Fluorinated NMR spectroscopy

The preparative reactions were conducted in sealed tubes in which — 1-3 g of the reagents had been placed. After the vessels had been maintained at the indicated temperatures for the designated times, the contents were removed, to be separated by fractional condensation and GLC. In addition to the (trifluoromethyl)Group 4A halides reported next, each sample contained unreacted (CFalaHg, the expected (tri-fluoromethyDmercuric halide, and the mercuric halide, identified by fluorine-NMR spectroscopy and mass spectrometry. 

MAS has been applied to a highly viscous cubic phase of a lyotropic LC formed by 1-monooleolyl-rac-glycerol and water in order to obtain liquid-like and 13C spectra.330 Deuterium, sodium, and fluorine NMR spectroscopy have been applied to study the phase behaviour of several dilute lamellar systems formed by low concentrations of an ra-hexadecylpyridinium salt, a sodium salt (e.g., NaBr, NaCl, or sodium trifluoroacetate), 1-hexanol, and D20.331 The 2H, 19F, and 23Na splittings were used to monitor the phase equilibria. The last two studies are motivated by the search of new lyotropic LC for the alignment of biomolecules. 

Using related chemistry, we have found these repetitive couplings to be facile and have used aromatic diboronic acids to generate simple terphenyl structures on solid support. We used gel-phase fluorine NMR spectroscopy to observe the incorporation of the appropriate groups (Scheme 34) in repeated palladium-catalyzed reactions at slightly elevated temperature (55°C). This unoptimized two-step process was complete in essentially 24 h. 

The first NMR spectrometers developed were continuous-wave (CW) instruments and they are still in use for proton and fluorine NMR spectroscopy. In these instruments the irradiation frequency is fixed and the magnetic field strength of the magnet is slowly and continuously changed. When the correct magnetic field for the fixed frequency is reached for a proton in a particular chemical environment, then an absorption peak appears in the recorder of the instrument. The area of this absorption peak is a function of the number of protons in the sample that are in this particular chemical environment. 

The complex formation in the binary Th(IV)-5-sulphosalicylate (SSA ) system and in the corresponding ternary fluoride system was investigated using potentiometry and proton and fluorine NMR spectroscopy. The equihbrium constants for the reactions... 

NMR spectroscopy is ideal for detecting charged fluorinated intermediates and has been applied to the study of increasingly stable carbocation and carbanion species. Olah [164, 165] has generated stable fluorocarbocations m SbFj/SOjClF at low temperatures The relatively long-lived perfluoro-rerr-butyl anion has been prepared as both the cesium and tris(dimethylamino)sulfonium (TAS) salts by several groups [166, 167, 168], Chemical shifts of fluonnated carbocations and carbanions are listed m Table 23. 

Infrared spectra and F-NMR spectroscopy showed the presence of IF5 and covalently bonded fluorine. Grafoil turns white upon intercalation with IF, this is reminiscent of graphite fluoride, CFi.ij (1,6). The IF, intercalate also evolves IF5 upon heating, but at much higher temperatures than C/IF5 this has been attributed to the lowered mobility of IF5 in the fluorinated matrix, which may no longer be planar. At 450°C, considerable amounts of fluorocarbons are evolved. 

Table 5 shows the most important NMR data of halophosphates. In the species containing fluorine, apart from the chemical shifts of the P-spectra, the F-spectra and the coupling constants Jpp are available for the discussion of bonding. The importance of NMR spectroscopy for purity control, for equilibrium measurement, as kinetical method in the investigation of reaction processes and for the identification of unstable compounds shall not be discussed here, though these apphcations are of great importance for the halophosphates too. 

Those of the readers who are already quite familiar with proton NMR spectroscopy are aware that, as the most electronegative element, fluorine substituents deshield proximate hydrogens more than any other atomic substituent because of their unique inductive influence. This fact is exemplified below in Scheme 2.11. 

Another possibility to find out more about the structure of these dendrimers was chosen by incorporating fluorine atoms. The use of 19F-NMR spectroscopy offered an additional tool to study the conformation of the dendrimer, especially with the fluorines attached close to the stereogenic centers [91 ]. Following our previously developed methods [92], fluorine-containing 1st- and 2nd-genera-tion chiral dendrimers such as 76 were synthesized (Fig. 24). 

In contrast, hydrolysis of the methyl derivative, Cp(OC)2Fe-SiMe(H)Cl (Id), yields the corresponding ferrio-silanol, Cp(OC)2Fe-SiMe(H)OH, as a short-lived intermediate identified by means of IR and NMR spectroscopy, which immediately reacts with Id to give the bis(ferrio)disiloxane 6. Conversion of 6 to the fluorine analogue [Cp(0C)2Fe-Si(Me)F]20, characterized by a linear Si-O-Si arrangement [6], is achieved via consecutive H/Cl- and Cl/F-exchange. 

Fluorine decoupled 13C NMR spectroscopy was utilized to confirm the structure of 7,8,9,10-tetrafluoro-6//-pyri-mido[2,l-3]quinazoline-6-one (see structure 206a, Equation (23), Section 12.04.2.6.5) rather than the angularly condensed isomer <2001TL1851>. 

NMR is an extremely powerful method for observing the environment of an atom of interest. The element most commonly studied is hydrogen bonded to another element, usually carbon, and is referred to as NMR spectroscopy (sometimes called HNMR, H NMR, P NMR, or proton NMR). The second most commonly studied element is carbon, specifically 13C, bonded to other carbon atoms and to hydrogen atoms. Other elements commonly measured include fluorine, nitrogen, and phosphorus. 

Solid-state nmr spectroscopy on a single crystal of KHF2 was interpreted in terms of a centred proton with f = 113.8 pm (Pratt and Smith, 1975). The same conclusion was reached for the other alkali-metal bifluorides (Ludman et al., 1977) and the displacement of the proton from the midpoint was estimated to be less than 60 pm. This technique cannot distinguish between a single minimum and a double minimum with fast transfer. Fluorine-19 nmr spectroscopy has been used to study solutions of HFj" in formic acid, and the data have been interpreted in terms of equilibria involving F , HF, HF2 and H2F2 (Coulombeau, 1977). 

Despite the success with gel-phase NMR spectroscopy the spectra obtained are of limited value because of the broad peaks, and indeed the NMR spectra of unlabelled samples need relatively long acquisition times (because of the low abundance of C) especially with lightly loaded resins such as TentaGel and Argo-Gel. NMR [206, 207] and NMR [208, 209] spectroscopy of gel beads have also been shown to be a convenient NMR technique for monitoring reactions of fluorine or phosphorous-containing molecules, respectively, attached to solvent-swollen polymer supports. 

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