Molecular Structure of Cyclobutane from Its Proton NMR in a Nematic Solvent

Abstract

The molecular structure of cyclobutane has been deduced from its proton nuclear magnetic resonance spectra in the nematic phase of $p$ , p′‐di‐n‐hexyloxyazoxybenzene. The observed spectra include satellites due to molecules containing carbon‐13 in natural abundance. Cyclobutane is found to be bent with a symmetry of $D_{\text{2d}}$ , but the molecule oscillates between the two equivalent bent conformers to give a spin Hamiltonian of symmetry $D_{\text{4h}}$ . From the observed spectra, it is not possible to determine both coordinates required to locate the carbon atoms, and an extra assumption must be made to determine the geometry. If the two C–H bond lengths to a carbon are assumed equal, then we find a bent geometry with a dihedral angle of 27°, with $\angle \mathrm{HCH}\text{\hspace{0.17em}=\hspace{0.17em}108.1°}$ , with the methylene group tilted 4° in a way which brings axial protons on the same side of the ring together. If the size of cyclobutane is scaled by taking the C–C bond length to have the electron diffraction value of 1.548 Å, then we find a C–H bond length of 1.133 Å. Other assumptions to fix the undetermined coordinate are explored.