Dynamics of Endoergic Bimolecular Proton Transfer Reactions: F-+ ROH → HF + RO-(R = H, CH3, CH3CH2, (CH3)2CH, and (CH3)3C)

Abstract

Guided ion beam tandem mass spectrometry techniques are used to measure reaction threshold energies for proton transfer of water, methanol, ethanol, 2-propanol, and 2-methyl-2-propanol with fluoride anion, F- + ROH → RO- + HF, where R = H, CH3, CH3CH2, (CH3)2CH, or (CH3)3C. The measured reaction threshold energy is an upper limit for the gas-phase acidity of the alcohol relative to hydrogen fluoride. Our guided ion beam measurements yield threshold energies that are consistently higher than those based on current literature gas-phase acidity values by 5−9 kJ/mol, indicating that the reactions have a small effective barrier in excess of the endoergicity. To help interpret the experiments, ab initio and density functional theory techniques are used to calculate the proton transfer reaction potential energy surfaces. No intrinsic barriers or double minima along the reaction path are found on the potential energy surfaces. Possible dynamical bottlenecks for translational activation are discussed in detail.