Christopher I. Ratcliffe

Proton spin-lattice relaxation and methyl group rotation

Abstract Proton spin-lattice relaxation times have been measured at 16, 31, and 59 MHz in 4-methyl-2,6-ditertiarybutyl phenol between 80 K and its melting point, 340 K. The variation of T 1 with temperature shows too distinct minima. The lower-temperature minimum has been analyzed in terms of relaxation by reorientation of four of the six t-butyl methyl groups with an average apparent activation energy of about 2.4 kcal mole −1 (104 meV molecule −1 ). The higher-temperature minimum has been analyzed in terms of relaxation by reorientation of the t-butyl groups about their C 3 axes with four of the six t-butyl methyl groups reorienting very rapidly, and the remaining two reorienting with correlation time similar to that of the t-butyl group. The activation energy for the higher-temperature minimum is 5.76 kcal mole −1 (250 meV molecule −1 ). Steric potential calculations are used to add weight to these assignments, and a number of peculiarities displayed by the lower-temperature minimum are discussed.

An NMR spin lattice relaxation study of several N2H62+ salts and 14N1H dipole-dipole contributions to relaxation

Abstract The NMR spin-lattice relaxation times ( T 1 ) at 31 MHz of N 2 H 6 Cl 2 , N 2 H 6 Br 2 , N 2 H 6 Br 2 ·2H 2 O, and N 2 H 6 SO 4 have been investigated over appropriate temperature ranges. Minima of T 1 for the halide salts have been identified as due to reorientation of both − NH 3 + groups at equal, or nearly equal, rates with activation energies 9.85, 7.83, and 8.59 kcal mol −1 for chloride, bromide, and hydrated bromide, respectively. A T 1 minimum for the orthorhombic phase of N 2 H 6 SO 4 has been assigned to reorientation of one − NH 3 + group, with an activation energy of 5.85 kcal mol −1 , while the other group reorients very much faster. A low-temperature phase transition in N 2 H 6 SO 4 has been observed. The low-temperature phase is metastable above the temperature of the phase transition and gives a T 1 minimum which has been assigned to reorientation of both − NH 3 + groups at very similar rates, with an average activation energy of 6.16 kcal mo −1 . The activation energies have been compared with the results of a previous inelastic neutron scattering study. A consideration of 14 Nue5f8 1 H dipole-dipole interactions in − NH 3 + has indicated that these may make a contribution of about 14% to the maximum relaxation rate.

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

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.

Nuclear magnetic resonance spin–lattice relaxation and barriers in ethylenediammonium chloride, bromide, iodide and sulphate

N.m.r. T1 has been studied as a function of temperature in the Cl–, Br–, I– and SO42– salts of the ethylenediammonium ion +NH3CH2CH2NH3+. The chloride and sulphate show phase transitions at elevated temperatures. The T1 minima of 21–22 ms (at 31 MHz) in all four salts may be interpreted in terms of —NH3+ reorientation. The observed activation energies for this motion, which vary between 41.3 and 8.6 kJ mol–1, have been related where possible to the nature of the groups environment.

Proton magnetic relaxation study of molecular motion in anilinium chloride, bromide, iodide and sulphate

Spin–lattice relaxation times of anilinium chloride, bromide, iodide, and sulphate have been measured at 31 MHz by pulsed n.m.r. spectrometry over appropriate temperature ranges between 80 and 550 K.Each halide salt has a minimum in T1 of 38 ms, which has been ascribed to reorientation of the –NH+3 group about its C3 axis. Activation energies for this motion are 37.1, 11.2 and 8.5 kJ mol–1 for the chloride, bromide and iodide, respectively. The iodide has a second minimum of 178 ms at much higher temperature (513 K) and this has been ascribed to reorientation of the phenyl ring among more than two (probably four) potential wells about the C–N axis. Although this minimum is not reached by the bromide before decomposition and melting occur, a similar mechanism is likely and one may estimate an activation energy for this motion as 96 kJ mol–1, compared with 75 kJ mol–1 for the iodide. Irregular features of the variation of T1 between about 200 and 300 K in the bromide and iodide can be interpreted in terms of a higher order phase change.The sulphate shows three minima in T1. The two at lower temperature are nearly equal at about 81 ms and have been attributed to relaxation by C3 reorientation of –NH+3 groups in two different crystallographic sites with activation energies of 11.2 and 27.4 kJ mol–1. The third minimum, at higher temperature, has been assigned to a pseudo-C4 reorientation of the phenyl group, with an activation energy of 59 kJ mol–1.The activation energy of the –NH+3 reorientation has been found to be proportional to the temperature of the corresponding minimum in T1, the constant of proportionality being related to τ°c for the motion. The activation energies of the –NH+3 motion are compared with results for other compounds and their variation in magnitude discussed in terms of hydrogen bonding strength and the symmetry of the environment of the group.

Proton spin-lattice relaxation study of molecular motion in tertiary-butylammonium chloride, bromide and iodide

The spin-lattice relaxation times (T1) of the Cl–, Br– and I– salts of (CH3)3CNH+3 and (CH3)3CND+3 have been measured over a range of temperatures by pulsed proton magnetic resonance. All six salts show two T1 minima. For the (CH3)3CND+3 salts these have been interpreted in terms of C3 reorientations of methyl groups (lower temperature minimum) and of the tertiary-butyl group about its C3 axis (higher temperature minimum). The —NH+3 group relaxes most efficiently in the same temperature region as the t-butyl group which consequently modifies the higher temperature minimum in the (CH3)3CNH+3 salts, and by considering the two sets of results the —NH+3 contribution can be separated. The T1 minimum values for the methyl group reorientations are all above the calculated value and the curves show unusual broadening, both effects decreasing in the order of salts Cl–, Br– to I–. This has been interpreted in terms of non-identical environments for the three methyl groups and confirmed by spin-lattice relaxation in the rotating frame (T1ρ measurements) and far infrared spectra. Activation energies have been worked out for all three types of motion. The iodide undergoes a phase transition at higher temperatures. This was checked by differential scanning calorimetry.

Inelastic neutron scattering studies of the torsional and librational modes of the anilinium halides, C6H5NH3X

The low-frequency modes of vibration of the anilinium ion in the salts C6H5NH3X (where X = Cl, Br or I) have been studied by inelastic neutron scattering, infrared and Raman spectroscopies. In addition, spectra of both the N- and C-deuterated analogues, and also of solid toluene, have been measured. Where possible we have identified the torsional modes of the C6H5— and —NH3+ groups. Barriers calculated from the NH3+ torsions have been compared with the energies of activation obtained from nuclear magnetic resonance studies.

Librational and torsional modes in hydroxylammonium, NH3OH+, salts studied by inelastic neutron scattering and infrared spectroscopy

Inelastic neutron scattering spectra of the hydroxylammonium halides and sulphate together with Raman and infrared data on these systems are reported. The two librational/torsional modes of the NH3OH+ ion about the N—O axis have been identified and, with a plausible assumption for the internal barrier, the external barriers to rotation calculated.

Inelastic neutron scattering studies and barriers to methyl rotation in alkali-metal methanesulphonate salts

The “torsional” motion in the methanesulphonate ion (CH3SO3–) in its alkali-metal salts has been investigated by means of incoherent inelastic neutron scattering spectroscopy. Infrared and Raman spectra in the low-frequency region were also obtained to aid in assigning torsional modes. The Cs+ salt shows a single torsional band, while the other salts show several bands which may be assigned to torsions. A model which takes account of the coaxial motions of the SO3 and CH3 groups has been used to calculate values for the sum of the internal and external barriers which hinder reorientation of the CH3 group.

Nuclear magnetic resonance spin–lattice relaxation studies of H3O+ and —CH3 group reorientations in para-toluenesulphonic acid hydrate

Proton spin–lattice relaxation times have been studied in para-toluenesulphonic acid hydrate, CH3C6H4SO3–H3O+, as a function of temperature between 80 K and its melting point. Two relaxation processes occur which have been attributed to three-fold reorientations of the oxonium ion, H3O+, and the methyl group, —CH3. Activation energies of 51.6 and 7.30 kJ mol–1, respectively, have been determined for these processes and they are discussed in terms of the interactions of the rotors with their environment in the lattice.