Objective
The proton dynamics in a number of solid heterocycles have been investigated. For hydrogen-1 fast magnetic field cycling nuclear magnetic resonance (NMR) spectroscopy experiments, a special field cycling spectrometer was constructed.
The X-ray crystal structure was examined for evidence of disorder in the N-H to N hydrogen bonding between the molecules, some of which form discrete n-mers (n = 2,3,4); selected examples were then enriched with nitrogen-15 and studied by nitrogen-15 (and in some cases carbon-13) cross polarization - magic angle spinning (CP-MAS) NMR spectroscopy. From changes in the appearance of the spectra with temperature, the presence of dynamic disorder could be inferred and the proton jump times measured. A smaller number of compounds were then studied by high speed MAS magnetization transfer and the frequency dispersion of the hydrogen-1 spin lattice relaxation time. Deuterated samples with a wide range of enrichment were also investigated in order to study the kinetic isotope effect as a function of temperature.
A wealth of new information has been obtained on electronic structure and mechanisms of proton transfer in the solid state. There is some evidence for a change in the transfer mechanism between the dimer of 3,5-diphenyl-4-bromo-pyrazole and the trimer of 3,5-dimethylpyrazole, in which the double and triple proton jumps, respectively, are believed to be concerted, and the tetramers of 3,5-phenyl-methyl-pyrazole and 3,5-diphenylpyrazole, in which there is some theoretical evidence for a stepwise mechanism. It has been shown that in the trimer of 3,5-dimethyl-pyrazole, the rate constant departs very significantly from an Arrhenius dependence at low temperatures, due, it is believed, to the existence of proton tunnelling as a major reaction pathway, for which the kinetic isotope effect is providing further evidence. Studies of nitrogen-15 chemical shift anisotropy and intermolecular proton transfer in host guest crystals and on surfaces have also been performed. Experiments to measure the N-H distance in several N-H to N hydrogen bonds have recently shown great promise.
Proton exchange and tunnelling in a number of crystals containing N-H...N N-H...O and O-H...O hydrogen bonds will be studied by a combination of the techniques of CPMAS NMR spectroscopy and quadrupole resonance and relaxation, so as to cover the widest possible range of exchange lifetimes. Additionnal studies will be carried out on molecules embedded in disordered organic glasses and other host materials. X-ray crystal structure determinations combined with LCAO-MO calculations on molecular aggregates will also be undertaken in an effort to correlate the dynamic, thermodynamic and structural factors.
Fields of science (EuroSciVoc)
CORDIS classifies projects with EuroSciVoc, a multilingual taxonomy of fields of science, through a semi-automatic process based on NLP techniques. See: The European Science Vocabulary.
CORDIS classifies projects with EuroSciVoc, a multilingual taxonomy of fields of science, through a semi-automatic process based on NLP techniques. See: The European Science Vocabulary.
- engineering and technology materials engineering crystals
- natural sciences physical sciences optics spectroscopy absorption spectroscopy
- engineering and technology materials engineering amorphous solids
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Coordinator
WC2R 2LS LONDON
United Kingdom
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