Nuclear Magnetic Resonance of the Aquated Proton. II. Chloroauric Acid Tetrahydrate. Phase Transitions and Molecular Motion

摘要

Proton and deuterium nuclear magnetic resonance and chlorinehyphen;35 nuclear quadrupole resonance of chloroauric acid tetrahydrate,H5O2plus;AuCl4minus;middot; 2H2Ohas been studied from 180 to 300deg;K. Phase transitions are observed at 290deg;Klpar;Solid Iharr;Solid IIrpar;and at 218deg;Klpar;Solid IIharr;Solid IIIrpar;in ordinary chloroauric acid tetrahydrate. In the deuterated compound the lowerhyphen;phase transition shifts upward by 33deg; to 252deg;K; an upperhyphen;phase transition was not detected below 300deg;K. The chlorinehyphen;35 NQR consists of two lines (1:1 intensity ratio) above the Solid IImdash;III phase transition and three lines (1:2:1 intensity ratio) below this transition. The proton spinmdash;lattice relaxation timeT1ofH5O2plus;AuCl4minus;middot; 2H2Oexhibits a minimum at 273deg;K which is assigned to the interbond jump motion of the bridging proton in theH5O2plus;ion. The activation energy for this process is 5.7 kcal moleminus;1in Solid II and 11.7 kcal moleminus;1in Solid III. Rotating frame relaxation times have minima at 200 and 233deg;K which are assigned to the interbond jump process and a proton exchange process, respectively. The proton exchange process involves rotation of the hydronium ion about the threefold axis and translational diffusion of water molecules between sites which differ in water molecule orientation by 90deg;. The deuteron resonance of a single crystal and polycrystallineD5O2plus;AuCl4minus;middot; 2D2Ois interpreted in terms of the deuterium exchange process. This process has an activation energy of 10.4 kcal moleminus;1in Solid II.