Substitution reactions of gaseous ions in a three-dimensional quadrupole ion trap

Substitution reactions between gaseous ions and neutral substrate molecules are of ongoing high interest. To investigate these processes in a qualitative and quantitative manner, we have constructed a device, with which a defined amount of a volatile substrate can be mixed with a defined amount of helium gas and added into a three-dimensional quadrupole ion trap. From the known inner volume of the device, the known ratio n_substrate :n_He of the mixture, and the determined absolute partial pressure of helium in the ion trap, we can derive the partial pressure of the substrate in the ion trap and, thus, convert the directly observable pseudo-first-order rate constants of the substitution reactions into absolute bimolecular rate constants. We have tested the device by investigating a series of S_N 2 reactions of Br^- and CF₃ CH₂ O^- anions as well as ligand exchange reactions of ligated Na⁺ cations. As the obtained results suggest, the described device makes it possible to determine the bimolecular rate constants of substitution reactions as well as other ion-molecule reactions with satisfactory accuracy and reliability.