Factors influencing Kinetic Isotope Effects in Some Proton-transfer Reactions in Aprotic Solvents

Article abstract of DOI:10.1039/P29860001117

Kinetic studies have been carried out on the proton-transfer reactions of (i) 4-nitrobenzyl cyanide (4-NBC) with tetramethylguanidine (TMG) and (ii) 4-nitrophenylphenylcyanomethane (4-NPPCM) with 1,8-bis(dimethylamino)naphthalene (DMAN) in acetonitrile.The reaction (i) has a moderate enthalpy of activation (ΔH⁺⁺ = 25 kJmol^-1) and a negative entropy of activation (ΔS⁺⁺ = -98 Jmol^-1K^-1).The deuterium kinetic isotope effect is large.The reaction (ii) of the more hindered molecule 4-NPPCM and DMAN has similar activation parameters (ΔH⁺⁺ = 26 kJmol^-1, ΔS⁺⁺ = -140 Jmol^-1K^-1), but the kinetic isotope effect is considerably larger; also ΔH_D⁺⁺ - ΔH_H⁺⁺ is larger (13 kJmol^-1) and A_H/A_D is small, indicating a considerable tunnelling effect.Evidence that the larger isotope effect are not artefacts of hydrogen exchange is presented.The tunnelling factor is estimated.