Thermoneutral isotope-exchange reactions of cations in the gas phase

Rate constants have been measured for reactions of the type AD/sub 2//sup +/ + MH ..-->.. MD + ADH/sup +/, where AD/sub 2//sup +/ is CD/sub 3/CND/sup +/, CD/sub 3/CDOD/sup +/, (CD/sub 3/COCD/sub 3/)D/sup +/, or (C/sub 2/D/sub 5/)/sub 2/OD/sup +/ and the MH molecules are alcohols, acids, mercaptans, H/sub 2/S, AsH/sub 3/, PH/sub 3/, or aromatic molecules. Rate constants are also presented for the reactions Ar/sub H/D/sup +/ + D/sub 2/O ..-->.. Ar/sub d/D/sup +/ + HDO, where Ar/sub H/D/sup +/ is a deuteronated aromatic molecule and Ar/sub D/D/sup +/ 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 to 420 K. The efficiencies of the isotope exchange reactions are interpreted in terms of estimated potential surface cross sections for the reactions AD/sub 2//sup +/ + MH ..-->.. (AD/sub 2//sup +/MH) ..-->.. (ADMHD/sup +/) ..-->.. (ADH/sup +/MD) ..-->.. ADH/sup +/ + MD. When the formation of the (ADMHD/sup +/) 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 (AD/sub 2//sup +/MH) ..-->.. (ADMHD/sup +/) 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.