Bimolecular Reactions of Trapped Ions. V. Ionic Chemistry of Simple Oxygen-containing Molecules

Abstract

The ion-molecule reactions occurring in a number of one-component and two-component systems involving oxygen-containing compounds have been studied using an ion trapping technique. The variable reaction time possible (up to 3.2 ms) has permitted a study of the sequential ion–molecule reactions occurring. In one-component systems (M = CH3CHO, (CH3)2O, or (CH3)2CO) the MH+ ion formed initially reacts to produce M2H+ in a rapid third order reaction (k in the range 10−24 to 10−25 cm6 molecule−2 s−1). No M2H+ was observed in ethylene oxide. In two-component systems a rapid proton transfer reaction M1H+ + M2 → M2H+ + M1 occurs when the proton affinity of M2 is greater than the proton affinity of M1; consequently, the third order solvation reactions normally will involve only reactions of M2H+.The reactions of isomeric C2H5O+ ions have been studied. The CH3OCH2+ ion reacts slowly by H− abstraction and/or CH3+ transfer. The CH3CH=OH+ ion rapidly (and essentially completely) transfers a proton to (CD3)2O and to (CH3)2CO. By contrast the C2H5O+ ion obtained by protonation of ethylene oxide does not react completely with (CD3)2O or (CH3)2CO. In the former case a quasi-equilibrium is reached while in the latter case there is evidence that the C2H5O+ ions have rearranged to a CH3OCH2+ structure by an unknown mechanism.Considerable detail has been obtained concerning the primary ion–molecule reactions occurring, particularly for systems containing ethylene oxide.