Small Host-Guest Systems in the Gas Phase: Tartaric Acid as a Host for both Anionic and Cationic Guests in the...

The ionization of tartaric acid (TA) in an atmospheric pressure chemical ionization corona discharge ion source was studied by ion mobility spectrometry (IMS) with zero air as the drift gas. Density functional theory was used for structural and thermodynamic analyses of the produced ionic clusters. Ion mobility spectra of TA were recorded in both positive and negative modes of CD with and without ammonia and chloroform as dopants in order to produce NH₄⁺ and Cl^-, respectively, as the reactant ions (RIs). In the absence of these dopants, the RIs were mainly H₃O⁺ and O₂^- in the positive and negative CD, respectively. TA solutions in water and methanol were injected into the ionization region of the IMS instrument, and the product cations TA·H⁺(H₂O), TA·H⁺(CH₃OH), TA·NH₄⁺, and TA·NH₄⁺(CH₃OH) were observed in the positive CD. Anionic clusters (TA-H)^-, (TA-H)^-·CH₃OH, (TA-H)^-·TA, TA·Cl^-, and (TA)₂Cl^- were produced in the negative CD. The anions TA·Cl^- and (TA)₂Cl^- were not produced in an air atmosphere, and we observed their peaks when pure oxygen was used as the drift gas. Optimized structures of the clusters showed that TA·NH₄⁺, TA·Cl^-, and (TA)₂Cl^- are small host-guest systems in the gas phase, with TA as a host. (TA)₂Cl^- is a weakly bonded complex (an anion-bound dimer) that was observed at atmospheric pressure. The proton-bound dimer TA·H⁺·TA was not produced in the positive CD, while the anionic dimer (TA-H)^-·TA was observed in the negative CD. This phenomenon was interpreted on the basis of the hydration of TA·H⁺ and (TA-H)^-.