Thermoneutral Isotope Exchange Reactions of Cations in the Gas Phase

Article abstract of DOI:10.1021/ja00403a005

Rate constants have been measured for reactions of the type AD₂⁺ + MH --> MD + ADH⁺, where AD₂⁺ is CD3CND⁺, CD3CDOD⁺, (CD3COCD3)D⁺, or (C2D5)2OD⁺ and the MH molecules are alcohols, acids, mercaptanes, H2S, AsH3, PH3, or aromatic molecules.Rate constants are also presented for the reactions Ar_HD⁺ + D2O --> Ar_DD⁺ + HDO, where Ar_HD⁺ is a deuteronated aromatic molecule and Ar_DD⁺ is the same species with a D atom incorporated on the ring.In all but two cases, the competing deuteron transfer is sufficiently endothermic that it cannot be observed under the conditions of the ICR experiments at 320 - 420 K.The efficiencies of the isotope exchange reactions are interpreted in terms of estimated potential surface cross sections for the reactions AD₂⁺ + MH --> 2⁺*MH> --> +> --> +*MD> --> ADH⁺ + MD.When the formation of the +> complex is estimated to be thermoneutral or slightly endothermic, the isotope exchange process is inefficient (probability of a reactive collision < 0.1) or does not occur.The most efficient isotope exchange reactions are observed for those systems for which it is estimated that the transformation 2⁺*MH> --> +> is exothermic.For most of the systems, trends in reaction efficiency appear to be related to factors such as dipole moments of reactant species (or for aromatic compounds, the electron-donating or -withdrawing properties of ring substituents) which influence the relative orientation of the two reactant species in the complex.