Tandem mass spectrometry study of protonated methanol-water aggregates

Protonated nanodroplets containing methanol (M) and water (W) solvent monomers (up to M23W41H+) have been generated using electrospray ionisation (ESI) in conjunction with both high-solvent and low-drying gas-flow rates in a triple quadrupole mass spectrometer. Under the conditions employed, pure methanol clusters (MmH+) dominate the low-mass region of the spectrum (m<7). When the number of methanol monomers exceeds seven, a rapid increase in the addition of water to the clusters was observed, and a 1:1 mole ratio is achieved for m=11 (M11W11H+). The variation of the summed ion current with the total number of monomers m+n=constant, where m and n represent the number of methanol and water molecules, respectively, was found to peak after repeated additions of five or six units, implying rings could be a structural feature of these mixed clusters [Garvey and coworkers, J. Am. Chem. Soc. 114 (1992) 3684]. The low-energy collision-induced dissociation (CID) chemistry of selected nanodroplets up to m/z=600 has also been investigated, and the gentle nature of this approach has enabled some characterisation of the outermost solvation shells. Unexpected ion-molecule chemistry involving substitution of outer shell methanol molecules by water (present as a small component of the CID gas) suggests that even in large MmWnH+ clusters some methanol molecules occupy positions at the droplet periphery. CID evidence was also found for competitive solvation of the proton, although dehydration of mixed clusters to yield methanol cores is by far the most thermodynamically favourable process.