Spin–spin coupling constants between side-chain and ring fluorine nuclei in some benzotrifluoride, benzal fluoride, and benzyl fluoride derivatives: coupling mechanisms

Abstract

A complete analysis of the 1H and 19F nmr spectra of 2,5- and 3,4-difluorobenzotrifluoride, together with multiple resonance experiments, yields the signs and magnitudes of the long-range 19F,19F and 1H,19F spin–spin coupling constants. The coupling mechanisms are discussed. In particular, the coupling over six bonds, [Formula: see text], whose sign is interpretable in terms of a σ–π mechanism, is too large in magnitude when compared to [Formula: see text], and [Formula: see text] in the analogous compounds. These latter three couplings are consistent in sign and magnitude with what is known about hyperfine interaction constants. The magnitudes of [Formula: see text] are reported for 4-fluorobenzotrifluoride, 3-amino-4-fluorobenzotrifluoride, 3-nitro-4-fluorobenzotrifluoride, as are 6JpF,F values for p-fluorobenzal fluoride and p-fluorobenzyl fluoride. In contrast to 6JpH,CH and 6JpF,CH it seems unlikely that, unless its coupling mechanism becomes more precisely understood, 6JpF,CF will be a reliable indicator of conformational preferences.

Signs and mechanisms of 13C, 19F spin–spin coupling constants in benzotrifluoride and its derivatives

“…Nevertheless, the magnitudes and signs of " J ( H , CF,) in benzotrifluoride are not strikingly different from " J ( H , CH,) in toluene (5). " J ( F , CH,) and " J ( F , CF,) in fluorine derivatives of toluene and benzotrifluoride, respectively, have also been discussed in some detail in terms of composite coupling mechanisms (6,7 ) .…”

“…A brief speculation then proposes eq. [6] where the u-IT contribution is written explicitly. Now, 7.46 sin2 0, the u-IT term, is too large by a factor of 2.67 (see above).…”

“…Furthermore, to obtain agreement with experiment, divide the u terms in eq. [6] by an arbitrary factor of 3.08 and obtain eq. [7].…”

Theoretical and experimental conformational preferences of the aldehyde and fluoroaldehyde groups in 2-methylbenzaldehyde, 2-trifluoromethylbenzaldehyde, and 4-chloro-2-methylbenzoyl fluoride. Proximate couplings

“…In another vein, considerable effort has been expended in the elucidation of so-called "throughspace" coupling constants (21)(22)(23)(24). These exist when the coupled nuclei, although separated by four or more formal bonds, approach each other rather closely; or when the bonds containing these nuclei are juxtaposed in certain orientations.…”