2H-NMR studies of supercooled and glassy aspirin

TL;DR

This study uses 2H-NMR to analyze molecular reorientations and tunneling in supercooled and glassy aspirin, revealing similarities with other glass formers and detailed insights into molecular dynamics at various temperatures.

Contribution

It provides new insights into the molecular dynamics of aspirin in supercooled and glassy states using 2H-NMR techniques, highlighting tunneling and relaxation behaviors.

Findings

Molecular reorientations above Tg are similar to other glass formers.

Spectra indicate rotational tunneling below 20 K.

Distribution of correlation times in the glass reveals complex dynamics.

Abstract

Acetyl salicylic acid, deuterated at the methyl group, was investigated using 2H-NMR in its supercooled and glassy states. Just above the glass transition temperature the molecular reorientations were studied using stimulated-echo spectroscopy and demonstrated a large degree of similarity with other glass formers. Deep in the glassy phase the NMR spectra look similar to those reported for the crystal [A. Detken, P. Focke, H. Zimmermann, U. Haeberlen, Z. Olejniczak, Z. T. Lalowicz, Z. Naturforsch. A 50 (1995) 95] and below 20 K they are indicative for rotational tunneling with a relatively large tunneling frequency. Measurements of the spin-lattice relaxation times for temperatures below 150 K reveal a broad distribution of correlation times in the glass. The dominant energy barrier characterizing the slow-down of the methyl group is significantly smaller than the well defined barrier in the crystal.