NMR Investigations of Solid (CH3)3CCOOH (Trimethylacetic Acid) and (CH3)3CCOOD.

摘要

(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 77K and the melting point (310K). 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 (E(a)) of 2.35 plus or minus 0.15 and 4.00 plus or minus 0.25 kcal/mole, respectively. In the high temperature plastic phase above the transition at 280K, overall molecular tumbling with an E(a)) of 6.0 plus or minus 0.6 kcal/mole governs T1, and self-diffusion with an E(a)) of 12 plus or minus 2 kcal/mole is evident from the spin-lattice relaxation time in the rotating frame (T1p). Also, it is found that T1p falls significantly below T1 in the 30K just below the transition. The deviation increases to as much as an order of magnitude as the transition temperature is approached, being about three-fold 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.