Kinetic-freezing dynamics of the supercooled liquid state in 2-cyclooctylamino-5-nitropyridine

Abstract

The nature of the supercooled liquid state, namely, either a kinetic freezing phenomenon or an equilibrium glass phase transition, has been studied in the organic material [2-N-(cyclooctylamino)-5-nitropyridine] (COANP) and the results were compared to a deuteron glass phase of ${\mathrm{Rb}}_{0.56}{(\mathrm{N}\mathrm{D}}_{4}{)}_{0.44}{\mathrm{D}}_{2}{\mathrm{PO}}_{4}$ (DRADP-44). In partially deuterated COANP the ${}^{1}H$ line-shape analysis shows a continuous broadening of the spectrum on cooling that is typical for a slowing-down molecular dynamics where the thermal motions gradually freeze out. The motions are characterized by a nonexponential decay of the autocorrelation function. Two types of motion are involved in the freezing scenario. On cooling the isotropic molecular reorientations with the Vogel-Fulcher-type dynamics freeze first. This is followed by the freezing of the Arrhenius-type intramolecular motions. The kinetic-slowing down dynamics is observed also in the gradual broadening of the deuteron spectrum. In the low-temperature part of the supercooled phase the frozen-in molecular orientations result in a strongly disordered amorphous structure where all molecular orientations are equally probable. The corresponding deuteron static NMR spectral shape in the supercooled phase equals that of a crystalline powder. The degrees of static and dynamic disorder have been observed spectroscopically separated by the two-dimensional (2D) NMR separation of static and dynamic line-shapes technique. It is found that the glass disorder in the high-temperature part of the supercooled state is purely dynamic, exhibiting no static component. This is different from the deuteron glass phase in DRADP-44 where a similar 2D separation experiment has shown that the static component of the disorder is present in both the fast and the slow motion regimes. The asymmetric static ${}^{87}\mathrm{Rb}$ line shape demonstrates that the glass disorder in DRADP-44 is weak and represents small deviations from the crystalline order. The supercooled state in COANP appears as a pure kinetic-freezing phenomenon when observed on the frequency scale of the NMR line shape.