The temperature dependences of methyl group and (CHS)4N+ ion reorientations in the tetramethylammonium salts [(CH S )4N+X-where X is CI, Hr, I] were studied by means of the proton spin-lattice relaxation times in the static and rotating frame (1'1 and TIP) ' The 1\ values calculated for the motions are in good agreement with the double minima observed for each compound. Activation energies and frequency factors were obtained for both motions from the temperature dependence of T, in the three salts. Rotating frame spin-lattice relaxation times were also measured over temperatures from 200 to 380 0 K for (CHs)4NCI, giving results in agreement with those for T,. A discontinuity in the T, of (CHs)4NCI was found near 418°K. This led us to make a preliminary DTA investigation with results indicating that a first-order phase transition may occur near 434°K and another definitely at 528± 1 0 K. The appearance of the phase above 434°K is accompanied by a modulation on the free induction envelope which we ascribe to piezoelectric properties. The phase can be supercooled to room temperature and remains for a considerable length of time. INTRODUCTIONThe motions of the tetramethylammonium (TMA) ion in the solid halides have been the subject of several recent NMR investigations. I-a In the most complete study,! the proton linewidths and second moments were measured for TMA chloride, bromide, and iodide relaxation measurements in order to resolve the two motional processes; the DTA was prompted by the unexpected discontinuity in our TI data for the compound. EXPERIMENTAL PROCEDURE AND RESULTSbetween 77 and 4000K. It was found that at the lowest The three TMA halides were obtained from comtemperatures the absorption is "rigid lattice," while mercial sources. The bromide and chloride came from above room temperature there is motional narrowing Eastman Organic Chemicals, Rochester, New York, by over-all tumbling of the TMA cation as well as by and the iodide (99+% pure) from Matheson, Colemethyl group rotation. At intermediate temperatures man and Bell. A quantity of each was dried over there is a region where motional narrowing is produced molecular sieves in a vacuum dessicator for a number of by methyl group rotation. Proton spin-lattice relax-days and then sealed under vacuum in a Pyrex tube. ation time (T I ) measurement on the TMA bromide The chloride was pumped and dried for longer periods yielded a single minimum which was attributed to the than the two other salts due to its stronger hygroscopic combined effects of methyl group reorientation and nature. Our samples were microanalyzed and were molecular tumbling. 2 Rotating frame spin-lattice re-found to be dry and pure. The proton TJ and Tip laxation times (TIp) of the TMA iodide showed the were' measured as a function of temperature between presence of two minima,a but the data were not analyzed 171 and 483°K at a frequency of 2S.3 MHz using the in any detail. apparatus lO and techniques ll • 12 described previously.1° At room temperature, these three compounds are The ...
The three phases of solid ammonium hexafluorophosphate (NH4PF6) have been studied by cw and pulsed NMR methods between 298° and 77°K. The proton and fluorine second moments of 1.8 and 1.9 G2, respectively, at 298°K are consistent with rapid, randomized reorientations of NH4+ and PF6- ions, while the larger values at 77°K require that there be little or no motional narrowing by reorientations of the PF6− ions even though the narrowing by NH4+ reorientations is still present. For both nuclear species the spin-lattice relaxation time (T1) studies of the two low-temperature phases reveal three distinct regions: (1) Between 109° and 110°K their relaxation is exponential with time. (2) Between 105° and 87°K the relaxation is nonexponential, but may be expressed as the sum of two exponential terms. At each temperature the same pair of time constants applies to both nuclear species. (3) At 77°K the relaxation is again exponential, with different T1's for protons and fluorine. The nonexponential behavior in region (2) is attributed to spin exchange between protons and fluorine nuclei. It results from modulation of the H–F dipole-dipole interactions by relatively slow reorientations of the PF6− ions. This interpretation was confirmed by a study of the fluorine T1 of ND4PF6, for which only exponential decays were observed. Activation parameters for the PF6- reorientation were determined from the fluorine T1 in ND4PF6. The parameters depend somewhat upon temperature, being 4.1 ± 0.2 kcal/mole and (3.7 ± 0.5) × 10−16 sec above 130°K and 4.4 ± 0.3 and (5.2 ± 1.5) × 10−17 below. A discontinuity occurs in the T1's at the 192°K phase transition, and in the high temperature NaCl phase the T1's are almost independent of temperature and probably are governed by spin-rotation interactions. DTA experiments indicate that no additional phase transitions occur in NH4PF6 between 300° and 473°K.
(CH3)3CCOOH (trimethylacetic or pivalic acid) and (CH3)3CCOOD have been investigated in the plastic and brittle modifications by pulsed and continuous wave proton magnetic resonance methods between 77 °K and the melting point (310 °K). For the low-temperature phase of (CH3)3CCOOD, the second moment and spin–lattice relaxation time (T1) of the protons are in agreement with a combination of methyl group (C3) and t-butyl group (C3′) reorientations having activation energies (Ea) of 2.35±0.15 and 4.00±0.25 kcal/mole, respectively. In the high-temperature plastic phase above the transition at 280 °K, overall molecular tumbling with an Ea of 6.0±0.6 kcal/mole governs T1, and self-diffusion with an Ea of 12±2 kcal/mole is evident from the spin–lattice relaxation time in the rotating frame (T1ρ). Also, it is found that T1ρ falls significantly below T1 in the 30° just below the transition. The deviation increases to as much as an order of magnitude as the transition temperature is approached, being about threefold larger for the protonated form of the acid than for the deuterated. This behavior is consistent with the slow onset of molecular tumbling. We suggest that the faster rate in the protonated compound may be attributed to the importance of quantum mechanical tunneling in the breaking and reforming of hydrogen bonds during the tumbling process.
Continuous wave and pulsed NMR studies were carried out on solid hexafluorobenzene (C6F6) from 270 to 77°K. The temperature dependence of the second moment exhibits two regions of motional narrowing and that of the spin-lattice relaxation time shows minima at 142.5 and 260°K. The spin-lattice relaxation in the rotating frame has a minimum at 172°K and decreases toward a second minimum at low temperature. Although the line shape is asymmetric in the entire temperature range, it differs between 100 and 145°K from that at higher and lower temperatures. This intermediate region line shape is interpreted as a superposition of two components from motionally inequivalent hexafluorobenzene molecules. The second moment and relaxation time data are consistent with a model in which two different sets of C6F6 molecules reorient around their sixfold axis at different frequencies. Using BPP expressions, we obtain from our T1 and T1ρ data the following parameters for the low- and high-temperature motions, respectively, Ea=2.85± 0.15 kcal/mole, τ0=(1.60± 0.1)× 10−13sec,Ea′=7.00± 0.40 kcal/mole, τ0′=(5.00± 0.3)× 10−15 sec.
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