Solvent, pressure, and deuterium isotope effects on proton tunnelling. The reaction between 2,4,6-trinitrotoluene and 1,8-diazabicyclo[5.4.0]undec-7-ene in aprotic solvents

Abstract

Proton tunnelling has been examined for the reaction between 2,4,6-trinitrotoluene and 1,8-diazabicyclo-[5.4.0]undec-7-ene in several aprotic solvents (acetonitrile, benzonitrile, 1,2-dichloroethane, and dichloromethane) in view of the solvent and pressure effects on the rate ratio k^H/k^D. The reaction rates have been measured in the range 1–1 000 bar, at 25 °C by a high-pressure stopped-flow method. The reaction rate ratios k^H/k^D in all these solvents are greater than the semiclassical limit of the primary kinetic isotope effect and show a significant tunnelling contribution. Further, with increasing pressure from 1 to 1 000 bar, the ratio k^H/k^D at 25 °C diminishes from 19.1 to 16.9 in acetonitrile, from 15.9 to 15.0 in benzonitrile, and from 29.9 to 22.5 in 1,2-dichloroethane, respectively. However, the increases of the effective mass of the transferred particle with pressure are calculated to be about the same in all these solvents.