Kinetic isotope effects and tunnelling corrections in the proton-transfer reactions between 4-nitrophenylnitromethane and some tertiary amines in aprotic solvents

Abstract

Kinetic isotope effects for the proton-transfer reaction between 4-nitrophenylnitromethane and the tertiary amine bases tri-n-butylamine, triethylamine and quinuclidine, in toluene, have been determined at temperatures between –15°C and +35°C by the stopped-flow technique. The rate ratios k^H/k^D for these bases at 25°C are respectively 14 ± 1, 11.0 ± 0.7 and 15.6 ± 0.6. The ratios of the pre-exponential factors A^D/A^H are respectively 1, 4 and 9. These values are interpreted in terms of tunnelling. Bell's equations for an unsymmetrical parabolic barrier give almost the same barrier width for all three bases. Earlier results for such reactions in acetonitrile are interpreted in terms of coupling of the motions of solvent molecules with that of the proton.