The proton affinity and absolute heat of formation of trifluoromethanol

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 CF3OH2+ in the flow tube of a flowing afterglow triple-quadrupole instrument. The isomeric cluster ion, (HF)CF2OH+, was shown to be more stable than CF3OH2+ by the base-catalyzed conversion of CF3OH2+ to (HF)CF2OH+ using either SO2 or OCS as the catalyst. The proton affinity of CF3OH at oxygen was determined from the enthalpy change for the endothermic proton transfer reaction CF3OH2+ + CO --> CF3OH + HCO+. The measured enthalpy change, 9.2 +/- 1.4 kcal mol(-1), was combined with the known value for the proton affinity of CO (141.9 kcal mol(-1)) to yield a value for the oxygen proton affinity of CF3OH of 151.1 +/- 1.7 kcal mol(-1). The dissociation energy for the loss of water from CF3OH2+ was measured to be 36.6 +/- 2.1 kcal mol(-1) by energy-resolved collision-induced dissociation. This value was used in a thermochemical cycle along with the measured proton affinity of CF3OH to derive the gas-phase heat of formation of CF3OH of -220.7 +/- 3.2 kcal mol(-1). This experimental value is slightly lower than, but in good agreement with, the 298 K heat of formation of CF3OH that is predicted by high-level molecular orbital calculations.