NMR investigations of solid (CH3)3CCOOH (trimethylacetic acid) and (CH3)3CCOOD

Abstract

(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.