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N.M.R. studies of 19 F chemical shifts and coupling constants in cyclobutane derivatives

1969; Taylor & Francis; Volume: 16; Issue: 3 Linguagem: Inglês

10.1080/00268976900100301

1362-3028

Richard R. Ernst,

Molecular spectroscopy and chirality

Abstract The 19F spectra of several substituted trifluorochlorocyclobutanes which commonly occur as mixtures of two cis-trans isomers have been investigated. Substituents of different bulkiness have been introduced to shift the conformational equilibrium of the puckering motion of the cyclobutane ring system. A complete assignment of the resonances of the two isomers was possible, based on chemical shifts and H-F coupling constants. Vicinal F-F coupling constants have been investigated for a possible dependence on the dihedral angle ϕ. It is found that the coupling constants change sign twice for 0 ⩽ ğf ⩽ 1>80°, being negative for small and large angles and positive for intermediate angles. The investigations were based on fluorine spectra with completely eliminated H-F couplings by means of noise decoupling.