Fluorine-19 NMR spectra

Figure 6.23 Fluorine-19 nmr spectra of mixtures of boron halides showing the presence of mixed fiuorohalogeno-boranes.

Fluorine-19 NMR spectra of fluorine containing criss-cross adducts have been extensively studied <79LA133,82LA845,82LA853). Table 5 contains data of several interesting examples. 

Figure 3.1 Fluorine-19 NMR spectra of several HFA adducts. From left HFA (free), butyl trifluoroacetate, trifluoroacetic acid, HFA dimethyl glyoxime, HFA (benzyl alcohol), HFA (methanol), HFA (ethanol), HFA (isopropanol), HFA ( er -butanol) and HFA H2O. The numbers on top are Hz downfield from CgFg lock. Reprinted with...

As with other nuclei, theoretical explanations for chemical shift are well developed but are nevertheless unable to predict with any certainty the structural influences on the chemical shift. Thus, there is still widespread reliance on empirical correlations for such problems, which is reflected in the discussion below. Despite the widespread use of fluorine-19 NMR spectroscopy, there are no published, widely applicable additivity rules that enable the chemical shift to be calculated conveniently as they are for carbon-13 spectra. A number of studies have concentrated on the correlation of fluorine chemical shifts in substituted aromatic fluorobenzenes, because of the well defined inductive and resonance... 

Fluorine-19 NMR has also been used to examine the fate of anesthetics in vivo. Figure 9 shows the spectra of two anesthetics in rabbit brain using a surface coil. The 70-Hz geminal fluorine-proton coupling is well resolved, however, the smaller fluorine-proton vicinal couplings are not resolved owing to the large line widths obtained by this technique. 

The S-0 stretching frequency in the IR spectra of 1,2-oxathietane 2-oxides is observed at 1150-1190cm . The carbonyl stretching frequency in 1,2-oxathietane-4-one 2-oxide is observed at 1840 and 1856 cm". The sulfonate IR absorptions in 1,2-oxathietane 2,2-dioxides are 1370-1408 and 1176-1235 cm", except for the fluorinated derivatives, the absorption of the tetrafluoro- 3-sultone being reported at 1470cm". Fluorine-19 (nmr) chemical shifts and coupling constants are useful in structural and conformational studies of... 

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). 

The fluorine-19 NMR spectrum of PCFE appears far more complicated. Figure 2 shows the spectra from two PCFE samples, one prepared at 60 °C (a) and one prepared at -80 °C in urea (b). Each backbone carbon is a pseudoasymmetric center in PCFE, compared to every second carbon in PVCF, so that the dispersion of fluorine-19 chemical shift from stereoirregularity is much larger. This dispersion is almost 15 ppm for PCFE, and is similar to the spread observed in the fluorine-19 NMR spectrum of poly(l,2-difluoroethene) (14). 

FIGURE 3. The NMR spectra of the two racemic diastereomers of lV-(4-methyl-2-pentyl)-a-methoxy-a-trifluoromethylphenylacetamide prepared from racemic a-methoxy-a-(trifluoromethyl)phenylacetic acid [MTPA, ( )-83] and racemic 4-methyl-2-pentylamine [( )-84] (A) 60-MHz proton spectrum in chloroform-4 with tetramethylsilane (TMS) as the internal standard (B) 94.1-MHz fluorine-19 spectrum in chloroform-4 with trifluoroacetic acid as the internal standard. Reprinted with permission from Reference 76. Copyright (1969) American Chemical Society... 

Now that the principles of NMR spectroscopy have been introduced, we will see how NMR spectra of the two most common nuclei—hydrogen and carbon-13—are obtained. The principles described for carbon-13 are applicable to many other spin- /2 nuclei, such as nitrogen-15, fluorine-19, silicon-29, and phosphorus-31. Topics to be discussed include the components of a typical NMR spectrometer, preparation of a sample, signal optimization techniques, spectral acquisition, selection of processing parameters, spectral presentation, and calibration of the spectrometer. 

The low-temperature 19 F NMR spectra of fluorosilicate anions 220,221, and 222 show equivalent fluorine atoms satellites due to 29Si-,9F coupling are observed at -58°C. 

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