Ion/molecule reaction kinetics using a modified Finnigan GCQ ion trap mass spectrometer: the energetics of the dehydration of proton-bound alcohol dimers

Abstract

A Finnigan GCQ ion trap mass spectrometer has been modified for the measurement of ion/molecule reaction rate constants. The rate constants for the reactions between CH 3OH 2 + and methanol, CH 3CH 2OH 2 + with methanol and ethanol, CH 3CH 2CH 2OH 2 + with methanol, ethanol, and n-propanol and protonated isopropanol with neutral isopropanol, all leading to products consisting of a protonated ether and neutral water, have been determined. All of the values, except that for the reaction involving isopropanol, are significantly lower than that predicted by the collision capture theory of Su and Chesnavich. Common to all the systems studied is an isomerization reaction that takes place in the proton-bound dimer that is initially formed in the reaction. This isomerization, which takes place via an internal S N2-type mechanism, leads to the formation of the protonated ether. The rate constants, in combination with Rice-Rampsberger-Kassel-Marcus modeling, allow the isomerization activation energies, E iso, for the proton-bound dimers to be estimated. The E iso values were found to decrease as the size of the alcohols increased, an observation consistent with the S N2-type rearrangement that has been predicted for these systems.