DYNAMIC HYDROGEN DISORDER IN SOLID TROPOLONE - A SINGLE-CRYSTAL NMR-STUDY OF THE HYDROXYL DEUTERONS

The orientation and temperature dependences of the deuterium NMR spect rum and spin-lattice relaxation time of the hydroxyl deuterons in sing le crystals of tropolone-d(1) are reported. The results are interprete d in terms of a dynamic hydrogen disorder model in which the hydrogen nuclei move in an asymmetric double well potential. According to this model, the hydrogen-bonded dimer structure as determined by X-ray diff raction constitutes a majority species in the tropolone crystal, compr ising more than 98% of the molecules at room temperature. However, the re also exists a tautomeric minority species formed by a concerted bac k and forth shifting of the hydroxyl hydrogens (deuterons) along the h ydrogen bonds to the nearby carbonyl oxygens. This process results in a modulation of the electric field gradient tensor at the site of the deuterons, thus providing an efficient relaxation mechanism. The conce ntration of the minority species is too low and its Lifetime is too sh ort to make its direct observation possible. Still, structural informa tion about this species and kinetic and thermodynamic parameters of th e hydrogen shift process can be derived by fitting the measured T-1 va lues to the above model. (C) 1997 Academic Press.