A correlation between slightly endothermic reactivity and ion–molecule association reaction rates

Primary binary reaction processes which are slightly endothermic can create relatively long-lived complexes in ion–neutral molecule collisions. The endothermic process absorbs translational energy, inhibiting separation to reactants, and in addition may facilitate access to an overall reaction potential well. The endothermicity must be less than the available collision energy, i.e., the electrostatic attractive energy and possibly some chemical interaction energy. An enhanced complex lifetime can result in enhanced rate constants for three-body ion–neutral association, radiative association, vibrational relaxation or complicated binary reactions. Application is made to the three-body association of hydrocarbon ions with H2, a selective H/D isotope effect in three-body association, the three-body association of Si+ with O2, the efficient vibrational relaxation of O+2 by CH4 and the fast low temperature reaction of O+2 with CH4 to produce protonated formic acid which involves the making and breaking of four chemical bonds.