Fluorine-19, decoupling

There is still another situation that leads to second order spectra and this one usually cannot be anticipated. For example, take a look at the proton spectrum of 3,3,3-trifluoropropene in Fig. 2.9. This spectrum is not the simple one that one would expect for a monosubstituted ethylene. However, the second order nature of this spectrum can be understood after examining the fluorine-decoupled spectrum, which is given in Fig. 2.10. The decoupled spectrum displays the expected multiplets from the ABC system, each proton appearing as a doublet of doublets. The second order spectrum seen in Fig. 2.9 derives from the fact that the protons at 5.98 and 5.93 are seen from the 19F frequency as... 

Proton and carbon spectra of 3,3,3-trifluoropropene are provided in Figs. 5.12 and 5.13 as specific examples of such spectra. The proton spectrum is more complicated than one would have expected based on a first-order analysis. However, a fluorine-decoupled spectrum becomes first order, as was depicted and discussed in Chapter 2, Section 2.3.5, Figs. 2.9 and 2.10. 

Fluorine decoupled 13C NMR spectroscopy was utilized to confirm the structure of 7,8,9,10-tetrafluoro-6//-pyri-mido[2,l-3]quinazoline-6-one (see structure 206a, Equation (23), Section 12.04.2.6.5) rather than the angularly condensed isomer <2001TL1851>. 

Figure 9. Fluorine-decoupled, MAS C-13 spectra of PTFE as a function of...

Figure 10. Fluorine-decoupled, MAS C-13 spectra of PCTFE as a function of temperature. Spectra were obtained with a CP time of 5.0 ms and an experiment repetition time of 2.5 s. Spectra represent a 2K FT in each case the number of FID accumulations varied from IK to 5K (41).

The spectrum was obtained with the proton and fluorine decoupling frequencies set on resonance for the respective and nuclei in PE and PTFE. The reasonably well-defined powder patterns imply effective decoupling and suggest the absence of any large Bloch-Siegert shifts, (e.g., see... 

General Features of PVF Spectra The proton- and fluorine-decoupled 22.62 MHz carbon-13 NMR spectra of PVF prepared from PVCF (a) and commercial PVF (b) are shown in Figure 3. There are five additional peaks present in spectrum (b) from the commercial polymer which are absent in spectrum (a). These are due to aregic monomer sequences, which have been assigned according to Tonelli et al. (16). Monomer sequence triads are resolved and are denoted by the binary regiosequence pentad notation in Table 1 (l = CFH, O = CH2). 

Fig. 4.5 Spectra (A) and (B) represent proton coupled and proton decoupled regions of the spectrum of the compound shown. Huorine atoms 2 and 3 couple together, so that in the proton decoupled spectrum (B), each appears as a doublet. (C) and (D) represent the proton spectrum with and without fluorine decoupling respectively. Spectra (E)-(G) show the proton difference spectra resulting firom steady state heteronuclear nOe experiments, with F irradiation at d F -140.9, -135.9 and -111.4ppm. In (E), nOe is seen firom F3 to proton 4, and (G) shows nOes from F9 to H8 and HIO - all examples of intra-ring nOes. Specttum (F) however, shows an inter-ring nOe from H2 to H8, revealing that the molecule can adopt a conformation in which the two aromatic tings are close in space.

D.W. Ovenall, J.J. Chang, Carbon-13 NMR of fluorinated compounds using wide-band fluorine decoupling, J. Magn. Reson. 1977,25, 361. 

Static and spinning fluorine-decoupled CP C-NMR spectra of highly crystalline poly(tetrafluoroethylene) at -120 C. 

Figure 10 8 400 MHz proton NMR spectrum of polyvinylfluoride with differential fluorine decoupling at -220 ppm. (Reprinted with permission from D.W. Ovenall and R.E. Uschold, Macromolecules, 1991, 24, 11, 323S. 1991 ACS [80])...

The resonance frequency of fluorine-19 lies close to that of the proton, so that the same measuring channel is used to observe it. 19F spectra with proton decoupling or proton spectra with 19F decoupling thus have special hard- and software requirements. 

But the most unambiguous and arguably the most elegant confirmation of structure would come in the shape of a hetero-nuclear NOE experiment. (First, you have to run a quick 19F spectrum in order to determine the relevant 19F resonance frequency and set the decoupler in the fluorine domain, of course.)... 

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