coupling with at 4.35 ppm (doublet,... 

Resonances from the SiHs protons in the H NMR spectrum of p2P NHSiH3. The pattern is a doublet of doublets of doublets of triplets, but because of the relationships between couphng constants many lines overlap Jpu = 8, /hh = 4, NH = 3, — 2 Hz, approximately. 

---