Reorientational dynamics of trimethoxyboroxine: A molecular glass former studied by dielectric spectroscopy and 11B nuclear magnetic resonance

Abstract

In this work, trimethoxyboroxine (TMB) is identified as a small-molecule glass former. In its viscous liquid as well as glassy states, static and dynamic properties of TMB are explored using various techniques. It is found that, on average, the structure of the condensed TMB molecules deviates from threefold symmetry so that TMB’s electric dipole moment is nonzero, thus rendering broadband dielectric spectroscopy applicable. This method reveals the super-Arrhenius dynamics that characterizes TMB above its glass transition, which occurs at about 204 K. To extend the temperature range in which the molecular dynamics can be studied, 11B nuclear magnetic resonance experiments are additionally carried out on rotating and stationary samples: Exploiting dynamic second-order shifts, spin-relaxation times, line shape effects, as well as stimulated-echo and two-dimensional exchange spectroscopy, a coherent picture regarding the dynamics of this glass former is gained.

Anion dynamics and motional decoupling in a glycerol–choline chloride deep eutectic solvent studied by one- and two-dimensional 35Cl NMR

“…This technique has been demonstrated for a range of halfinteger quadrupolar nuclei including 11 B, 17 O, and 87 Rb. 49,79,80 For 35 Cl, obviously only 2D spectroscopy in the absence of an external static magnetic field has been performed, so far. 81–83…”

“…Here, we have in mind the stimulated-echo methods that were developed for (other) quadrupolar nuclei in order to examine the dynamics in the milliseconds regime. [47][48][49] The present results regarding the dynamics of the Cl À ions will be compared with the time constants reported 24,25 for the choline cations as well as for glycerol in glyceline and of course also with its overall response that has been studied over a wide dynamic range by means of dielectric spectroscopy 15,43,50 and shear rheology. 51…”

“…Here, we have in mind the stimulated-echo methods that were developed for (other) quadrupolar nuclei in order to examine the dynamics in the milliseconds regime. 47–49…”

Dielectric Relaxation Spectroscopy in Synthetic Rubber Polymers: Nitrile Butadiene Rubber and Ethylene Propylene Diene Monomer

“…e glass transition temperature T g for DRS can be obtained from the temperature dependence of f 0 for an α-relaxation process measured at different frequencies. e T g at DRS is defined as the temperature at which f 0 of the α-relaxation process reaches 1.6 mHz, which was empirically defined in many other prior studies, by extrapolating the fitting of equation ( 4) in Figure 9(a) [15,[40][41][42][43]. T g for DRS was determined to be 242.1 K (±3.2), as shown in Figure 9(a), which was compared with the T g determined by DSC measurements (Figure 9(b)), where T g for DSC is usually defined as the half-height of the heat capacity increase ((1/2)ΔC p , also called the "temperature of half-unfreezing") [15].…”

Time Scales of the Quasitetrahedral Motion in KMnO4 Observed by 17O Central-Transition NMR Spectroscopy

“…Conversely, in the present study, the second-order quadrupolar interactions dominate the exchange-driven modulation of the NMR frequencies. On the basis of the recent development of the central-transition stimulated-echo technique, an efficient exploitation of the second-order quadrupolar interaction allowed us to obtain detailed insights concerning the dynamics in a number of crystalline , and amorphous , materials. Applying this approach to KMnO 4 , we will find an energy barrier of about 0.35 eV for the reorientational dynamics of the manganate group in this substance.…”