Energy-Resolved Collision-Induced Dissociation Studies for Ion Thermochemistry: Bond Dissociation and Competitive Threshold Measurements

Abstract

Energy-resolved competitive threshold collision-induced dissociation (TCID)measurements for radical species relevant to combustion were made using a guided ion beam tandem mass spectrometer. These ion thermochemistry methods can be used to obtain thermochemical properties of these neutral radicals through the use of thermochemical cycles. The high reactivity of radical species often makes them challenging to directly measure. Study of the peroxyformate anion (HC(O)OO−) revealed a singlet-triplet crossing in the oxygen-oxygen bond dissociation channel. Analysis of the thresholds gives the first experimentally determined enthalpies of formation for both peroxyformic acid and the peroxyformyl radical. We have also studied the TCID of linear and branched hexanol (C6H13OH) complexed with fluoride and acetylide to determine their gas-phase acidities. The role of conformation on the gas-phase acidities of 1-hexanol and 3, 3-dimethyl-1-butanol was explored using modified microcanonical statistical rate theory (RRKM) models with internal rotations treated as one-dimensional hindered rotors. These six-carbon primary alcohols have the same “intrinsic” gas-phase acidity, defined as deprotonation without conformational relaxation, but differ in observed acidity due to conformational stabilization of the anion.