William R. Davidson

On the measurement of gas-phase ion-molecule equilibrium constants in an ion cyclotron resonance spectrometer

Abstract The technique used in measuring ion-molecule equilibrium constants, in particular proton-transfer equilibrium constants, with a high pressure drift cell ion cyclotron resonance spectrometer is described. Experimental and kinetic problems which occur in such measurements and their effect on the determination of accurate equilibrium constants are discussed. Non-attainment of equilibrium may result from either slow proton-transfer rates or fast competitive side reactions. With proper experimental technique, errors in free energies caused by such kinetic problems may usually be kept below 0.2 kcal mol−1. A number of examples are given. While the analysis is applied to high pressure drift cell ICR, most of the constraints are applicable to equilibrium measurements with any type of apparatus.