Collision-induced dissociation studies of protonated alcohol and alcohol—water clusters by atmospheric pressure ionization tandem mass spectrometry. 1?Methanol

Cluster size distribution and collision-induced dissociation (CID) studies of protonated methanol and protonated methanol—water clusters yield information on the structure and energetics of such ions. Ions were formed at atmospheric pressure in a corona discharge source, and were subjected to CID in the center quadrupole of a triple quadrupole mass spectrometer. Cluster ions containing up to 13 molecules of methanol and/or water were observed and examined using CID experiments. The CID of all (CH₃OH)_n · H₂O · H⁺ clusters, where n ? 8, showed that water loss was statistically favored over methanol loss and that the preferred dissociation channel involved loss of water with methanol molecules. These results support a model employing a chain of hydrogen-bonded solvent molecules rather than one in which fused rings of ligands surround a central hydronium ion. However, CID of larger clusters, where n ? 9, showed that loss of one methanol was equal to or less than loss of water, reflecting a change in structure.