High Pressure Nmr Studies of the Dynamic Structure of Liquids
Hasha, Dennis Lloyd
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https://hdl.handle.net/2142/67292
Description
Title
High Pressure Nmr Studies of the Dynamic Structure of Liquids
Author(s)
Hasha, Dennis Lloyd
Issue Date
1981
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Physical
Language
eng
Abstract
The high pressure-high resolution nuclear magnetic resonance technique at high fields is developed and used to study the conformational isomerization of cyclohexane in solution. The solvents used in this study are methylcyclohexane-d(,14), carbon disulfide and acetone-d(,6). It is found that the activation parameters obtained from the temperature dependence of the rate constant are independent of solvent. However, the coalescence temperature in the methylcyclohexane-d(,14) solvent is 1.5(DEGREES)C higher than in the other two solvents. It is observed that ring inversion in cyclohexane is accelerated when pressure is applied. This pressure dependence which is non-linear is more pronounced in acetone-d(,6) and carbon disulfide than in the more viscous methylcyclohexane-d(,14) solvent. This behavior provides the first experimental proof of the breakdown of transition state theory, thus confirming recent predictions of stochastic models for isomerization reactions in condensed phases. In these models it is proposed that there are dynamical effects on isomerization because the reaction coordinate is coupled to the surrounding medium. This leads to the dependence of the transmission coefficient upon the collision frequency, which in the absence of electrostatic effects reflects the actual coupling of the reaction coordinate to the surrounding medium. In classical transition state theory the transmission coefficient is assumed to be unity and independent of the thermodynamic state. The solvent viscosity which is proportional to the collision frequency is measured under the same experimental conditions. The solvents used enabled the transition between the inertial and diffusive regimes to be observed.
The transport behavior of cyclohexane, methylcyclohexane, n-butane and n-pentane are measured over a wide range of temperature and pressure. The rough hard sphere model is used to obtain hard-core diameters and rotational-translational coupling constants. The Stokes-Einstein equation, relating self diffusion and shear viscosity, is found to be valid in the slip limit.
Many of the liquids for which transport coefficients have been measured at elevated pressures form so-called plastic crystals upon freezing. A simple correlation of transport properties for these liquids (for convenience we use the term plastic liquids) is presented. In the absence of strong attractive forces i.e., hydrogen bonding, the transport coefficients have the same dependence on reduced density. The self diffusion and fluidity (= 1/(eta)) are described by the same reduced density dependence. The thermal conductivity which is less sensitive to the nature of the potential and dynamics of the dense liquid is a weaker function of reduced density than the other transport properties. Comparison with other liquids reveals that the correlation is unique to plastic liquids.
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