Nuclear Magnetic Resonance Spectroscopy. Conformational Properties of Cyclobutanes. Variation of Geminal Fluorine—Fluorine Chemical-Shift Differences with Temperature

Abstract

The temperature dependence of the geminal fluorine-fluorine chemical-shift differences in a variety of unsymmetrically substituted cyclobutanes is interpreted in terms of a classical equilibrium between axial and equatorial conformations. With the assumption that the conformations have the same entropy, free-energy differences between the conformers are calculated and discussed. The axial conformer in monosubstituted cyclobutanes may be nearly planar. The absence of temperature effects in the spectra of a cyclobutene and a cyclobutanone indicates that these systems are statically planar. The angle of puckering of I,1-difluoro-3-phenylcyclobutane is calculated by the dipole moment method to be about 27º.

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