I. Conformational Properties of Cyclobutanes. II. Nitrogen-15 Magnetic Resonance Spectroscopy

Citation

Lambert, Joseph Buckley (1965) I. Conformational Properties of Cyclobutanes. II. Nitrogen-15 Magnetic Resonance Spectroscopy. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/RHP6-YT07. https://resolver.caltech.edu/CaltechETD:etd-09252002-123537

Abstract

The temperature dependence of the geminal fluorine-fluorine chemical-shift differences in a variety of unsymmetrically substituted cyclobutanes has been interpreted in terms of a classical equilibrium between axial and equatorial conformations, for which free energy differences have been calculated. The absence of this effect for cyclobutenes and cyclobutanones indicates that these systems are nearly planar. The angle of puckering of 1,1-difluoro-3phenylcyclobutane has been calculated by the dipole-moment method to be about 27°. The observation of substantial temperature variations of the vicinal hydrogen-fluorine coupling constants is consistent with the presence of an equilibrium between conformers. The magnitude of these couplings as a function of the dihedral angle appears to follow a relationship similar to that described by Karplus for proton-proton coupling constants.

Nitrogen-15 magnetic resonance spectroscopy has been investigated for the first time. The absolute magnitude of the chemical shift and its variation with protonation have been interpreted in terms of the predominance of the paramagnetic term in the shielding expression. A correlation between the energy of the n → π* transition and the magnitude of the chemical shift is considered. An empirical relationship between the hybridization of nitrogen and the coupling between directly bonded nitrogen-15 and hydrogen is derived, and the limitations are discussed. Contributions to the coupling from orbital motion are invoked in order to explain the significant deviation of the coupling in ketimines from the behavior predicted by this relationship. The temperature dependence of the proton spectrum of nitrogen-15-labeled ketimines is interpreted as resulting from a degenerate, bimolecular exchange of the imine protons. Separate geometrical isomers of an imine unsubstituted at nitrogen s-butylphenylketimine are observed for the first time.

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