An experimental examination of Biswas–Bagchiʹs prediction on the viscosity dependence of the rate of activated barrier surmounting in viscous liquids

Biswas and Bagchi predicted that dynamic solvent effects on barrier-surmounting reactions render their rate constant kobs proportional to (log η)−1 asymptotically as the solvent viscosity η exceeds 1 Pa s. This was tested for thermal Z/E isomerization of azobenzenes and N-benzylideneanilines by varying η up to 107 Pa s under pressure. The prediction was not confirmed: over 100-fold η variation, kobs appears to follow A+(log η)−1 with A<0, but this form is inappropriate as an asymptotic form, approaching a negative constant A. Over the whole region of η≳100 Pa s, kobs rather obeyed η−β with 0<β<1.