Monitoring reagent modification of charged SnxOy nanoclusters using Fourier transform ion cyclotron mass spectrometry

Abstract

The ion–molecule reactions of laser-generated radical clusters Sn_xO_x+1⁻ (x = 1–6), Sn_xO_x−1⁺ (x = 1–6) and Sn_xO_x⁺ (x = 2, 3) with the reagents H₂S and CH₃OH have been investigated using FTICR-MS. For the hypermetallic cations Sn_xO_x−1⁺, the rates of the sulfur-for-oxygen substitution reactions with H₂S appear to be sensitive to LDA/DFT-predicted oxygen binding modes, with μ₃-O modes relatively inert with respect to μ-O conformations. The reactions of the hypometallic anions Sn_xO_x+1⁻ with H₂S were dominated by proton transfer, although S-for-O substitution was observed to be kinetically competitive. The rates of the proton transfer were found to vary with both the reagent and cluster anion, and an analysis of the reaction kinetics for Sn_xO_x+1⁻ afforded the relative cluster proton affinities:PA(Sn₂O₃⁻) > PA(SnO₂⁻) ≫ PA(Sn₃O₄⁻) > PA(Sn₅O₆⁻) ≈ PA(Sn₆O₇⁻) ≈ PA(CH₃O⁻) = 381 ± 2 kcal mol⁻¹ > PA(Sn⁻) = 352 ± 10 kcal mol⁻¹Ion–molecule reaction results for the hypermetallic cations x = 2–5 with CH₃OH are suggestive of gas-phase coordination chemistry, with each cluster undergoing one slow association reaction. A recurrent theme of the chemistry of ionic Sn_xO_y is the initial activation or generation of O_ySn_x^−/+–OH bonds by a radical mechanism involving hydrogen or hydroxyl abstraction from the reagent. The resulting cluster-hydroxyl bonds are relatively labile with respect to conversion to alkoxy ligands.