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 Ca2+ 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 ROH2+. 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 Ca2+ 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.