Fluorine proton coupling

Of the other two protons, the signal at 8.82ppm, (Hb) shows only a 2.9 Hz coupling which is also found in IT, whilst Ha exhibits two small couplings (2.0 and 1.7 Hz), the smallest of these also appearing in Hc. These observations lead to the conclusion that the fluorine-proton couplings in this molecule are as given in Table 6.2. 

Table 6.2 Fluorine-proton couplings in 3-fluoro nicotinic acid.

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 magnitude of the fluorine—proton coupling constants have been invoked to establish the conformation of l,3,4-tri-0-benzoyl-2-deoxy-2-fluoro-jS-D-ribose as (66). The magnitude of the proton-proton... 

The highly stereospecific inter-proton /(H3,CHO) coupling in benzaldehyde derivatives was found to be perturbed by the intramolecular hydrogen bond in salicylaldehyde, reducing such couphng about 25%. Similarly, the fluorine-proton coupling J(F3,CHO) in 3-fluorosalicylaldehyde is decreased about 50% from the corresponding value in 3-fluorobenzaldehyde. ... 

The spectral properties of pentafluorophenylcopper te-tramer are as follows infrared (Nujol) cm. 1630 medium 1391 medium 1353 medium 1275 medium 1090,1081, and 1071 strong triplet 978 strong 785 medium fluorine magnetic resonance (tetrahydrofuran with trichlorofluoromethane as internal reference) 8 (multiplicity, number of fluorines, assignment, coupling constant J in Hz.) 107.2 (20-line multiple , 2, ortho F), 153.4 (triplet of triplets, 1, para F, J= 1.3 and 20), 162.3 (17-line multiplet, 2, meta F). Absorptions at 820-900, 1100-1125, and 1290 cm.- in the infrared spectrum and at 8 3.05 in the proton magnetic resonance spectrum indicate that dioxane is still present. 

Usually, a careful analysis of the combination of fluorine, proton, and carbon NMR chemical shifts and spin-spin coupling constants will provide definitive information regarding the structure of disubstituted fluoroaromatics. 

A couple of examples of other heterocycles, a pyrazole and a pyrida-zine, are given in Scheme 5.58, with their fluorine, proton, and carbon NMR data. 

V ( C, F). This coupling shows the same angular dependences as the corresponding fluorine-proton or proton-proton counterpart, thus permitting the development of Karplus-type relationships " the sign was assumed to be positive. " ... 

A number of interacting factors precludes any generalisations to be made but it should be noted that the clear distinction of the proton couplings to axial and equatorial fluorine atoms in pentavalent compounds is again obtained. The same order is found as for 2/(H-P-F) namely 13/(H-F ax) 3/(H-Feq). ... 

An example of an aromatic fluorine-containing compound can be found in Figure 6.7, where we have recorded the l9F spectra (both proton-coupled and decoupled) of fluorobenzene along with the H and l3C spectra. Once again we find a singlet for the fluorine atom in the ptoton-decoupled spectrum and a complex multiplet for the fluorine atom in the proton-coupled spectrum. The fluorine atom couples differently to the ortho-, meta-, and para-protons in this mono-substituted compound. Coupling constants for proton-fluorine can be found in Appendix F of Chapter 3. 

Fluorine, like hydrogen, has a spin of 1/2 and couples with the vicinal proton. Fluorine-hydrogen coupling constants are significantly larger than those for vicinal hydrogens. 

S H one, leading to the usual inversion of the proton signal. If the dipolar coupling between the proton and fluorine spins is an appreciable relaxation process for the fluorine nuclei, then when the proton performs an H+ transition in relaxing towards the thermal equilibrium distribution, there will be a probability that the fluorine spin will perform a simultaneous F+ transition due to the term H+F+ in the fluorine-proton dipolar interaction Hamiltonian. Thus in addition to any fluorine enhancement due to the direct fluorine electron interaction, there will be an additional positive enhancement due to the indirect three-spin interaction. 

The scalar interaction, like the quadrupole coupling and the shift tensor, depends on the atomic number. There is a complete set of isotropic scalar coupling data from high resolution solution NMR. Typical proton coupling may be 10 s of Hz while fluorine couplings can be as big as a few kHz, for example, in TeFg. 

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