Cyclohexane, pressure effects inversion

The study of the pressure effects on the conformational inversion of cyclohexane in different solvents represents the second illustrative example of the application of high resolution, high pressure FT NMR. 

There were two main motivations for our study. First, we wanted to follow the effects of pressure on the conformational inversion of cyclohexane in different solvents acetone-dg, carbon disulfide and perdeuterated methylcyclohexane. Secondly, in view of recent theoretical research in the area of stochastic models for isomerization reactions as proposed by Montgomery, Chandler and Berne ( 8) and by Skinner and Wolynes ( 9) we attempted to provide the first experimental proof of the theoretical prediction of large pressure effects on the transmission coefficient, or otherwise stated, a large colllslonal contribution to the activation volume for an isomerization reaction. Our results show that we were successful in both aspects. 

Substrates such as H2 which can form strong agostic complexes are expected to inhibit the oxidation of more weakly bound substrates by sequestering the active oxidant (Scheme 9). In fact, the oxidation of cyclohexane was inhibited by the presence of H2 [137]. On removal of the H2 from the reaction system, the rate of cyclohexanol production returns to its original value in the absence of H2, demonstrating that the inhibition is reversible. The pressure at which the rate was half that of the uninhibited reaction (P1/2) was 1.4 atm at 0°C [137]. When H2 was replaced by D2, the P1/2 value became smaller (1.0 atm) [137]. Such an inverse isotope effect can be compared with the value (0.78) reported for H2/D2 binding to a tungsten... 

---