Carbon fluorine-decoupled

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. 

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). 

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

The standard carbon experiment decouples protons from carbons to enhance sensitivity. In the absence of proton decoupling, each carbon resonance is split by one more than the number of protons attached to that carbon. Since, unlike for protons, the natural abundance of carbon-13 is only 1.1%, it is critical to maximize sensitivity in order to detect carbon in a reasonable amount of time. The most commonly used proton decoupling scheme is WALTZ-16. " Usually, protons are the only nuclei that are decoupled in a standard carbon experiment. Other spin-1/2 nuclei, such as fluorine, are not decoupled and thus split each carbon resonance by one more than the number of attached fluorines (e.g., see Figure 31). This can be particularly advantageous for identifying fluorinated carbon entities. Since the carbon signals are often too weak to observe in a small number of scans, one needs to make a reasonable approximation for the spectral... 

Figure 2.12 shows the carbon NMR spectrum for 1,2-dibromofluoroethane. As usual, the protons are decoupled, so that the only couplings that one can see are those between fluorine and carbon. Two signals are observed at 33.8 and 89.1 ppm, with one-bond and two-bond coupling constants of 257 and 23.5 Hz, respectively. (The multiplet at 77 ppm derives from the solvent, CDC13.)... 

Fluorine-19 NMR data were acquired at a frequency of 188.22 MHz with a Varian XL-200 spectrometer. Typically, 100 transients were accumulated from a 5% polymer solution by volume in dimethylformamide-d7 placed in a 5 mm sample tube at 120 C with internal hexafluorobenzene as a reference ( = 163 ppm). A sweep width of 8000 Hz was used with 8 K computer locations (acquisition time 0.5s) and a 5.0 s delay between 90 pulses (9.0 s duration). Proton heteronuclear coupling was removed by broad-band irradiation centered at 200 MHz. A modified Bruker WH-90 spectrometer allowed carbon-13 NMR spectra to be obtained with simultaneous proton and fluorine-19 broadband decoupling (13). 

In 4-ethyl-l,l-difluoro-4-methylcyclohexane at room temperature the proton-decoupled C spectrum shows averaged coupling of fluorines to the nearest three types of carbon, and in 1,1-difluorocyclohexane the triplet for C-3 at room temperature ( /cjc = 4.7 Hz) becomes a doublet ( /cf = 9.5 Hz) at -90 °C, probably owing to a stereospecific coupling involving the equatorial fluorine. ... 

It will be remembered that NMR spectra of F-containing systems is routine (Chapter 2, VI [7] and Chapter 5, C.II). Indeed, the value to structure detennination of fluorine-containing compounds should be clear. However, it is important to remember (Chapter 5, C.II) that F has spin 1/2 and will thus couple with protons ( H) and carbon ( C). Furthermore, while noise decoupling of protons to yield C spectra in which different carbons appear as single lines at their different chenaical shifts is common, H decoupling will not, of course, remove coupling between F and C. 

With the application of a triple-resonance scheme [52], which simultaneously broad-band decouples both proton and fluorine nuclei while observing carbon nuclei, it was possible to obtain CNMR spectra of fluoropolymers that were free of both — H and J-couplings. As an example, the triple... 

Organic compounds that contain C, H, O, Cl, and Br will show only singlets when the proton decoupler is turned on. The oxygen, chlorine, and bromine atoms will not couple to a carbon-13 atom under normal conditions. However, when the organic compound has a fluorine atom attached to a carbon-13 atom, you will observe heteronuclear coupling even though the proton decou-... 

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