Proton Magnetic Resonance Studies of Solid Triethylenediamine—Molecular Structure and Motions

Abstract

Proton magnetic resonance (PMR) studies of solid triethylenediamine (TEDA) were made between 77° and 420°K. Measurements of linewidth, second moment, and spin—lattice relaxation time were determined. Rigid‐lattice second moments yielded a C–H distance of 1.09±0.02 Å but could not distinguish between the D_3h and D₃ molecular configurations. A line narrowing at about 190°K was explained in terms of a restricted molecular reorientation about the N–N axis plus a wobbling of the axis, the activation energy being about 7 kcal/mole. At the temperature of transition to a plastic crystal (351°K) the second moment dropped discontinuously to a value characteristic of general molecular reorientation. Simultaneously, molecular self‐diffusion became effective as a line‐narrowing agent. The activation energy for diffusion was found to be 16.2 kcal/mole, in good agreement with the lattice energy of 14.8 kcal/mole, as estimated from the heat of sublimation. A high‐resolution experiment failed to reveal any interconversion between the D₃ and D_3h molecular configurations. Theoretical expressions for the second moment for isotropic molecular reorientation are given in terms of the density, the molecular mass, and the number of protons per molecule for simple cubic, fcc, bcc, and hcp structures. Within 0.5% the expressions are the same for the last three lattices, and the simple cubic lattice expression is only 13% larger.