Proton Magnetic Resonance Studies on Hexamethylenetetramine—Structure, Molecular Reorientation, and Impurity Diffusion

Abstract

Nuclear magnetic resonance measurements on protons in solid hexamethylenetetramine have led to information about the molecular structure and motions and diffusion of impurity water molecules in the solid. The information obtained is consistent with previous work indicating that the solid contains distorted molecules with a C–H distance of 1.13 A in a body‐centered cubic crystal structure. From temperature dependences of a linewidth transition at 375°K, and a relaxation time minimum at 550°K, a rotational activation energy of 18 kcal/mole was computed. Sublimation, with a latent heat of 18 kcal/mole, occurs at the same temperatures as the rotation. Intermolecular C–H···N hydrogen bonds are the source of both the stability of the crystal and the barrier to rotation. The energy of these bonds appears to be 1.2 kcal/mole per bond. It was determined that a water impurity was present, in an amount of about 0.1% by weight, and was diffusing in the solid. The diffusion produced an anomalous minimum in T₁. Rough analysis of the T₁ minimum, using the theories of Bloembergen, Purcell, and Pound and of Torrey, yielded values of the infinite temperature diffusion coefficient D₀ for the water of 10^—3 to 10^—1 cm²/sec and an activation energy of 5 to 7 kcal/mole.