High Pressure Nmr Studies of the Dynamic Structure of Liquids
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.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign