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Free keywords:
Crystals; Dyes; Glasses; Polymers; Proton Transfer; Spectroscopy, Nuclear Magnetic Resonance
Abstract:
This paper deals with the question of how symmetric double minimum potentials of isolated bistable molecules are affected in the ordered or disordered solid state, in a timescale of slow molecular motion. It is shown that high resolution solid state NMR under conditions of magic angle spinning (MAS) and cross-polarization (CP) can contribute to answering this question. In particular, variable temperature high resolution solid state 15N CPMAS NMR spectra (CP = cross polarization, MAS = magic angle spinning) of 15N enriched tetraaza[14]annulene dyes in the crystalline state and imbedded in disordered glassy polystyrene are presented. These dyes interconvert fast between different tautomeric states whose gasphase degeneracy is lifted in the solid. By contrast to the crystalline state where all dye molecules experience the same solid state perturbation i.e. the same equilibrium constants of tautomerism, molecules dissolved in the glass exist in a multitude of different inequivalent sites with different solid state perturbations. This effect leads to a broad distribution of equilibrium constants of dye tautomerism. This interpretation is confirmed by NMR lineshape simulations and two-dimensional 15N CPMAS NMR experiments by which the temperature dependent distribution can be characterized. High temperature experiments in the region of the glass transition give insights into the process leading to motionally averaged symmetric double minimum potential of tautomerism within the NMR timescale. Thus, solid solution NMR studies provide information about elementary steps of molecular rearrangements in a timescale of slow solvent reorientation. In addition, dyes such as tetraaza[14]annulenes can be used as novel NMR probes for microscopic order and motion in glasses.
