| Popis: |
Spin–lattice relaxation times, T1, have been measured over appropriate temperature ranges for sulphamic acid (NH3SO3), caesium trifluoromethanesulphonate (CF3SO3Cs) and caesium methanesulphonate (CH3SO3Cs). Reorientation about the C3v axis of the three-spin proton or fluorine system in each substance accounts for the observed relaxation rates. Activation energies have been obtained for all the reorientations and in the cases of sulphamic acid and caesium methanesulphonate, whose crystal structures are known, related to the hydrogen bonding in NH3SO3 and to the C—H⋯O and C—H⋯H interactions in the caesium salt. Even though a 3-fold rotor in sulphamic acid reorients in a very roughly 5-fold environment, the external barrier is not especially small due to a departure from perfect 5-fold symmetry in the environment. Failure to obtain close agreement between barrier height and activation energy should probably be attributed to an oversimplification in the assumptions about the form of the barrier potential. Singularly good agreement is obtained between the n.m.r. activation energy (17.5 kJ mol–1) for the caesium salt and the rotational barrier derived from inelastic neutron scattering results. The activation energy for the reorientation of the CF3 group in CF3SO3Cs is markedly dependent on temperature in a manner which is overall non-linear. |
