Ab initio dynamics and photoionization mass spectrometry reveal ion–molecule pathways from ionized acetylene clusters to benzene cation

Abstract

The growth mechanism of hydrocarbons in ionizing environments, such as the interstellar medium (ISM), and some combustion conditions remains incompletely understood. Ab initio molecular dynamics (AIMD) simulations and molecular beam vacuum-UV (VUV) photoionization mass spectrometry experiments were performed to understand the ion-molecule growth mechanism of small acetylene clusters (up to hexamers). A dramatic dependence of product distribution on the ionization conditions is demonstrated experimentally and understood from simulations. The products change from reactive fragmentation products in a higher temperature, higher density gas regime toward a very cold collision-free cluster regime that is dominated by products whose empirical formula is (C 2 H 2 ) n + , just like ionized acetylene clusters. The fragmentation products result from reactive ionmolecule collisions in a comparatively higher pressure and temperature regime followed by unimolecular decomposition. The isolated ionized clusters display rich dynamics that contain bonded C 4 H 4 + and C 6 H 6 + structures solvated with one or more neutral acetylene molecules. Such species contain large amounts (>2 eV) of excess internal energy. The role of the solvent acetylene molecules is to affect the barrier crossing dynamics in the potential energy surface (PES) between (C 2 H 2 ) n + isomers and provide evaporative cooling to dissipate the excess internal energy and stabilize products including the aromatic ring of the benzene cation. Formation of the benzene cation is demonstrated in AIMD simulations of acetylene clusters with n > 3, as well as other metastable C 6 H 6 + isomers. These results suggest a path for aromatic ring formation in cold acetylene-rich environments such as parts of the ISM.ion-molecule reactions | polycyclic aromatic hydrocarbons | molecular dynamics | quantum chemistry | photoionization mass spectrometry

From atoms to aerosols: probing clusters and nanoparticles with synchrotron based mass spectrometry and X-ray spectroscopy

“…To elucidate ion-molecule growth dynamics in hydrocarbons, we chose mixed acetylene-ethylene clusters 53 and pure acetylene clusters. 54 The experimental set up essentially has two controls -one can tune the photon energy to precisely ionize a moiety and secondly, change the ionization distance to probe various regions of an expansion as demonstrated earlier with methanol. This work focused mainly on the formation of (C2H2)(C2H2), (C2H2)(C2H4), and (C2H4)(C2H4) dimers.…”

“…Guided by these earlier results, we now focus on the decomposition mechanisms of larger acetylene clusters leading to ring formation. 54 This is illustrated in figure 6, where ion-molecule reactions with in-source ionization can lead to fragmentation products. Notice the absence of m/z 52 and 78, which are acetylene dimer and trimer respectively.…”

“…Indeed, we have measured ion-induced growth of water clusters in a supersonic expansion as described earlier for methanol 24 and acetylene clusters. 54 The mass spectra shows non equilibrium distributions dependent on the distance between the nozzle and ionization source, analysis of which should allow access to kinetics and thermodynamics of these processes. Modelling the fluid dynamics of the supersonic expansion will be crucial for such an enterprise.…”

Molecular growth upon ionization of van der Waals clusters containing HCCH and HCN is a pathway to prebiotic molecules

“…43,44 Mass spectrometry has been applied to characterize the products resulting from photoionization of acetylene clusters, and peaks with mass corresponding to intact clusters with the formula (C2H2)n + (where n is an integer), were observed. 45 In a recent study, we used ab initio molecular dynamics (AIMD) to model the results of those experiments. 45 Our results showed that chemically bonded molecules can be formed subsequent to ionization of neutral van der Waals clusters.…”

“…45 In a recent study, we used ab initio molecular dynamics (AIMD) to model the results of those experiments. 45 Our results showed that chemically bonded molecules can be formed subsequent to ionization of neutral van der Waals clusters. Upon ionization, the clusters are able to spontaneously rearrange, and undergo geometric relaxation.…”