An experimental study of the influence of hydration on the reactivity of the hydroxide anion in the gas phase at room temperature

Abstract

Flowing afterglow measurements of rate constants and ionic products are reported which explore the influence of step-wise hydration on the reactivity of the hydroxide anion in the gas phase at 296 ± 2 K. Rate constants have been measured for the reactions of OH−•(H2O)n and/or OD−•(D2O)n with CO2, SO2, CH3OH, C2H5OH, C2H2, CH3COCH3, CH3NO2, and HCN, in most instances for values of n from 0 to 3. The reactions with CO2 and SO2 involve ligand addition or exchange and the results of their study corroborate earlier measurements. The remaining reactions proceed by proton transfer. For these reactions the results establish trends in reactivity as a function of step-wise hydration when relative acidity is preserved and when a reversal occurs in the relative acidity upon hydration. The fast hydrated acid–base reactions were observed to establish interesting hydrated product ions many of which underwent secondary reactions involving the exchange of H2O or D2O for the reagent molecule.

Formation of positive product ions from substances with low proton affinity in high kinetic energy ion mobility spectrometry

“…However, as shown in several studies, 17,18,52–54 the rate constants of the ligand‐switching reactions typically decrease with increasing water cluster size n . Therefore, increasing the sample gas humidity and thus the humidity in the reaction region results in a decrease in the sensitivity.…”

“…This may be achieved when the hydration energy of the analyte molecules is comparable with that of H 2 O. Thus, the permanent dipole moment and the polarizability of the analyte molecule seem to be key parameters to predict whether ligand‐switching reactions occur 10,17,18,52 …”

“…Thus, the permanent dipole moment and the polarizability of the analyte molecule seem to be key parameters to predict whether ligand-switching reactions occur. 10,17,18,52 However, as shown in several studies, 17,18,[52][53][54] the rate constants of the ligand-switching reactions typically decrease with increasing water cluster size n. Therefore, increasing the sample gas humidity and thus the humidity in the reaction region results in a decrease in the sensitivity. In particular, IMS instruments operated at ambient pressure thus suffer from a sample gas humidity-dependent detection of analytes.…”

The Ionic Hydrogen Bond

“…Ionic hydrogen bonds can also affect the kinetics by solvating the ions. For example, the reactivity of the hydroxyl anion was observed to be affected by solvation by H 2 O molecules …”

Acid−Base Catalyzed Reactions in Ionic Water Clusters

“…They determined the absolute rate constant of the reaction of OH - (H 2 O) with acetaldehyde to be (3 ± 1) × 10 -9 cm 3 molecules -1 s -1 , nearly equal to the rate constant of the reaction of naked OH - ( k abs = (3.1 ± 0.8) × 10 -9 cm 3 molecules -1 s -1 ). With the help of deuterated measurements Stewart et al and the group of Bohme , were able to show that the proton transferred to the OH - originates from the methyl group. The resulting CH 2 CHO - ion can be formally written as an equilibrium between two ionic forms, aldehydic and enolate: In the presence of a second acetaldehyde (or another molecule containing a carbonyl bond), a nucleophilic addition of the anion to the carbonyl group of the nonionized molecule can take place, forming an anion of dimer containing both alcoholic and aldehydic groups, “aldol”.…”