The Solvation of Ca²⁺ with Gas Phase Clusters of Alcohol Molecules

Abstract

A comprehensive examination of how the identity of an alcohol molecule can change the behavior of a solvated, alkaline earth dication has been undertaken. The metal dication of Ca 2+ has been clustered with a range of different alcohols to form [Ca(ROH) n ] 2+ complexes, where n lies in the range 2–20. Following collisional activation via electron capture from nitrogen gas, complexes for n in the range 2–6 exhibit a switch in reaction product as a function of n . For low values, solvated CaOH + is the dominant fragment, but as n increases beyond 4, this is displaced by the appearance of solvated CaOR + . A separate study of unimolecular metastable decay by [Ca(ROH) n ] 2+ complexes found evidence of charge separation to form CaOH + (ROH) n −1 + R + . For two isomers of butanol, the n = 3 complexes were found to follow parallel, but different metastable pathways: one leading to the appearance of CaOH + and another that resulted in proton abstraction to form ROH 2 + . These differences have been attributed to the precursor complexes adopting geometries where one ROH molecule occupies a secondary solvation shell. Comparisons were made with a previous study of magnesium complexes; [Mg(ROH) n ] 2+ show that the difference in second ionization energy Mg + (15.09 eV) as opposed to Ca + (11.88 eV) influences behavior. A complex between Ca 2+ and 1-chloroethanol is shown to favor the formation of CaCl + as opposed to CaOH + as a unimolecular charge separation product, which is attributed to differences in bond energy in the precursor molecule. Electronic supplementary material The online version of this article (10.1007/s13361-019-02263-x) contains supplementary material, which is available to authorized users.