Application of Low-Field NMR to the Unambiguous Assignment of Spin-Coupling Parameters. III. Other Pentafluorobenzenes
1969; American Institute of Physics; Volume: 51; Issue: 10 Linguagem: Inglês
10.1063/1.1671839
ISSN1520-9032
AutoresW. B. Moniz, Ernest Lustig, Elizabeth Hansen,
Tópico(s)Advanced NMR Techniques and Applications
ResumoThe 6.14-MHz fluorine NMR spectra were employed in removing ambiguities in the signs and assignments of coupling constants in C6F5–Cl, –Br, –CN, –COOH, –COCl, –OH, –NH2, and –SO2Cl. Except for C6H5OH, selection of the correct parameter set entailed only a visual comparison of the low-field spectrum and computed spectra based on permutations of the nonassignable parameters from high-field studies. The correct parameter sets were subsequently computer refined to obtain best fits to the 6.14-MHz spectra. However, in the case of C6F5OH, the solvent dependence of the spectrum necessitated computer iterative analysis of the 6.14-MHz data to arrive at the correct parameter set. For C6F5COCl, high-precision 94.1-MHz spectra were of use in eliminating all but one uncertainty in the assignments. The Abraham–MacDonald–Pepper linear relationship involving the ortho–ortho substituent contribution and the Hammett σp constant was modified and extended to yield for the compounds C6F5–X the relationship J26(Hz) = − 9.5σp(X) − 2.3, from which several J26's and one σp were derived.
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