| Gas-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·OC4H 9]- 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 Deltaacid H0(n-butanol)= 1567.0 +/- 3.5 kJ/mol, Delta acidH0(isobutanol)= 1555.8 +/- 3.3 kJ/mol, DeltaacidH0(sec-butanol)= 1556.0 +/- 3.1 kJ/mol, and DeltaacidH0(tert-butanol)= 1567.2 +/- 1.2 kJ/mol. |
