Fluorine chemical shift for

There will usually not be much variation observed in fluorine chemical shifts for the three most common solvents used for obtaining NMR spectra, that is CDC13, DMSO-d6, and acetone-, as can be seen in the data presented in Table 2.3 for spectra of a series of typical fluorine-containing compounds in various solvent. The variation in fluorine chemicals shifts for these three solvents is no more than 1 ppm. Thus, in reporting chemical shifts in this book, no mention of specific solvent will be made, although the vast majority of spectra will have been measured in CDC13. 

The fluorine chemical shifts for fluoropyrimidines, 2-fluoropyrazine, 2-fluoro quinoxaline, 4-fluoroquinazaline, 5-fluorouracil, 5-fluoro-cytosine, and cyanuric fluoride are provided in Scheme 3.82. The fluorine of 2-fluoropyrimidine is considerably deshielded relative to... 

A trifluoromethyl group attached to a cyclohexane ring is unremarkable with respect to its chemical shift, absorbing at -75 ppm, with a 3/fh = 8 Hz (Scheme 5.2). There are no data available for triflu-oromethylcyclopentane or cyclobutane. The fluorine chemical shift for trifluoromethylcyclopropane reflects additional shielding, such CF3 groups appearing the farthest upheld of any CF3-substituted hydrocarbon. 

Indeed, examining the fluorine chemical shifts for the series of 1,1,1-trifluorobutyl amines in Scheme 5.20 indicates that there is the same lack of consistent trend that was observed for the analogous alcohols. 

Moreover, it is known that molten alkali fiuoride mixtures (LiF, NaF, KF) behave as a bath of polarizable spheres containing free cations (Li, Na" ", K ) and free F anions [15]. In that case, the fluorine chemical shift for a given composition is given by ... 

Partially fluorinated surfactants also have been investigated by NMR. Muller and Birkhahn [27] found the fluorine chemical shift for anionic terminally fluorinated surfactants, CF3(CH2)sCOONa and CF3(CH2)ioCOONa, to be essentially independent of ionic strength. An added electrolyte depressed cmc but did not affect the chemical shifts for the surfactant anions in the micelle. They explained their results by the formation of prolate-shaped spheroid micelles in the presence of sufficient electrolyte. An alternative explanation that electrolytes do not increase the micellar size was considered to be unlikely. 

The rehability of these analytical methods may be questionable when chemical shift differences of derivatives are of the same magnitude as variations encountered from solvent, concentration, and temperature influences. Reported fluorine chemical shift ranges for tnfluoroacetylated alcohols (1 ppm), p-fluorobenzoylated sterols (1 ppm), and p-fluorobenzoylated ammo acids (0.5 ppm) are quite narrow, and correct interpretation of the fluonne NMR spectra of these denvatized mixmres requires strict adherence to standardized sampling procedure and NMR parameters. 

The chemical shifts for P—F compounds have been reviewed.The compounds differ from most other organophosphorus compounds because Sp becomes more positive as the electronegativity of the atoms attached to phosphorus increases. The effect is at a maximum for P" compounds. They behave normally with regard to an increase in shielding with increase in co-ordination number and therefore the P" compounds are the least shielded. Thus the largest negative values (— 190 to — 250) are observed for compounds of the type YPFj. With the new value of Sp of + 80 for PFg, the variation of Sp with the number of fluorine atoms in P compounds is now shown to be fairly consistent. The value of Sp has also been reported for a series of aminohalogeno P compounds. -... 

If one wishes to obtain a fluorine NMR spectrum, one must of course first have access to a spectrometer with a probe that will allow observation of fluorine nuclei. Fortunately, most modern high field NMR spectrometers that are available in industrial and academic research laboratories today have this capability. Probably the most common NMR spectrometers in use today for taking routine NMR spectra are 300 MHz instruments, which measure proton spectra at 300 MHz, carbon spectra at 75.5 MHz and fluorine spectra at 282 MHz. Before obtaining and attempting to interpret fluorine NMR spectra, it would be advisable to become familiar with some of the fundamental concepts related to fluorine chemical shifts and spin-spin coupling constants that are presented in this book. There is also a very nice introduction to fluorine NMR by W. S. and M. L. Brey in the Encyclopedia of Nuclear Magnetic Resonance.1... 

The typical chemical shift for primary n-alkyl fluorides is -219, but the values for primary alkyl fluorides vary between -212 for ethyl fluoride and -226 for 2-ethyl-l-fluorobutane (Scheme 3.1). As mentioned above, alkyl branching leads to shielding of fluorine nuclei. 

The isomeric 1- and 2-fluoronaphthalenes have fluorine chemical shifts of -124 and -116 ppm, respectively. A full analysis of the proton and carbon spectra of 1-fluoronaphthalene is given in Scheme 3.56. NMR data for a number of other fluoropolycyclic aromatic compounds are available.7... 

The Pentafluorophenyl Group. Fluorine NMR chemical shifts for ortho, meta, and para fluorines can vary considerably (see Chapter 6 for more complete details). Scheme 3.59 provides data for one example, that of pentafluorotoluene. 

The rules governing trends in chemical shift for hydrocarbon CF2 groups are virtually the same as those that govern monofluoroalkanes. Thus, the fluorine nuclei within primary CF2 groups (that is, CF2H groups) are the most shielded, with secondary CF2 groups (i.e., those... 

Fluorine chemical shift data are also given in Scheme 4.31 for silanes bearing a CF2-halogen group. 

Figure 4.10 provides the 19F NMR spectrum of 1,1-difluorobutene. The chemical shifts for its Z- and E-fluorines are -92.8 and -90.8ppm, respectively, with the geminal 2JFF coupling constant being 50 Hz, and the trans 3/HF coupling constant being 25.5 Hz. The cis coupling was too small to be seen in the fluorine spectrum, but was determined to be 2.7 Hz from the proton spectrum shown in Fig. 4.11. The magnitudes of these vicinal F—H coupling constants are considerably diminished as compared to those of monofluoroalkenes.

Fluorine chemical shift and coupling constant data are provided in Scheme 4.47 for all of the hydrofluoroethylenes. 

Examples providing fluorine chemical shift data for trifluoromethyl imidazoles and a benzimidazole bearing a trifluoromethyl group are given in Scheme 5.53. 

Scheme 6.26 provides chemical shifts for all of the fluorines in a representative group of perfluorocarbons. The various environments exhibited should allow one to estimate the chemical shift for almost any fluorine in a perfluorocarbon system. The fluorine chemical shifts of four-, five- and six-membered ring perfluoroalicyclics are quite consistently in the range of -133 to -134 ppm, but as usual, fluorines on a cyclopropane ring appear at a much higher field than those of other fluorinated alicyclics, perfluorocyclopropane having a chemical shift of -159 ppm. 

The chemical shifts for all fluorines, in two representative examples of perfluoro-l-alkenes are given in Scheme 6.32. F—F coupling constants... 

More pertinent to the interests of fluoro-organic chemists, a number of compounds bearing a single N—F bond have become useful electrophilic fluorination reagents, i.e., behaving as effective sources of F+. The structures of some of them are given below, along with the chemical shifts for the N—F fluorine substituent. 

Scheme 7.14 provides fluorine NMR data for some organic SF3 compounds, each of which exhibits peaks of vastly different chemical shift for their axial and their equatorial fluorines. The peaks in the fluorine NMR spectrum of CH3SF3 are given in Fig. 7.1 as an example of this type of compound. 

Thanks to the presence of the fluorine substituent on the salicylic ring, the 19F NMR measurements could be performed. Similar values of the chemical shifts for the ligand ( 75.2 ppm) and the complexes (from 70.2 up to — 72.8 ppm) suggested that the fluorine atom was not involved in bonding. 

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