Complexes of 2,6-dimethylpyridine with water in condensed phases and the dynamical co-operative interactions involving hydrogen bonds

Abstract

The system 2,6-dimethylpyridine–water possesses peculiar phase properties. Three solid phases: C₇H₉N, H₂O and C₇H₉N·H₂O are mutually immiscible, and the liquid system shows a closed miscibility gap. However, the liquid system remains single-phase in the temperature range from the melting point up to 307 K. The present thermodynamic and IR spectroscopy studies showed that the hydrogen bond co-operativity determines the properties of the system in the condensed phases. In the crystals of the hydrate, H/D isotopic self-organisation was observed. This non-conventional phenomenon consists of a non-random distribution of the hydrogen isotopes in the neighbouring hydrogen bonds. This has never been reported for the multicomponent systems. The effects of dynamical couplings involving hydrogen bonds between water molecules suggested a way in which the C₇H₉N·H₂O molecules were arranged in the crystal lattice: chain-like structures with an alternating sequence of the rings. The pronounced co-operativity of the hydrogen bonds, as well as the stability of the hydrates, led to the explanation of the limiting miscibility of the liquids in terms of interactions between the molecules of H₂O and C₇H₉N·H₂O. In a similar way to crystals, the 1 : 1 complexes probably associate thanks the O–H⋯O bonds. Thus, water molecules are necessary to cause aggregation in the liquid mixture. The higher the O–H⋯N bond’s energy, the stronger is the cooperativity and the more stable are the aggregates. Consequently, the propensity to phase separation is also stronger.