Encasing of Na⁺ ion in dimer‐formed acetic acid clusters

Abstract

Peaks with anomalous abundance found in the mass spectra are associated with ions with enhanced stability. Among the scientific community focused on mass spectrometry, these peaks are called 'magic peaks' and their stability is often because of suggestive symmetric structures. Here, we report findings on ionised Na-acetic acid clusters [Na(+) -(AcA)n ] produced by Na-doping of (AcA)n and UV laser ionisation. Peaks labelled n = 2, 4, 8 are clearly distinguishable in the mass spectra from their anomalous intensity. Ab initio calculations helped elucidate cluster structures and energetic. A plausible interpretation of the magic peaks is given in terms of (AcA)n formed by dimer aggregation. The encasing of Na(+) by twisted dimers is proposed to be the origin of the enhanced cluster stability. A conceivable dimer-formed tube-like closed structure is found for the Na(+) -(AcA)8 .

Photoionization, Structures, and Energetics of Na‐Doped Formic Acid–Water Clusters

“…In the supersonic expansion in the vacuum, carboxylic acid vapors easily aggregate forming pure acid or mixed acid‐water clusters detected in our apparatus as Na‐doped clusters by laser UV ionization [13–15] . In particular, in ref [14] we demonstrated that the ion signals of Na−(FA) n are produced by one‐photon ionization, so the cluster ionization potentials were correctly assigned in the laser UV range.…”

UV Photoionization of Sodium‐Doped Formic Acid Clusters

“…[25] While the expansion in helium produces a narrow distribution predominantly peaked at low masses and almost constant by varying the sampling zone of the beam, when using heavier carrier gases the mass distribution changes sharply by varying the time delay between the valve opening and the laser firing. In fact, moving from the front towards to the centre of the gas pulse, gradually the amount of cooling increases and thus the cluster size increases.…”

“…In a previous paper, we studied Na-acetic acid clusters and showed that their formation is very sensitive to the expansion conditions, in particular to the carrier gas type. [25] While the expansion in helium produces a narrow distribution predominantly peaked at low masses and almost constant by varying the sampling zone of the beam, when using heavier carrier gases the mass distribution changes sharply by varying the time delay between the valve opening and the laser firing. In fact, monomer and dimers are detected at the front of the beam and heavy clusters are detected just after few microseconds.…”

“…A schematic of the experimental apparatus is shown in Figure 1, whereas the full technical details of the experimental setup of the Na-doping are reported in. [8,[25][26][27] For clarity, only a brief description of the basic details of apparatus setup is given here. The main hardware is composed of two vacuum chambers, independently evacuated by two turbomolecular pumps (1000 l/sec) and connected to each other through a 3 mm hole.…”

“…Full technical details are reported in. [25,26] The laser was an optical parametric oscillator (OPO) (Opotek, RADIANT 355, 410-2500 nm, peak energy 30 mJ @400 nm, 4 ns pulse duration, linewidth 4-9 cm À1 ) equipped with doubling and mixing non-linear crystals to extend the tuning range up to 210 nm. The laser energy was measured with a pyroelectric energy sensor (Gentec-EO mod.…”