The relative motion of ions in solution. II. An NMR relaxation study of attractive ions in water at low ionic strength J. Chem. Phys. 80, 6267 (1984); 10.1063/1.446729 Study of adsorbed water: Electric potential calculation and molecular orientation in the two layer hydrate of a Mg vermiculite J. Chem. Phys. 67, 5252 (1977); 10.1063/1.434702 NMR study of adsorbed water. I . Molecular orientation and protonic motions in the twolayer hydrate of a Na vermiculiteIn the electrically neutral interlayer space of endellite, the water molecule does not exhibit a preferential orientation such as that observed in phyllosilicates with water hydrating the interlayer cations (see, for example, the first paper in this series). The influence of the neutral surface is to provoke an anisotropy of the molecular motions which can be separated into a rapid 180 0 flipping motion about the C 2 axis and a slow tumbling about an axis perpendicular to C 2' The activation energy of the former motion is 4.6 kcal mole-I, whereas that of the latter is 3.6 kcal mole-I. The proton and deuteron spin-spin and spin-lattice relaxation times can be accounted for by this simple model. Two other types of experimental results complement this model. The observed infrared OH frequencies suggest weak hydrogen bonds between water molecules and surface lattice hydroxy Is, whereas strong hydrogen bonds lock water molecules to each other. The molar heat capacity of adsorbed water parallels that of ice up to the temperature (130 OK) where a torsional motion about the C 2 axis is transformed into the 180 ° flipping motion.2018

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six polar hydroxy sites. Sorbitol, in contrast, appearsAcknowledgmerti. The financial assistance of the to be a struchre breaker, as is indicated by the mono-National Science Foundation is gratefully aeknowlronic negatiw erilhalpies over the entire concentration edged. J. K. S. wishes to thank Professor Y, Marcus range investigated, This behavior of ART, is similar at> the Hebrew University for his hospitality. for XaCE in acjil~aoixs hydrogen peroxideJ5 ureaJ6 and %,hi& differ d r o~i d e s .~ in position of one of their six hy-bility of Inorganic and Organic Compounds,'' Supplement to 3rd Publication costs assisted by the Muage Royal de E'Afrigue CentraleThe proton motions in the methanol-silica gel (Xerogel) systems have been deduced from the measurement of the spin-lattice (TI) and spin-spin (T,) proton relaxation times in the systems C€LOD-XOD, CDIOH-XOK, and CH,OH-XOH, where XOH and XOD stand for the Xerogel-OH or Xerogel-OD surfaces, respectively. In CH30D-XOD, only one TI or T2 is observed. The experimental data may be accounted for by a laandational motion of CH,OD, the methyl. group rotating very rapidly around the triad axis. In CD30H-XOH, one TI aiid two Tt are measured. TI is the superimposition of two functions. The rate of exchange of nuclei between the two populations is fast with respect to T1-l but slow with respect to T2-I. The long TZ represents the diffusion motion observed in the first system, while the short T2 is assumed to be due to a proton exchange process between the hydroxylic protons of C&OH and XOH or between CDIOI~ molecules. Moreover, it has been possible to compute the observed Ti for the CHIOH-XOH system from the data obtained for the two simpler cases. At 25", the surface diffusion coefficient of adsorbed methanol is reduced by two orders of magnitude with respect to that in the liquid for a degree of coverage e ^I 0.6. For 1 < e < 2, the lowering is by a factor of 10. For e 2 0.7, the surface diffusion coefficient at 25' is the same as that in the liquid state at -85". More than three layers should be probably required to find a diffusion coefficient in the adsorbed state close to that in the liquid state. In the assumed exchange process a proton is transferred from the surface into a cluster of adsorbed species where it jumps from a molecule to another before coming back on the surface: [-OH + CB301-I 3 1 0-+ CH10H2+; CH30H2+ + CH80J1 -+ CH30H + C€InOIh+; CH3OH2" + -0-[ -+ ] -OH + CHaOH. The ratio of the exchange to the molecular diffusion jump frequency ( v e / u d ) is 6 x 10-3 for e = 1.7 and 2.5 x 10-2 for e = 0.8. This means that the number of times a methanol molecule is protonated during its stay on an adsorption site increases at decreasing degree of coverage. For 0 = 0.8, u./ud is very close to the value observed in the liquid protonated methanol al, 25" The activation energies of both the diffusion and the proton exchange processes are dose to each other (-6 kcal at 8 = l), and there are approximately equal to half the isosteric heat of adsorption. Both these motio...

Deuteron longitudinal (Ti) and transversal (T2) relaxation times have been measured for CD3OH adsorbed on the hydroxylic surface of a Xerogel silica gel (the CD3OH-XOH system) and for the related CH3OD-XOD system. These data complement those obtained and discussed earlier for the proton resonance.In the CD3OH-XOH system, is associated with a coverage independent correlation time (jm) = 2.9 X 10-16 exp(5.4 kcal mol-1//??7) sec which has been associated with a molecule tumbling on an adsorption site. T2 seems to be mostly influenced by reorientational effects even though no doublet splitting is apparent in the decay of the magnetization following a 90°p ulse. In the CD3OD-XOD system, has contributions from a molecular diffusion and an exchange term, the latter becoming increasingly important at lower degrees of coverage. At = 1.7, the correlation time associated with and T2 is indeed in the range of those obtained primarily for molecular diffusion: (rod) = 3.15 X 10-14 exp(5.5 kcal mol-1/ RT) sec. For smaller , T2 is influenced by another process and with analogy to what was found for the proton resonance, it appears that this may be a deuteron exchange such as CHsOD2+ + CH3OD. If this is the case, then the quadrupole coupling constant is significantly lower (perhaps 50%) than that used for molecular diffusion. For the two systems studied here, approximately the same distribution of correlation times considered in the proton resonance study is used to account for the experimental data.