Molecular Reorientation in the Plastic Crystalline Phase of Tris

Abstract

Carbon-¹³ N.M.R. spectra of tris(hydroxymethyl)aminomethane (Tris) have been measured between 407 and 461 K. Proton-decoupled ¹³C N.M.R. spectra of solid Tris between 407 K and its melting point are relatively sharp (v_½ < 30 Hz) indicating rapid overall molecular reorientation in this temperature range. It was not possible to detect a ¹³C N.M.R, signal for Tris below 407 K.

The observed ¹³C N.M.R. spin-lattice relaxation times appear continuous across the solid ↔ liquid phase transition. From the temperature dependence of T₁, a rotational activation energy of 51.6 ± 6 kJ mol^-1 is calculated, which indicates that the molecules must expend considerable energy in reorienting.

The N.M.R. results are discussed in relation to previous differential scanning calorimetry and X-ray diffraction data which indicate that Tris undergoes a solid ↔ solid transition at 407 K.