Nuclear magnetic resonance and dielectric spectroscopy of a simple supercooled liquid: 2-methyl tetrahydrofuran

Abstract

The small-molecule glass former methyl tetrahydrofuran ͑MTHF͒ was investigated using dielectric spectroscopy, spin-lattice relaxometry, multidimensional stimulated-echo nuclear magnetic resonance techniques, and field gradient diffusometry. We show experimentally that MTHF nicely fits into the pattern of related small-molecule glass-forming liquids, including the existence of a high-frequency contribution to the dielectric loss, the appearance of a pronounced translational enhancement, the dominance of small average rotational jump angles, and the existence of short-lived dynamical heterogeneity.

From heterogeneous probe rotation to the hydrodynamic limit

“…(3), it is near the largest time constant s lim of the probability density g HN (s) of relaxation times s. The 'average' relaxation time is closer to s h c h % s h /2 [30]. This s ex % s lim result is compatible with most estimates on the time scale of rate exchange, see for example the compilation by Qi et al [31]. Significantly longer exchange times relative to the structural relaxation time s a have been observed only by deep photobleach [32] and single molecule [33] experiments, with values approaching s ex % 10 3 s a only at temperatures very close to T g .…”

Fast stochastic librations and slow small-angle rotations of molecules in glasses observed on nitroxide spin probes by stimulated electron spin echo spectroscopy

“…2) and for o-terphenyl below 230 K (Figs. [3][4][5][6], no relaxation is seen, i.e., W(T) is near zero. Above these temperatures W(T) becomes non-zero, and experimental points form straight lines, which shift upwards with increasing temperature.…”

“…This suggestion of course requires appropriate theoretical consideration, which is however beyond the scope of the present paper. It is also interesting to note that mentioned s 0 values are of the same order as values for relaxation times for dielectric b-relaxation [1][2][3][4].…”

“…Data on these motions have been obtained by NMR [4][5][6][7]28,29], ESE [14], and other techniques. It has been suggested that small-amplitude angular motions, with angles of 2-15°, may be superimposed on large-angle jumps, and the presence of small-angle librations has also been postulated [6,12,14,28,29].…”

“…Molecular motions in glassy supercooled liquids have been studied using dielectric relaxation [1][2][3][4], nuclear magnetic resonance (NMR) [4][5][6][7], electron paramagnetic resonance (EPR) [8][9][10][11], and many other techniques. However, the microscopic nature of the phenomenon of glass transition is still unclear.…”