19F NMR spectrum

The assumption is made at present that elemental combustion analysis for carbon, hydrogen, and fluorine provides a good approximation to the extent of incorporation of fluoroalkyl residues, i.e. alcohols and ethers. We have ruled out trifluoromethylcarbonyl groups since no evidence is seen for their presence in either the infrared spectra or the 19F-NMR spectra. Thus, our values for percent modification reflect the best fit of the combustion data to an idealized stoichiometry for the product in Equation 1, where (m+n+o) = 100, and the percent modification (% mod.) is given by the expression [100 x (m+o)/(m+n+o)], equivalent to the number of fluoroalkyl residues per one hundred methylenes. An appropriately normalized formula was used to fit the data for polypropylene (sample 10). 

To aid in the characterization of the modified polymer samp os 1-10, and of the mixtures 11 and 12 obtained from functionalizing the two cycloalkanes, we prepared the model fluoroalkanols 13 and 14 as shown in Equation 2 (11-12), and the model fluoroalkyl ethers 15 and 16 as shown in Equation 3 1 3). Compounds 13 and 15 were previously isolated by- the Dupont group (8) from their mixture 11. Products 13-16 proved to be good comparison compounds for the infrared spectra, and particularly- for the 19F-NMR spectra. The I9F-NMR signal for the fluoroalkanol is approximately 1 ppm downfield from that of the fluo-roalky.l ether, which is enough to get semiquantitative ratios of... 

It is possible to synthesize alkylcobalamins containing halogenated alkyl groups. Of these, the compounds studied most extensively are the halo-methylcobalamins (745). For the various fluorine containing derivatives which have been prepared, 19F NMR spectra have been obtained and the results are shown in Table 6. Chemical shifts of a number of fluorome-thanes are included for comparison. 

As is the case for 11 spectra, but not for 13C spectra, the intensities of individual signals in 19F NMR spectra constitute an accurate measure of the relative number of fluorine atoms responsible for such signals. 

Indeed, the three isomeric difluorobenzenes are not readily distinguishable based on the appearance of their 19F NMR spectra (Figs. 3.17-3.19), all run in benzene-d6-... 

Phosphorous (V) fluorides have some interesting structural and dynamic features that influence their 19F NMR spectra. All such compounds appear to have a trigonal bipyramid structure, which features two axial and three equatorial sites. Hexafluorophosphate, PF6 , is of course octahedral with all sites equivalent (Scheme 7.6). 

There are some unique structural aspects of some of the sulfur fluorides that will need to be discussed in order to understand the 19F NMR spectra. The geometry of tetracoordinate group VI compounds is predicted on the basis of Gillespie s electron-pair repulsion theory to be trigonal bipyramid, with an electron pair occupying one of the equatorial sites.2 Thus, the SF3 substituent as well as the molecule SF4 have structures as depicted in Scheme 7.12, with nonequivalent (axial and equatorial) fluorines, and thus their 19F NMR spectra consist of two 19F signals, with the fluorines being coupled if the system is scrupulously dry. 

The CF3 groups not only had interesting effects upon certain synthetic steps on the way to such dendrimers [93], but also could be used as probes the 19F-NMR spectra which were measured in different solvents and at different temperatures exhibited a clear-cut difference between the inner CF3 groups and those close to the periphery. 

Fig. 8 Representative solid-state 19F-NMR spectra of 19F-labelled gramicidin S (substituted at both Leu positions with 4F-Phg), embedded in macroscopically oriented ghost membranes (left column) and DMPC bilayers (right column) at a peptide-to-lipid ratio of about 1 40 [in (F) the lipid was 1 1 DMPC/cholesterol]. Depending on temperature, the peptide can assume different alignment states, which are strikingly similar in the native membranes and the model bilayers...

The kinetic parameters of tautomeric equilibrium of 5(6)-fluoro-6(5)-R-benzofuroxans were derived from the measurement of the temperature dependence of the 111 and 19F NMR spectra (Scheme 3) <2004RJ01167>. 

In a NMR tube, to a solution of the epoxy alcohol (2.5 mg) in CDCI3 (0.5 mL) was added 4-dimethylaminopyridine (5 mg) and (R)-(+)-a-methoxy-a-(trifluor-omethyl)phenylacetyl chloride (5 mg). The mixture was allowed to stand overnight at room temperature. The reaction was monitored by TLC to ensure complete consumption of the starting material. H and 19F NMR spectra were carried out on the crude reaction mixture. In the 19F NMR spectrum, each enantiomer gave a signal an additional signal at —71.8 ppm was ascribed to residual MTPA. (19F NMR (250 MHz, CDCI3) 8 - 70.7 (s, (2R,3R)-enantio-mer) —72.0 (s, (25 ,3.S)-enantiomer)). 

It has been established by a variety of techniques that aromatic cyanate esters cyclotrimerize to form cross-linked cyanurate networks.1 Analogously, the fluoromethylene cyanate monomers cure to cyanurate networks. In addition to the 19F-NMR spectra shown in Figure 2.3, evidence includes an up-field shift of the methylene triplet (1H-NMR, 0.21 ppm 13C-NMR, 9.4 ppm), the disappearance of the cyanate functional group (IR, 2165 cm4 13C-NMR, 111.9ppm) and the appearance of the cyanurate functional group (IR, 1580 and 1370 cm4 13C-NMR, 173.6 ppm).9 Typically, monomers are advanced to prepolymers by thermal treatment at 120°C or just above the melting point. The prepolymers are then cured at 175°C and are postcured at 225°C. 

FIGURE 28. 283-MHz 19F NMR spectra of isomers of 8-F-rhodopsin in CFIAPS before (lower) and after photoirradiation (upper) (a) 11-cis (pulse delay, D5 = 5.0 s, number of acquisitions, NA = 5200, line broadening, LB = 80 Hz) (b) 9-cis (D5 = 50 ms, NA = 160000, LB = 80 Hz). Disappearance of the excess 9-cis aldehyde was due to repeated formation and bleaching of pigment during the irradiation process. Reprinted with permission from Reference 48. Copyright (1996) American Chemical Society... 

Not unexpectedly the 5a,9a-dihydro-l-benzazepine (64) on heating isomerizes to the conjugated 2,3-dihydro isomer (78JOC315). Likewise, 19F NMR spectra reveal that after 24 hours at 25 °C, 30% of the unconjugated bis-(trifluoromethyl)dihydroazepine (65) (see Section 5.16.4.2.1(ii)) is converted via prototropic shift to the conjugated 2,3-dihydro-3iT-azepine (67JA605). 

Similar to the zinc insertion methodology for the preparation of fluorinated vinylzinc reagents (Section II.C), zinc reacts quantitatively with bromo- and iodopentafluorobenzene in coordinating solvents such as THF, Et20, DME, DG (diglyme), DMF, DMAC and DMSO to give bis(pentafluorophenyl)zinc (equation 72)68. These workers also obtained equilibrium constants for the Schlenk equilibrium between the mono/bis species via 19F NMR spectra. 

Determined by 19F NMR spectra of isolated crude reaction mixtures. 

Fig. 5.19F NMR spectra of commercial BF3 NH2C2H5 samples (A = Harshaw, B = Alfa, C = K. and K., D = Pfaltz and Bauer)...

---