Electron-driven ionization of large methanol clusters in helium nanodroplets

Abstract

The electron-driven ionization of helium droplets doped with pure methanol and ethanol clusters has been investigated for the first time using high resolution mass spectrometry. Large clusters are readily accessible by this route, with up to 100 alcohol molecules seen in the present study. The mass spectra for the doped helium droplets show many similarities with previous gas phase mass spectrometric studies of methanol and ethanol clusters. Thus the dominant ion products, at least for small clusters, are the protonated species H⁺(CH₃OH)_n and H⁺(C₂H₅OH)_n. Likewise intra-cluster reaction is observed to produce H⁺(H₂O)(CH₃OH)_n and H⁺(H₂O)(C₂H₅OH)_n ions. However, in helium droplets the observation of consecutive intra-cluster reactions is seen with product molecules containing up to five water molecules. The evidence points towards the proton locating on H₂O to form H₃O⁺, rather than the alcohol, despite the higher proton affinity of the latter. The behaviour of the H⁺(H₂O)_m(ROH)_n ion signals as a function of cluster size is consistent with the most stable cluster structures arising from a central H₃O⁺ ion surrounded by two or more complete five-membered rings with the constituents held in place by hydrogen bonds.