Competitive Threshold Collision-Induced Dissociation Measurements: Determination of the Gas-Phase Acidities of the Butanol Isomers

Abstract

ABSTRACTGas-phase acidities of the four butanol isomers,1-butanol(n), 2-butanol (sec), 2-methyl-1-propanol (iso) and 2-methyl-2-propanol (tert) were determined using energy-resolved competitive threshold collision-induced dissociation. Using a guided ion beam mass spectrometer, reaction cross sections for dissociation reactions of butanol-fluoride [C4H9O•H•F]- and methanol butoxide [CH3O•H•OC4H9]- complexes were measured as a function of collision energy. Gas-phase acidities are then determined via modeling of reaction cross sections near the threshold using statistical rate theory. The modeling gives an enthalpy difference which is equal to the gas-phase acidity difference with respect to the reference acid (HF, or CH3OH or H2O). The measured gas-phase acidities at 0 K are ΔacidH0(n-butanol)= 1567.0 ± 3.5 kJ/mol, ΔacidH0(isobutanol)= 1555.8 ± 3.3 kJ/mol, ΔacidH0(sec-butanol)= 1556.0 ± 3.1 kJ/mol, and ΔacidH0(tert-butanol)= 1567.2 ± 1.2 kJ/mol.