Intracluster ion-molecule reactions of Ti⁺ with C ₂H₅OH and CF₃CH₂OH clusters: Influence of fluorine substituents on chemical reactivity

A laser ablation-molecular beam/reflectron time-of-flight mass spectrometric technique was used to investigate the ion-molecule reactions that proceed within Ti⁺(ROH)_n (R = C₂H₅, CF₃CH₂) heterocluster ions. The mass spectra exhibit a major sequence of cluster ions with the formula Ti⁺(OR) _m(ROH)_n (m = 1, 2), which is attributed to sequential insertions of Ti⁺ into the O-H bond of C₂H₅OH or CF₃CH₂OH molecules within the heteroclusters, followed by H eliminations. The TiO⁺ and TiOH⁺ ions produced from the reactions of Ti⁺ with C₂H₅OH are interpreted as arising from insertion of Ti⁺ into the C-O bond, followed by C₂H₅ and C₂H₆ eliminations, respectively. When Ti⁺ reacted with CF ₃CH₂OH, by contrast, considerable contributions from TiFOH⁺, TiF₂⁺, and TiF₂OH ⁺ ions were observed in the mass spectrum of the reaction products, indicating that F and OH abstractions are the dominant product channels. Ab initio calculations of the complex of Ti⁺ with 2,2,2-trifluoroethanol show that the minimum energy structure is that in which Ti⁺ is attached to the O atom and one of the three F atoms of 2,2,2-trifluoroethanol, forming a five-membered ring. Isotope-labeling experiments additionally show that the chemical reactivity of heterocluster ions is greatly influenced by the presence of fluorine substituents and cluster size. The reaction energetics and formation mechanisms of the observed heterocluster ions are discussed.