Proton-Fluorine Coupling Constants

The satellite spectrum (at -70 C) of tetrafluoroethylene has been analysed. Two alternative sets of parameters were obtained  

Recently reported spectral parameters for polyfluoro-furans and -thiophens - provide an interesting comparison with those of 1- and 5-chloropentafluorocyclopentadiene (see Table 5). The F— F coupling y, s is substantial in each case and is of largest magnitude in the case of the thiophens. 

The ortho couplings V ,s and /4,s are rather variable, are smallest for the thtophens and, where determined, are of opposite sign to and Vj.g. The couplings /a,4 are very variable for the furans and thiophens. 

Cooper has shown that in polyfluorobenzenes large solvent effects are observed, in particular for /fp. Thus, for example, in 1,2-dibromotetra-fluorobenzene, /,4 ranges from -19-92 (10% in perfluoroalkane solvent) to —22-50 Hz (in pyridine), while A.b changes from -17-99 to -20-39 for the same solvents. It was suggested that c tain discrepancies in the literature values of, in pmticular, Jff could be accounted for by such solvent efiects. 

The suggested additive substituent contributions to /ff and /pf in fluoro-benzenes ( /ff in p zra-difluorobenzene has recently been determined as 17-779 Hz compared with the standard 181 Hz used to calculate other esuunples) may be used to calculate such couplings to within some 0-5 Hz of experimental values, and it is not clear how much such deviations are due to experimental inaccuracies and how much due to failures of strict additivity. 

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Table A3.8 Characteristic coupling constants of protons with other nuclei (Hz) Proton-fluorine coupling constants...

The geminal proton-fluorine coupling constant 64.90 Hz in fluorocyclopropane was determined by Scherr and Oliver the trans = 9.87 Hz and its cis counterpart HgF = 21.02 Hz were found to have the same relative sign. Other examples of values... 

C. Thibaudeau, J. Plavec, and J. Chattopadhyaya, A new generalized Karplus-type equation relating vicinal proton-fluorine coupling constants to H-C-C-F torsion angles, J. Org. Chem., 63 (1998) 4967 1984. 

From a comparison of proton-fluorine coupling constants and bond angles in a series of cyclic organic molecules, it has been found that V( H, F) generally decreases as the H-C-C-F bond angle increases. In addition, the dihedral angle dependence of V( H, F) is very similar to the dependence of V( H, H), i.e. ( H, F) has a maximum value when the dihedral angle 0 between H and F is 0° and 180°,... 

The symbols t, h refer to proton-fluorine couplings with increasing number of the corresponding fluorine atom. Partial listing of coupling constants in A is given, too. 

Table 4.70. Carbon-Fluorine Coupling Constants of Isomeric Fluoropyridines and of 2-Fluoropurine (in Hz) [469]. If available, Couplings of the Protonated Heterocycles are Printed in Parentheses.

Proton to Fluorine Coupling Constants (V, and longer distances)... 

The section on Spectroscopy has been retained but with some revisions and expansion. The section includes ultraviolet-visible spectroscopy, fluorescence, infrared and Raman spectroscopy, and X-ray spectrometry. Detection limits are listed for the elements when using flame emission, flame atomic absorption, electrothermal atomic absorption, argon induction coupled plasma, and flame atomic fluorescence. Nuclear magnetic resonance embraces tables for the nuclear properties of the elements, proton chemical shifts and coupling constants, and similar material for carbon-13, boron-11, nitrogen-15, fluorine-19, silicon-19, and phosphoms-31. 

Give a clear indicaUon of solvent, concentration, and temperature. These parameters have a much greater effect on chemical shifts and coupling constants for fluorine than for protons. 

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. 

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... 

Although the spectra of fluorine containing compounds are non-exceptional, other than for the F—H coupling, and are highly predicable, typical proton chemical shift and coupling constant data will be provided within each chapter for each class of fluoroorganic compound. 

H and 13C NMR Data. The examples in Scheme 3.3 provide insight into expected proton and carbon chemical shift and coupling constant data for primary alkyl fluorides. It can be seen that the influence on both proton and carbon chemical shifts diminishes rapidly as one moves away from the site of fluorine substitution. 

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