Pressure effects on thermal isomerizations in highly viscous media. The first clear-cut example of viscosity-induced retardation of “slow” thermal reactions |
| |
| Authors: | Kevin Cossticks Tsutomu Asano Nobuyoshi Ohno |
| |
| Institution: | 1. Department of Chemistry, Faculty of Engineering , Oita University , Oita, 870-11, Japan;2. ICI Fibres , JSPS Fellow (1990-91), PO Box 90, Wilton, Middlesbrough, Cleveland, TS6 SJE, England;3. Department of Chemistry, Faculty of Engineering , Oita University , Oita, 870-11, Japan;4. Department of Mechanical Engineering, Faculty of Science and Technology , Saga University , Saga, 840, Japan |
| |
| Abstract: | Abstract Kinetic effects of pressure on thermal Z/E isomerizations of 4-(dimethylamino)-4′-nitroazo-benzene (DMNAB) and N-4-(dimethylamino)benzylidene]-4-nitroaniline (DMBNA) were studied by flash photolysis in three viscous solvents; glycerol triacetate (GTA), 2-methyl-2,4-pentanediol (MPD) and “Traction Fluid B” (TFB). In all cases studied, the pressure effects observed at the beginning of pressurization were qualitatively similar to the ones observed in less viscous solvents. The results strongly suggest that the conventional understandings of the kinetic effects of pressure based on the transition state theory (TST) are valid in these thermal unimolecular reactions. At higher pressures, however, pressure-induced viscosity increase resulted in strong retardation of both of the isomerizations. The diffusion-controlled rate constants obtained by substituting the observed and the TST-expected rate constants to I/kobs = l/kTST + l/kdif showed inverse fractional dependence on the solvent viscosity. |
| |
| Keywords: | Thermal isomerization kinetic pressure effect diffusion viscosity |
|
|
