Dynamic Hydrogen Disorder in Solid Tropolone. A Single-Crystal NMR Study of the Hydroxyl Deuterons

The orientation and temperature dependences of the deuterium NMR spectrum and spin-lattice relaxation time of the hydroxyl deuterons in single crystals of tropolone-d 1 are reported. The results are interpreted 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 diffraction constitutes a majority species in the tropolone crystal, comprising more than 98% of the molecules at room temperature. However, there also exists a tautomeric minority species formed by a concerted back and forth shifting of the hydroxyl hydrogens (deuterons) along the hydrogen 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 concentration of the minority species is too low and its lifetime is too short to make its direct observation possible. Still, structural information about this species and kinetic and thermodynamic parameters of the hydrogen shift process can be derived by fitting the measured T 1 values to the above model.