The Proton Affinity and Absolute Heat of Formation of Trifluoromethanol

Abstract

The proton affinity and absolute heat of formation of trifluoromethanol have been derived from translational energy threshold measurements for reactions involving oxygen-protonated trifluoromethanol. The reaction of ionized iodotrifluoromethane with water was used to prepare CF{sub 3}OH{sub 2}{sup +} in the flow tube of a flowing afterglow triple-quadrupole instrument. The isomeric cluster ion, (HF)CF{sub 2}OH{sup +}, was shown to be more stable than CF{sub 3}OH{sub 2}{sup +} by the base-catalyzed conversion of CF{sub 3} OH{sub 2}{sup +} to (HF)CF{sub 2}OH{sup +} using either SO{sub 2} or OCS as the catalyst. The proton affinity of CF{sub 3}OH at oxygen was determined from the enthalpy change for the endothermic proton transfer reaction CF{sub 3}OH{sub 2}{sup +} + CO {yields} CF{sub 3}OH + HCO{sup +}. The measured enthalpy change was combined with the known value for the proton affinity of CO (141.9 kcal mol{sup -1}) to yield a value for the oxygen proton affinity of CF{sub 3}OH. The dissociation energy for the loss of water from CF{sub 3}OH{sub 2}{sup +} was measured by energy-resolved collision-induced dissociation. This value was used in a thermochemical cycle along with the measured proton affinity of CF{sub 3}OH to derive the gas-phase heat of formation of CF{sub 3}OH. Thismore » experimental value is slightly lower than, but in good agreement with, the 298 K heat of formation of CF{sub 3}OH that is predicted by high-level molecular orbital calculations. 37 refs., 7 figs., 3 tabs.« less