Protonenresonanzuntersuchungen über innermolekulare Bewegungen, Phasenumwandlungen und Entglasungsvorgänge in festen Äthern

Abstract

Proton magnetic resonance investigations on solid dimethylether (DM) , diethylether (DE) , dipropylether (DP), diisopropylether (DIP), dibutylether (DB), dimethoxymethane (DMM), diethoxymethane (DEM), dimethoxyethane (DME), diethoxyethane (DEE), acetaldehyde (ACA) and diethylketone (DEK) yielded information on molecular motion, solid state phase transitions and deglassing processes. The temperature-dependence of the spin-lattice relaxation time and the second moment was studied between the melting point and 77 °K using radiofrequency pulse techniques. Both spin-lattice relaxation and line shapes are governed by dipolar interactions which are modulated in time by hindered rotations of CH3-groups. The parameters for this relaxation mechanism are given in detail. Concerning the general results it was possible to distinguish between three cases : a) substances that form only one crystalline phase in the solid state (DM, DIP, DEM, DME, ACA, and DEK); b) ethers that form only a vitreous state (DP and DEE) and c) ethers that form both crystalline and vitreous states depending on the way in which the liquid was cooled. In the last case a pronounced decrease of the second moment of the absorption line was observed near the phase transition from the vitreous to the polycrystalline state. This means that within a small range of temperatures far below the melting point some kind of melting of the glass occurs before the crystal is formed.