Chemistry of Mg+ and Mg2+ in Association with Methanol Clusters

Abstract

A combination of the “pickup” technique and a double focusing, high-resolution mass spectrometer have been used to generate and study the chemistry of cluster ions of the form Mg+(CH3OH)n and [Mg(CH3OH)n]2+, for n in the range 1−20. The singly charged ions exhibit a switch in intensity between Mg+(CH3OH)n and Mg+OCH3(CH3OH)n-1 at n = 3, which is attributed to an increase in stability of the polarized ion [Mg2+−OCH3-] as more methanol molecules are added to the complex. No doubly charged ions are observed for n =1, and the mass spectrum is dominated by parent ions rather than fragments when n ≥ 2. For each of the two types of ion, the principal unimolecular (metastable) decay channel is the loss of methanol. With the introduction of a collision gas, a very wide range of reaction products is observed for the smaller singly charged ions, with the main fragments being Mg+OCH3, Mg+OH, and Mg+H, which are formed, in some cases, in the presence of additional methanol molecules. Collisional fragmentation of the doubly charged ions is most often accompanied by Coulomb explosion in which charge separation follows the breaking of covalent bonds. However, in some instances the measured kinetic energy releases for the latter process are very much lower than expected, and it is suggested that excess energy is being carried away by very light fragments, e.g., H and H2. Some of the reactions observed for [Mg(CH3OH)n]2+ ions can be accounted for by assuming a range of anion transfer processes.