Study of the interplay among non-covalent interactions in structural determination of gas phase cluster ions.

摘要

Vibrational spectroscopy in the region from 2600 to 4000 cm−1 was performed on cluster ion beams in a triple quadrupole mass spectrometer. Spectroscopic shifts of the O-H stretch were used to identify and determine the relative strength of intermolecular interactions and thereby characterize cluster structure. In the case of the fluoride anion solvated by methanol, deuterated (d₁ and d₃) methanol and ab initio calculations were essential in determining that fluoride is surface solvated, where the methanol molecules aggregate to one side of the ion.; In Li+(CH₃OH)_n and Li+(H ₂O)_n clusters the onset of hydrogen bonding occurs with four solvent molecules but only as a minor structural isomer. Significant hydrogen bonding occurs with five solvent molecules. Large Cs+(H ₂O)_n clusters were investigated to study the more stable magic number clusters, especially n = 20. The magic number clusters do not seem to gain their stability through structures with more H-bonds than their relatively less stable neighbors. It is possible that the structures of the magic number cluster are less strained.; The cesium and lithium ions were also investigated with benzene and water. Unimolecular dissociation rates were used to determine the most labile species and therefore a possible structural change. In the vibrational spectra of Li+(Benzene)_n(Water)_1–4 clusters, π-H bonding bands dominated, while water-water hydrogen bonding features dominated the spectra of Cs+(Benzene)_n(Water)_1–4 clusters. The lithium ion retains a full solvent shell of four waters, while the cesium ion becomes partially dehydrated. This study exemplifies the utility of vibrational spectroscopy to characterize intermolecular interactions with different ions and solvents.