Kinetic hydrogen isotope effects in the reaction between 2,4,6-trinitrotoluene and 1-ethylpiperidine in acetonitrile. The effect of pressure

Abstract

The effect of pressure on the kinetic isotope ratio k^H/k^D and tunnelling factor Q has been examined for proton and deutron transfer from 2,4,6-trinitrotoluene (TNT) to 1-ethylpiperidine in acetonitrile. The reaction rate has been measured in the pressure range 1–1000 bar and in the temperature range 15–35 °C by means of a high-pressure stopped-flow method. Even at 1000 bar, the kinetic isotope ratio k^H/k^D(16.4), the activation energy difference E_a(D)–E_a(H)(10.0 kJ mol^–1) and the ratio of Arrhenius pre-exponential factor A(D)/A(H)(4.0) are large, suggesting a considerable tunnelling contribution. However, the tunnelling factor Q(H), calculated according to Bell's equation, hardly changes with pressure (being 2.39 at 1 bar and 2.26 at 1000 bar). Tunnelling phenomena and related pressure effects are discussed in relation to the curvature of potential-energy barriers. The present results lead us to conclude that large changes of k^H/k^D with pressure do not always confirm the occurrence of tunnelling.