A nuclear magnetic resonance study of solid cyclobutane

Abstract

The absorption spectra and spin-lattice relaxation times have been measured for the proton magnetic resonance in solid cyclobutane in the temperature interval 115 °K to the melting point 182.3 °K. At temperatures below 130 °K it is found that the lattice is effectively rigid. Between 130 °K and the heat capacity transition point temperature of 146 °K a restricted form of motion of the molecules in the lattice affects the absorption spectrum. It is shown that the form of motion is probably reorientation of molecules possessing D4h symmetry about their fourfold axes. The possibility that molecules spend an appreciable time in a non-planar configuration with D2d symmetry is not excluded. At the transition temperature a discontinuous change in the form of the molecular motion is found to occur. Between the transition point and the melting point isotropic reorientation of the molecules takes place, combined with self-diffusion through the lattice. From the spin-lattice relaxation time measurements it is possible to estimate the activation energies for molecular reorientation in the low temperature solid phase and for self-diffusion in the high temperature solid phase. These activation energies are found to be 10.5 ± 1 0 kcal mole-1 and 4.1 ± 0.6 kcal mole-1, respectively. Reorientation rates are also estimated.