Fluorine-19, heteronuclear coupling

As we have already pointed out in the section dealing with heteronuclear coupling that it is not always necessary to confirm the presence of a particular hetero atom by acquiring the NMR spectrum of that nucleus. More often than not, the hetero atom will have a clear signature in the proton or carbon spectrum. Fluorine and phosphorus are both examples of nuclei that couple to protons over two, three, four and even more bonds. 

Figure 9.15—Heteronuclear couplings. Monofluoroacetone. Top H NMR spectrum in which coupling with the fluorine atom can be observed (2J = 47.5 Hz, 3J = 4.1 Hz). Bottom l9F NMR spectrum. The single fluorine atom present in this molecule leads to a triplet from coupling with the CH2 group and to a quartet from coupling with the three protons of the methyl group (coupling constants can be obtained using FSiCI, as a reference).

The use of phosphonodifluorodithioacetate as a 2ji component in cycloadditions with a variety of dienes provides simultaneous direct access to fluorine and phosphorus containing 3,6-dihydro-2/7-thiopyrans, for example, 217 and their 3-ones 218. Complex signals result from the heteronuclear couplings <2002TL2033>. 

Figure 15.18 NMR spectra of monofluoroacetone. An example of heteronuclear coupling. Above, the H NMR spectrum. The presence of the fluorine atom leads to a doublet for the methyl group Qj = 4.1 Hz) as well as for the CHj ( / = 47.5 Hz). Below, spectrum of F. The single fluorine atom of this molecule leads to a triplet with the CHj group and a quadruplet with the methyl (explanation in next section). The resulting signal is therefore a triplet of quadruplets. With the aid of table 15.4 the coupling constants can be calculated and compared for the two spectra (the scale for the chemical shifts is positioned with respect to FSiClj.

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). 

Heteronuclear coupling is observed when there are fluorine atoms in an organic com-... 

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-... 

Heteronuclear coupling of to and to was discussed in Chapter 6, Sections 6.15 and 6.16. Fluorine-19 is the only natural isotope for this element, and while phosphorus has more than one isotope, phosphorus-31 is the most abundant. This section will cover couplings to F and Since F and P each have a nuclear spin of 1/2, the principles discussed earlier in this chapter apply directly to interpreting resonances coupled to these elements. 

Fluorinated analogues of naphthyl xyloside in a variety of solvents Type of vicinal heteronuclear coupling measured. 196... 

To monitor the effective aluminium-fluorine dipolar couplings in cryolite, as a function of temperature, amplified 2D separated local field (SLF) NMR experiments under the action of fast MAS (which is in turn desirable for the simple elimination of the homonuclear couplings) were recorded by Kotecha et al [52]. In such SLF MAS experiments rotor-synchronized pulses were applied to achieve a net heteronuclear dipolar evolution with variable amplification factors xN of the Al- interaction along the indirect domain (x2 SLF, x4 SLF, and x8 SLF), followed by observation of aluminium s central-transition F]-decoupled evolution along the direct domain [53]. 

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.

The fluorocarbon-based polymers have some unique NMR characteristics because of the nuclei which produce special homo- and heteronuclear J-couplings. Table 24.2 summarizes typical J-couplings found in the organic polymers discussed in this review. Although not specifically dealing with fluoropolymers, a useful and comprehensive compilation of J-couplings in fluorinated compounds can be found in a publication by Foris [3]. 

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