Nuclear magnetic resonance spin–lattice relaxation in sulphamic acid and some sulphonate salts in the solid state

Abstract

Spin-lattice relaxation times, T I , have been measured over appropriate temperature ranges for sulphamic acid (NH3S03), caesium trifluoromethanesulphonate (CF3S03Cs) and caesium methanesulphonate (CH3S03Cs). Reorientation about the C3, 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 NH3S03 and to the C--He * -0 and C--Ha * .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-') 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 CF3S03Cs is markedly dependent on temperature in a manner which is overall non-linear.

Variable-Temperature 17O NMR Studies Allow Quantitative Evaluation of Molecular Dynamics in Organic Solids

“…It is clear that the calculated energy barriers are overestimated compared with the experimental E a values. We attribute this overestimation primarily to thermal factors that are not taken into account by the DFT calculations, which assume a temperature of 0 K. For example, thermal vibrations experienced by the molecules in the crystal lattice and fast rotation of NH 3 + groups 94 would allow a certain amount of structural relaxation to occur during the SO 3 − rotation process. We attempted to account for this by rerunning the calculations to include the geometry optimization of hydrogen atoms in close proximity to the sulfonate groups such as those from the NH 3 + group, or all hydrogen atoms.…”

Study of the reorientational motions of NH3 groups in crystalline sulphamic acid by quasi-elastic neutron scattering and heat capacity measurements

“…In this respect Ratcliffe et al (13) have underlined that the distribution of neighbouring oxygen atoms around NH, group is near a five-fold environment. If this were a perfect pentagon the first term to contribute would be (V,,/2) (1 -cos 15 0), i.e., it would generate a quasi-free rotation at room temperature.…”

“…Nuclear magnetic resonance spin lattice relaxation measurements in NH,SO, (13) have been interpreted in terms of NH, reorientations. An activation energy E, of 11.3 kJ mol-' is reported while a preexponential factor 71: = 7.5 X lo-'' s is given.…”

“…W e shall then discuss and compare our results with those of the literature: Raman (1 1, 12), nmr (13), particularly in terms of potential barriers originating from intra-and intermolecular interactions. Finally, we shall present some results of potential barrier calculations performed with the WMIN Busing's program (14) by using coulombian, Van der Waals, and hydrogen bonding atom-atom potentials.…”

“…The sample was purified by repeated recrystallizations from water-methanol mixture and dried in high vacuum. For heat capacity experiments, 13.99 g of the sample was used and loaded in the calorimetric vessel in which air was replaced with a few kPa of helium gas at 300 K. For IQENS experiments, powder samples were contained in circular thinwalled aluminum cans (5 cm in diameter). The thickness of the sample was adjusted to ensure a transmission of about 90%.…”