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

A Finnigan GCQ ion trap mass spectrometer has been modified for the measure ment of ion/molecule reaction rate constants. The rate constants for the re actions between CH3OH2+ and methanol, CH3CH2OH2+ with methanol and ethanol, CH3CH2CH2OH2+ with methanol, ethanol, and n-propanol and protonated isopro panol with neutral isopropanol, all leading to products consisting of a pro tonated ether and neutral water, have been determined. All of the values, e xcept that for the reaction involving isopropanol, are significantly lower than that predicted by the collision capture theory of Su and Chesnavich. C ommon to all the systems studied is an isomerization reaction that takes pl ace in the proton-bound dimer that is initially formed in the reaction. Thi s isomerization, which takes place via an internal S(N)2-type mechanism, le ads to the formation of the protonated ether. The rate constants, in combin ation with Rice-Rampsberger-Kassel-Marcus modeling, allow the isomerization activation energies, E-iso, for the proton-bound dimers to be estimated. T he E-iso values were found to decrease as the size of the alcohols increase d, an observation consistent with the S(N)2-type rearrangement that has bee n predicted for these systems. (C) 2001 Elsevier Science B.V.