Hydrated Formic Acid Clusters and their Interaction with Electrons

Abstract

We investigate ion formation in hydrated formic acid (FA) clusters upon collision with electrons of variable energy, focusing on electron ionization at 70 eV (EI) and low‐energy (1.5–15 eV) electron attachment (EA). To uncover details about the composition of neutral clusters, we aim to elucidate the ion formation processes in FAM·WN clusters initiated by interaction with electrons and determine the extent of cluster fragmentation. EI predominantly produces protonated [FAm+H]+ ions, and in FA‐rich clusters, the stable ring structures surrounding H3O+ ions are formed. In contrast, EA leads to a competition between the formation of intact [FAm·Wn]− and dissociated [FAm·Wn–H]− fragment ions, influenced by the cluster size, level of hydration, and electron energy. Our findings reveal a predisposition of lowenergy EA towards forming [FAm·Wn]−, while higher electron energies tend to favor the formation of [FAm·Wn–H]− due to intracluster ion‐molecule reactions. The comparison of positive and negative ion spectra suggests that the mass spectra of FA‐rich clusters may indicate their actual size and composition. On the other hand, the more weakly bound water evaporation from the clusters depends strongly on the ionization. Thus, for the hydrated clusters, the neutral cluster size can hardly be estimated from the mass spectra.

A Vacuum Ultraviolet Photoionization Mass Spectrometry and Density Functional Calculation Study of Formic Acid–Water Clusters

“…Our results support this stability finding and additionally find that clusters containing one water more than the stable cluster (FA 1 W 6 H + , FA 2 W 5 H + , FA 3 W 4 H + , FA 4 W 3 H + , FA 5 W 2 H + ) have diminished stability. The recent work of Li et al and earlier theoretical work cited therein and in the introduction suggest that the stability of these clusters arises from their cyclic structures. Indeed, Li et al observe very similar mass spectral distributions in a 98% v/v FA water beam upon electron impact ionization at 70 eV.…”

“…FA is also important in astrochemistry, and was recently found to be present in interstellar ices, and can take part in the formation of amino acids such as glycine. , To date there have been many studies, both experimental and theoretical, including quantum chemical calculations, ,− aiming at understanding the mechanisms at play. Early work employed mass spectrometry to probe the thermodynamics and structure of FA clusters employing collision-induced ionization, ,,, electron impact , and VUV photoionization . Very recently an electron impact ionization study of negatively and positively charged FA water clusters was reported coupled to theoretical calculations of the stability of neutral, cationic and anionic clusters .…”

“…Early work employed mass spectrometry to probe the thermodynamics and structure of FA clusters employing collision-induced ionization, ,,, electron impact , and VUV photoionization . Very recently an electron impact ionization study of negatively and positively charged FA water clusters was reported coupled to theoretical calculations of the stability of neutral, cationic and anionic clusters . While some clusters were identified to be more stable, particularly in cyclic geometries, no systematic attempt was made to correlate the stability of these clusters with that of the appearance of enhanced intensities in the mass spectra, also known as “magic numbers”, as the emphasis of the study was to identify the distribution of neutral FA water clusters.…”

“…29 Very recently an electron impact ionization study of negatively and positively charged FA water clusters was reported coupled to theoretical calculations of the stability of neutral, cationic and anionic clusters. 28 While some clusters were identified to be more stable, particularly in cyclic geometries, no systematic attempt was made to correlate the stability of these clusters with that of the appearance of enhanced intensities in the mass spectra, also known as "magic numbers", as the emphasis of the study was to identify the distribution of neutral FA water clusters. The consensus that emerged, is that for FA clusters with water, open chain structures with water bound to the end are favored for n < 4, while the hydronium ion acts as a nucleation site beyond n = 4, and the open chains tend to take cyclic structures.…”

“…While this method of introduction is different from earlier reports 27−29 where FA and water were mixed in the same reservoir, our results are very similar to these reports, particularly in the case of the measurements using electron impact ionization. 27,28 The gas mixture passes through a 100 μm nozzle source and supersonically expands into a vacuum chamber, yielding the desired FA water molecular beam. This beam is extracted through a skimmer into a differentially pumped interaction region where the clusters are exposed to and ionized by VUV synchrotron radiation.…”