Biswajit Bandyopadhyay

Unexpected Chemistry from the Reaction of Naphthyl and Acetylene at Combustion‐Like Temperatures

The hydrogen abstraction/acetylene addition (HACA) mechanism has long been viewed as a key route to aromatic ring growth of polycyclic aromatic hydrocarbons (PAHs) in combustion systems. However, doubt has been drawn on the ubiquity of the mechanism by recent electronic structure calculations which predict that the HACA mechanism starting from the naphthyl radical preferentially forms acenaphthylene, thereby blocking cyclization to a third six-membered ring. Here, by probing the products formed in the reaction of 1- and 2-naphthyl radicals in excess acetylene under combustion-like conditions with the help of photoionization mass spectrometry, we provide experimental evidence that this reaction produces 1- and 2-ethynylnaphthalenes (C12 H8 ), acenaphthylene (C12 H8 ) and diethynylnaphthalenes (C14 H8 ). Importantly, neither phenanthrene nor anthracene (C14 H10 ) was found, which indicates that the HACA mechanism does not lead to cyclization of the third aromatic ring as expected but rather undergoes ethynyl substitution reactions instead.

Vacuum Ultraviolet Photoionization of Complex Chemical Systems.

Tunable vacuum ultraviolet (VUV) radiation coupled to mass spectrometry is applied to the study of complex chemical systems. The identification of novel reactive intermediates and radicals is revealed in flame, pulsed photolysis, and pyrolysis reactors, leading to the elucidation of spectroscopy, reaction mechanisms, and kinetics. Mass-resolved threshold photoelectron photoion coincidence measurements provide unprecedented access to vibrationally resolved spectra of free radicals present in high-temperature reactors. Photoionization measurements in water clusters, nucleic acid base dimers, and their complexes with water provide signatures of proton transfer in hydrogen-bonded and π-stacked systems. Experimental and theoretical methods to track ion-molecule reactions and fragmentation pathways in intermolecular and intramolecular hydrogen-bonded systems in sugars and alcohols are described. Photoionization of laser-ablated molecules, clusters, and their reaction products inform thermodynamics and spectroscopy that are relevant to astrochemistry and catalysis. New directions in coupling VUV radiation to interrogate complex chemical systems are discussed.

Probing Methanol Cluster Growth by Vacuum Ultraviolet Ionization

The ability to probe the formation and growth of clusters is key to answering fundamental questions in solvation and nucleation phenomena. Here, we present a mass spectrometric study of methanol cluster dynamics to investigate these two major processes. The clusters are produced in a molecular beam and ionized by vacuum ultraviolet (VUV) radiation at intermediate distances between the nozzle and the skimmer sampling different regimes of the supersonic expansion. The resulting cluster distribution is studied by time-of-flight mass spectrometry. Experimental conditions are optimized to produce intermediate size protonated methanol and methanol-water clusters and mass spectra and photoionization onsets and obtained. These results demonstrate that intensity distributions vary significantly at various nozzle to ionization distances. Ion-molecule reactions closer to the nozzle tend to dominate leading to the formation of protonated species. The protonated trimer is found to be the most abundant ion at shorter distances because of a closed solvation shell, a larger photoionization cross section compared to the dimer, and an enhanced neutral tetramer precursor. On the other hand, the protonated dimer becomes the most abundant ion at farther distances because of low neutral density and an enhanced charged protonated monomer-neutral methanol interaction. Thomsons liquid drop model is used to qualitatively explain the observed distributions.

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

Significance The formation of benzene and its cation constitutes a likely gateway to polycyclic aromatic hydrocarbons, which are the bridge to larger carbonaceous material such as soot in combustion processes and interstellar dust. Our paper reports computational and experimental results that address the long-standing puzzle of how ion–molecule reactions involving small unsaturated organics, such as acetylene, which is widespread in the interstellar medium, can lead to benzene cation. We present insights into the facile way in which C6H6+ products, including benzene cation, can be accessed after ionization of cold isolated neutral clusters, and show that there is a catalytic role for what are nominally spectator acetylene molecules. 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 (C2H2)n+, just like ionized acetylene clusters. The fragmentation products result from reactive ion–molecule collisions in a comparatively higher pressure and temperature regime followed by unimolecular decomposition. The isolated ionized clusters display rich dynamics that contain bonded C4H4+ and C6H6+ 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 (C2H2)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 C6H6+ isomers. These results suggest a path for aromatic ring formation in cold acetylene-rich environments such as parts of the ISM.

Proton transfer in acetaldehyde-water clusters mediated by a single water molecule

Proton transfer in aqueous media is a ubiquitous process, occurring in acid-base chemistry, biology, and in atmospheric photochemistry. Photoionization mass spectrometry coupled with theoretical calculations demonstrate that a relay-type proton transfer mechanism is operational for single-water-molecule-assisted proton transfer between two acetaldehyde molecules in the gas phase. Threshold photoionization of acetaldehyde-water clusters leads to proton transfer between the formyl groups (-CH[double bond, length as m-dash]O) of one acetaldehyde molecule to another, and the subsequent formation of cationic moieties. Density functional theory computations reveal several plausible pathways of proton transfer in mixed cluster cations. Among these pathways, water-mediated proton transfer is energetically favored. Mass spectra and photoionization efficiency curves confirm these theoretical findings and also demonstrate the increased stability of cluster cations where acetaldehyde molecules are symmetrically bonded to the hydronium ion.

Probing Ionic Complexes of Ethylene and Acetylene with Vacuum-Ultraviolet Radiation

Mixed complexes of acetylene-ethylene are studied using vacuum-ultraviolet (VUV) photoionization mass spectrometry and theoretical calculations. These complexes are produced and ionized at different distances from the exit of a continuous nozzle followed by reflectron time-of-flight mass spectrometry detection. Acetylene, with a higher ionization energy (11.4 eV) than ethylene (10.6 eV), allows for tuning the VUV energy and initializing reactions either from a C2H2(+) or a C2H4(+) cation. Pure acetylene and ethylene expansions are separately carried out to compare, contrast, and hence identify products from the mixed expansion: these are C3H3(+) (m/z = 39), C4H5(+) (m/z = 53), and C5H5(+) (m/z = 65). Intensity distributions of C2H2, C2H4, their dimers and reactions products are plotted as a function of ionization distance. These distributions suggest that association mechanisms play a crucial role in product formation closer to the nozzle. Photoionization efficiency (PIE) curves of the mixed complexes demonstrate rising edges closer to both ethylene and acetylene ionization energies. We use density functional theory (ωB97X-V/aug-cc-pVTZ) to study the structures of the neutral and ionized dimers, calculate their adiabatic and vertical ionization energies, as well as the energetics of different isomers on the potential energy surface (PES). Upon ionization, vibrationally excited clusters can use the extra energy to access different isomers on the PES. At farther ionization distances from the nozzle, where the number densities are lower, unimolecular decay is expected to be the dominant mechanism. We discuss the possible decay pathways from the different isomers on the PES and examine the ones that are energetically accessible.

Correlative Imaging and Spectroscopy of Particles in Liquid

Correlative imaging and spectroscopy has been widely used in biological and medical sciences. Its power of providing a holistic view of the system of interest makes it an intense research topic that often utilizes correlative optical microscopy, cryo electron microscopy, and a variety of spectroscopy techniques. With the advent of liquid handling in vacuum, in situ electron microscopy has become increasingly popular in studying complex systems, adding another palette in correlative imaging and spectroscopy. This work presents an example of characterization of particles suspended in liquid in a vacuum compatible microfluidic sample holder using a suite of tools including scanning electron microscopy (SEM), transmission electron microscopy (TEM), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and synchrotron vacuum UV (VUV) single photon ionization mass spectrometry (SPI-MS), highlighting the advantage of multiscale analysis in material sciences.

Structures and Infrared Spectroscopy of Metal Cation-Borazine Complexes

Borazine (B3N3H6) is known as ‘inorganic benzene’ because of a planar B3N3 ring with equivalent B-N distance. The lone pair from N in the ring delocalize to the adjacent p-orbital of B which leads to a conjugated system. Even though metal-benzene complexes have been studied extensively as models for cation- π interactions and organometallic bonding, similar systems with borazine is relatively scarce. Here, we present a density functional study on metal cation-borazine complexes focusing on geometric and electronic structures and their effects on infrared spectra. We have chosen Al, V, Mn, and Zn cations with various d-configurations which provide models for study donor-acceptor complexes. Among these four metal complexes, Al + and Mn + prefer to bind to π-cloud on top of the borazine ring. On the other hand, V + and Zn + bind to B and N, respectively. Infrared spectra of these complexes show four major bands: N-H-, B-H stretches and B-N-B ring and scissoring modes. Interactions of Al and Mn barely shift these band positions in the respective complexes as compared to those in isolated borazine, because of less cation-π interactions. On the other hand, V + and Zn + significantly perturb the borazine ring resulting shifts in