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(+)(CH(3)OH)(n) and H(+)(C(2)H(5)OH)(n). Likewise intra-cluster reaction is observed to produce H(+)(H(2)O)(CH(3)OH)(n) and H(+)(H(2)O)(C(2)H(5)OH)(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 H(2)O to form H(3)O(+), rather than the alcohol, despite the higher proton affinity of the latter. The behaviour of the H(+)(H(2)O)(m)(ROH)(n) ion signals as a function of cluster size is consistent with the most stable cluster structures arising from a central H(3)O(+) ion surrounded by two or more complete five-membered rings with the constituents held in place by hydrogen bonds.
Highlights
Proton solvation is of central importance in solution chemistry
It was established early on that the dominant products obtained from ionization of small methanol clusters, whether by electron impact or photoionization, are the protonated cluster ions, H+(CH3OH)n
The electron impact ionization of helium droplets doped with large methanol and ethanol clusters has been investigated for the first time
Summary
In aqueous solutions protons have high mobilities which are explained by the Grotthus mechanism,[1] whereby the proton is transported along a network of hydrogen bonded water molecules via the facile interchange of two different ion cores, the Eigen (H9O4+) and Zundel (H5O2+) cations.[2,3] The critical role implied here for H+(H2O)n clusters has made the Zundel and Eigen ions, along with their more solvated analogues, the target of numerous experimental and theoretical studies One of the challenges in cluster science is to explore how structure and chemistry change with cluster size, and thereby draw parallels with behaviour in bulk solutions. Major structural changes between the neutral methanol clusters and the corresponding cations deliver excess energy into the latter which leads to rapid ejection of an OCH3 entity, i.e It was established early on that the dominant products obtained from ionization of small methanol clusters, whether by electron impact or photoionization, are the protonated cluster ions, H+(CH3OH)n.9–11 Similar to water clusters, major structural changes between the neutral methanol clusters and the corresponding cations deliver excess energy into the latter which leads to rapid ejection of an OCH3 entity, i.e
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