Kinetic Study of Proton Tunneling in the Reaction between 2,4,6-Trinitrotoluene and 1,8-Diazabicyclo[5.4.0]undec-7-ene in Benzonitrile

The proton tunneling has been examined for the reaction between 2,4,6-trinitrotoluene and 1,8-diazabicyclo[5.4.0]undec-7-ene in benzonitrile in view of the solvent effect on the rate ratio kH⁄kD as well as on the various activation parameters. The reaction rate ratio kH⁄kD diminishes from 18.3 at 15 °C to 14.5 at 35 °C. The difference in the activation energies EaD–EaH is 8.8 kJ mol−1 and the ratio of Arrhenius preexponential factors is 2.5. All these values are greater than the semiclassical limits of the primary kinetic isotope effects. According to Bell’s equation the remarkable isotope effect is due to considerable contribution from tunneling in benzonitrile. The present results, being compared with those in other aprotic solvents, are discussed in relation with the effects of solvent polarity, specific acid–solvent interaction, and geometrical bulkiness in the vicinity of the reaction site.