Proton-coupled spectra

Compound 1 contains one phosphorus atom, so that the broad-band proton decoupled spectrum is extremely simple it consists of only one line at 8.5 ppm. This spectrum is shown in Fig. 22, together with the proton-coupled spectrum. 

The proton-coupled spectrum is much more informative. We can see immediately which protons show a measurable coupling with the phosphorus atom, because the pattern is clearly identifiable as two triplets separated by 28.7 Hz. This, as we have already seen in Table 1, is the two-bond coupling between the phosphorus and the methine proton. The triplets (intensity 1 2 1)... 

O NMR spectroscopic studies in HF-SbF5 with O-enriched hydronium ion have indicated strong 170-1 H coupling (Figure 4.1). In the proton-coupled spectrum, a quartet is observed at 8170 9 0.2 (with reference to S02 at 505 ppm) with i7o—1H = 106 1.5 Hz. 

As mentioned in Section 3.7.5, the l3C nucleus does not show coupling in XH NMR spectra (except for 13C satellites) due to the low natural abundance of 13C (1.1%) however, the same cannot be said about the reverse. The XH nucleus is >99% in natural abundance and effectively couples to the 13C nuclei. Because of the large. /CH values for 13C— H ( 110-320 Hz) and appreciable 2/CH, 3/CH values for l3C—C—3H and 13C—C—C— H (-0-60 Hz) couplings, proton-coupled I3C spectra usually show complex overlapping multiplets that are difficult to interpret (Figure 4.1a) the proton-coupled spectrum of cholesterol is hopelessly overlapped and difficult to decipher. 

FIGURE 6.2 Top, the proton-decoupled 15N (30.4 MHz) NMR spectrum of formamide in CDC13 referenced to external NH3. Second, the proton-decoupled 15N NMR spectrum (30.4 MHz) of ethylenediamine in CDC13. The proton-coupled spectrum is shown in the inset. Third, the proton-decoupled 15N spectrum (30.4 MHz) of pyridine in CDC1> Fourth, the proton-decoupled 15N (30.4 MHz) spectrum of quinine in CDC13. 

An example of an aromatic fluorine-containing compound can be found in Figure 6.7, where we have recorded the l9F spectra (both proton-coupled and decoupled) of fluorobenzene along with the H and l3C spectra. Once again we find a singlet for the fluorine atom in the ptoton-decoupled spectrum and a complex multiplet for the fluorine atom in the proton-coupled spectrum. The fluorine atom couples differently to the ortho-, meta-, and para-protons in this mono-substituted compound. Coupling constants for proton-fluorine can be found in Appendix F of Chapter 3. 

The proton decoupled 29Si spectrum of tetram-ethylsilane (TMS) is shown at the top of Figure 6.9 with the proton coupled spectrum for comparison as an inset. TMS is the obvious choice for a 29Si reference compound and we set it at zero ppm. The proton-coupled spectrum is quite interesting because the 29Si nucleus is coupled to 12 equivalent protons in TMS. First order rules predict a multiplet with 13 peaks. There are 9 peaks clearly visible and 11 with a little imagination we do not see the full 13 peaks because the outer ones are too weak and are lost in the noise. 

The proton coupled spectrum reveals a larger one-bond coupling constant (. /SiH) of about 215 Hz. The coupling pattern derived from the two- and three-bond coupling is complex but the pattern might serve as a starting point in the interpretation of 29Si spectra of reaction products. 

The most complete information about the proton-carbon interactions is of course contained in the fully proton-coupled spectrum. Unfortunately for molecules of any complexity there may be so much overlapping that complete interpretation is not possible. One of Ernst s two-dimensional experiments (Fig. 20) provides a solution by displaying the fully coupled resonances from different sites... 

FIG. 26. The high-field regions of spectra of l-dimethylamino-2-melhylpropene. (a) Proton-decoupled spectrum, (b)-(d) Multiplet sub-spectra corresponding to the three methyl resonances P, Q, and R respectively, (e) Full proton-coupled spectrum, showing overlapping and interference from acetone-df, and tetramethylsilane. From ref. 23. 

The NMR spectrum of quinazoline in [ Hj,]DMSO shows two sharp signals at 97.8 (Nl) and 86.5 (N3) ppm with respect to external neat nitromethane. This assignment is in accordance with INDO/S-SOS shielding calculations. The more-shielded signal in the proton-coupled spectrum has a doublet splitting of 13.8 Hz, whereas the less-shielded signal has a multiplet structure. 

Figure 5-21 The proton-coupled spectrum of pyridine, (a) with INEPT, (b) with NOE only, and (c) unenhanced, all on the same scale. (Reproduced with permission from G. A. Morris and R. Freeman,...

Coupling to H can be deactivated using proton broadband decoupling techniques. The respective notations are P-NMR for the proton coupled spectrum, and P- H -NMR for the proton decoupled spectrum. This notation is analogous to that used in C-NMR spectroscopy. 

Siloxanes represent another large class of silicon compounds for which 29Si-NMR data would be useful. The compound HMe2SiOSiMe2H was chosen as a representative siloxane with a more complex spin system due to the Si-H moieties. The proton coupled spectrum (Fig. 4) consists of a major doublet (J = 204 Hz), for which INEPT and DEPT parameters were set, and a minor septet of doublets. In the INEPT spectrum (Fig. 4a) the major doublet shows the expected INEPT intensities (+1 — 1), while the minor splittings show normal coupling intensities. A subtle but important distortion is present in the INEPT spectrum the expected septet of doublets... 

FIGURE 7.2. The proton-decoupled ISN NMR spectrum (30.4 MHz) of cthylencdiamine in CDC13. The proton-coupled spectrum is shown in the inset. 

FIGURE 7.6. The proton-decoupled 19F NMR spectrum (338.6 MHz) of fluoroacetone in CDClj. The proton-coupled spectrum is shown as an inset. 

The proton-decoupled 31P NMR spectrum of diethyl chlorophosphate is found in Figure 7.12 along with the proton-coupled spectrum. Also included with this figure are the lH and 13C NMR spectra for comparison. The details of the proton and carbon spectra are left to the reader to work out. It is important to note, however,... 

FIGURE 4.4 Ethyl phenylacetate. (a) The proton-coupled spectrum (20 MHz), (b) The proton-decoupled spectrum (20 MHz). (From Moore, J. A., and D. L. Dalrymple, Experimental Methods in Organic Chemistry, W. B. Saunders, Philadelphia, 1976.)... 

In many cases it would be helpful to have the information about the attached hydrogens that a proton-coupled spectrum provides, but frequently the spectrum becomes too complex, with overlapping multiplets that are difficult to resolve or assign correctly. A compromise technique called off-resonance decoupling can often provide multiplet information while keeping the spectrum relatively simple in appearance. 

In an off-resonance-decoupled or proton-coupled spectrum, a monosubstituted benzene ring shows three doublets and one singlet. The singlet arises from tbe ipso carbon, which has no attached hydrogen. Each of the other carbons in the ring ortho, meta, and para) has one attached hydrogen and yields a doublet. 

The proton-coupled spectrum of the —CHD2 group is more complicated as both hydrogen (spin= )... 

---