Spin coupling constants fluorine-proton

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... 

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. 

Spin-spin coupling involving and proton and fluorine is observed in the satellite spectrum of fluorobenzene. Values of the directly bonded C-H and C-F coupling constants are shown in Table XXVIII. The dispersion mode n.m.r. spectrum of fluoro-benzene (Fig. 7) also exhibits a two-bond F coupling, not... 

The NMR spectrum of methylene protons in PVDF consists of a quintet due to the coupling with four vicinal fluorine nuclei. The value of Jobs is determined from this splitting. PVDF is known to take three preferred con-formers as shown in Fig.l. The observed vicinal spin coupling constant is an average over the three preferred conforraers as given by... 

Simulated 500 MHz proton NMR spectra of C2H3F. Top - ab initio, bottom - experimental values of shielding and spin-spin coupling constants. The labels define the position of the relevant proton with respect to the fluorine. The shielding constants and the F rmi contact contributions to the coupling constants were computed at the CCSD level, the other theoretical values are taken from Helgaker et al. (1997), where the experimental data were also discussed... 

The F nucleus is almost as suitable as the proton for NMR work and the large fluorine chemical shifts minimize second order effects, thereby permitting meaningful deductions to be made about molecular structure. The F spectra of many of the recently described fluorocarbon-metal derivatives have been recorded, and the chemical shifts and spin coupling constants listed. References to these studies are collected in Table VIII. 

Fluorine NMR data can be collected readily on most spectrometers, requiring only minor adjustments to instrumentation used to run proton samples The fluonne-19 nucleus is easily detected (relative abundance, 100%, spin, 1/2) and generates a wealth of spectral information to assist in structure elucidation To take full advantage of all the spectral evidence available, lH, 13C, and 19F chemical shifts and coupling constants should be acquired and correlated... 

The pairs of fluorines in all of these molecules, except those in 1,1-difluoroethene, would also be magnetically equivalent. In order to be magnetically equivalent, nuclei that are chemically equivalent must have identical coupling constants to any other particular nucleus in the molecule, and it can be seen that the two protons in 1,1-difluoroethene do not have the same spatial relationship with respect to a given fluorine substituent. For example, the Fa substituent has a cis relationship to Ha, but a trans relationship to Hb (Scheme 2.30). A spin system such as this one is represented as an AA XX system, which contrasts with the A2X2, A2X, and A2XY systems in Scheme 2.29 wherein both fluorines in each of these systems have identical 2/HF coupling constants. 

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. 

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