Gas-phase Studies Research Articles

Gas-phase CO2 activation with single electrons, metal atoms, clusters, and molecules

In this review, the history and outlook of gas-phase CO2 activation using single electrons, metal atoms, clusters (mainly metal hydride clusters), and molecules are discussed on both of the experimental and theoretical fronts. Although the development of bulk solid-state materials for the activation and conversion of CO2 into value-added products have enjoyed great success in the past several decades, this review focuses only on gas-phase studies, because isolated, well-defined gas-phase systems are ideally suited for high-resolution experiments using state-of-the-art spectrometric and spectroscopic techniques, and for simulations employing modern quantum theoretical methods. The unmatched high complementarity and comparability of experiment and theory in the case of gas-phase investigations bear an enormous potential in providing insights in the reactions of CO2 activation at the atomic level. In all of these examples, the reduction and bending of the inert neutral CO2 molecule is the critical step determined by the frontier orbitals of reaction participants. Based on the results and outlook summarized in this review, we anticipate that studies of gas-phase CO2 activations will be an avenue rich with opportunities for the rational design of novel catalysts based on the knowledge obtained on the atomic level.

Gas-phase laboratory study on PAH/amino-acid cluster cations

ABSTRACT As an important class of carbon reservoirs in the interstellar medium (ISM), polycyclic aromatic hydrocarbon (PAH) molecules play an important role in the evolutionary network of prebiotic molecules. Here, the experimental evidence of astronomically relevant amino-acid derivatives – PAH/amino-acid clusters – is provided, and we study their ion–molecular collision reactions in the gas phase. With the initial molecular precursors dicoronylene (DC, C48H20)/alanine (Ala, C3H7NO2) and DC/isoleucine (Ile, C6H13NO2), the experiments indicate that PAH–amino-acid cluster cations (e.g. (Ala)C48H$_{(0-19)}\, ^+$ and (Ile)C48H$_{(0-19)}\, ^+$) and graphene–amino-acid cluster cations (e.g. (Ala)nC$_{48}\, ^+$ and (Ile)nC$_{48}\, ^+$, n = 1, 2, 3, 4) are efficiently formed in a strong interstellar radiation field. In addition, the structure of clusters and the binding energy of their formation pathways are studied by a quantum chemistry calculation method: gas-phase reactions (ion–molecule reactions) between PAH cations with amino acids readily occur (exothermic energy around 2.0–4.7 eV), and these newly formed clusters have a complex molecular configuration (C–O and C–N bond type). These laboratory studies provide a cluster growth pathway (through an ion–molecule reaction) towards the formation of amino-acid derivatives in a bottom-up process and insight into their chemical-evolution behaviour, opening up aromatic-based chemistry that is available to the species (dehydrogenated PAHs or graphene molecules) that formed from the photofragmentation process of PAHs in interstellar environments. When conditions are suitable (e.g. have similar molecular abundance spatial distributions in the ISM), amino-acid derivatives can form efficiently, and newly built large PAH/amino-acid clusters may be widespread in space.

A gas-phase study on the cyclometallation of a series of Cp*Ir(III) complexes bearing bidentate pyrimidine ligands

A concerted approach of synthesis and gas phase experiments characterizes the relative cyclometallation barriers of a series of cationic η5-Cp* iridium(III) compounds. The common feature of the investigated compounds is a bidentate N,N’-donor ligand possessing a pyridine site functionalized with a 5-butylpyrimidin-2-yl ring in the 2-position. In addition, the pyridine ring was functionalized with electron-donating or -withdrawing groups. The compounds were characterized by means of NMR and IR spectroscopy and elemental analysis. For the measurements of the relative barriers of the cyclometallation, collision-induced dissociation (CID) experiments were carried out, which revealed a clear dependence of the relative barriers from the nature and the position of the substituents at the pyridine ring could be worked out.

Single-shot 3D imaging of hydroxyl radicals in the vicinity of a gliding arc discharge

Plasma-related studies in gas phase are challenging to carry out due to plasma’s transient and unpredictable behavior, excessive luminosity emission, 3D complexity and aggressive chemistry and physiochemical interactions that are easily affected by external probing. Laser-induced fluorescence is a robust technique for non-intrusive investigations of plasma-produced species. In this letter, we present 3D distributions of ground state hydroxyl radicals (OH) radicals in the vicinity of a glow-type gliding arc plasma. Such radical distributions are captured instantaneously in one single camera acquisition by combining structured laser illumination and a lock-in based imaging analysis method called FRAME. The interference of plasma emission is automatically subtracted by the FRAME technique. In addition, the orientation of the plasma discharge can be reconstructed from the 3D data matrix, which can then be used to calculate 2D distributions of ground state OH radicals in a plane perpendicular to the orientation of the plasma channel. Our results indicate that OH distributions around a gliding arc are strongly affected by gas dynamics. We believe that the ability to instantaneously capture 3D transient molecular distributions in a plasma discharge, with minimal plasma emission interference, will have a strong impact on the plasma community for in situ investigations of plasma-induced chemistry and physics.

The influence of the methionine residue on the dissociation mechanisms of photoionized methionine-enkephalin probed by VUV action spectroscopy

VUV action spectroscopy has recently gained interest for the study of peptides and proteins. However, numerous aspects of the fundamental processes involved in the photodissociation are yet to be understood. It can, for example, be expected that sulfur-containing amino-acid residues have a significant impact on the dissociation processes following photoionization because of their potential involvement in the transport of electron holes in proteins. In order to investigate the influence of the sulfur-containing methionine residue on the VUV photodissociation of a small peptide a VUV action spectroscopy study of gas-phase protonated methionine-enkephalin and leucine-enkephalin in the photon energy range of 6–35 eV was performed. The results show that upon non-ionizing photoexcitation, the fragmentation patterns of the two peptides are nearly identical, whereas significant differences were observed upon photoionization. The differences between the fragment yields and the identified specific dissociation channels for methionine-enkephalin could be explained by the high electron hole affinity of sulfur, which efficiently directs the radical to the methionine side chain. Additionally, for both peptides the presence of the intact photoionized precursor ions was confirmed by their isotopic patterns and the stability of these species could be evaluated.Graphic abstract

Confinement effects on C–H and C–F stretching vibrational frequencies of difluoromethane in cold inert gas matrixes: a combined infrared spectroscopy and electronic structure theory study

Mid-infrared spectra of difluoromethane (DFM) have been recorded by confining the molecule in the matrix cages of pure argon, nitrogen and in the mixed matrixes of argon and nitrogen obtained by mixing the two gases in definite proportions. The measured spectra depict distinct confinement effects in terms of the infrared spectral shifts of the C–H and C–F stretching vibrational fundamentals as compared to the free molecules in the gas-phase, and also some remarkable changes in band appearances and relative intensities. The signs of the spectral shifts are observed to be different for the $$\upnu _{\text {C--H}}$$ and $$\upnu _{\text {C--F}}$$ transitions. Pronounced blue shifts are observed for the two $$\upnu _{\text {C--H}}$$ modes in pure nitrogen matrix, which progressively reduce in magnitude in the mixed matrixes, and is least in the pure argon matrix. On the other hand, distinct red shifts of the $$\upnu _{\text {C--F}}$$ vibrations are evident in pure nitrogen matrix, which are reduced in magnitude in the mixed matrixes, and become even less pronounced in pure argon matrix. The effect of Fermi resonances in the $$\upnu _{\text {C--H}}$$ region, that has motivated several previous investigations of DFM including high resolution gas phase studies, becomes explicit upon comparison of the matrix isolation spectrum with the gas phase spectrum. Electronic structure calculations using several DFT and DFT-D methods, in conjunction with NBO analysis, have been carried out for various sizes of $$\hbox {DFM-N}_{2}$$ and DFM-Ar complexes in order to understand the underlying interactions responsible for the observed shifts. The calculations are found to satisfactorily reproduce the observed variations in magnitude and direction of the shifts of both the $$\upnu _{\text {C--H}}$$ and $$\upnu _{\text {C--F}}$$ transitions for the change of the medium. It is inferred that the inert gas matrix environment exerts an electronic re-organization within the DFM molecule, and the consequent rehybridization of the carbon-centric hybrid orbital of the C–H and C–F bonds is responsible for the observed spectral shifts.

Gas-phase studies of chemically synthesized Au and Ag clusters.

Atomically precise Au and Ag clusters protected by monolayers of organic ligands have attracted growing interests as promising building units of functional materials and ideal platforms to study size-dependent evolution of structures and physicochemical properties. The use of gas-phase methods including mass spectrometry, ion mobility mass spectrometry, collision-induced dissociation mass spectrometry, photoelectron spectroscopy, and photodissociation spectroscopy will reveal novel and complementary information on their intrinsic geometric and electronic structures that cannot be obtained by conventional characterization methods. This Perspective surveys the recent progress and outlook of gas-phase studies on chemically synthesized Au/Ag clusters.

Computational modelling of nanotube delivery of anti-cancer drug into glutathione reductase enzyme

Density functional theory method combined with docking and molecular dynamics simulations are used to understand the interaction of carmustine with human glutathione reductase enzyme. The active site of the enzyme is evaluated by docking simulation is used for molecular dynamics simulation to deliver the carmustine molecule by (5,5) single walled carbon nanotube (SWCNT). Our model of carmustine in the active site of GR gives a negative binding energy that is further refined by QM/MM study in gas phase and solvent phase to confirm the stability of the drug molecule inside the active site. Once released from SWCNT, carmustine forms multiple polar and non-polar hydrogen bonding interactions with Tyr180, Phe209, Lys318, Ala319, Leu320, Leu321, Ile350, Thr352 and Val354 in the range of 2–4 Å. The SWCNT vehicle itself is held fix at its place due to multiple pi-pi stacking, pi-amide, pi-sigma interactions with the neighboring residues. These interactions in the range of 3–5 Å are crucial in holding the nanotube outside the drug binding region, hence, making an effective delivery. This study can be extended to envisage the potential applications of computational studies in the modification of known drugs to find newer targets and designing new and improved controlled drug delivery systems.

Gas-phase studies of the retinal protonated Schiff base chromophore

Gas-phase studies of the retinal protonated Schiff base chromophore are reviewed. The use of action spectroscopy has solidified the understanding of the spectral-tuning mechanisms of this important chromophore. Ion-mobility spectrometry and gas phase femtosecond pump-probe spectroscopy studies indicate that several of the remarkable photo-isomerization properties of the chromophore such as its specificity and ultrafast nature are intrinsic properties of the chromophore. With a firm understanding of the properties of the isolated retinal chromophore in terms of spectroscopy and dynamics, the influence of the protein is becoming better understood.

Gas-phase study of adduct and exchange complexation reactions between two and tridimensional macrocyclic ligands using ion mobility spectrometry and mass spectrometry

A gas-phase study concerning the interaction between macrocyclic ligands and different molecular ion guests (H3O+, NH4+, CH3NH3+) was performed using ion mobility spectrometry and mass spectrometry. 18-crown-6 (18C6), diaza18-crown-6 (DA18C6) and cryptand [222] (C222) ligands were selected for the investigation of structural properties on the nature of complexes formed. Characterization of complexes was performed by ESI-TWIM-TOFMS. In this study, the importance of the type of donating atom in the ring on complex formation was investigated by comparison of the ion mobility spectra of 18C6 and DA18C6 at various temperatures. Different complexation behaviors of ligands showed that the nature of species formed strongly depends on the structural features of the host. DA18C6 does not demonstrate the adduct formation that is observed for 18C6. This is attributed to the high proton affinity of DA18C6, which serves to preferentially remove a proton from any reactant ion upon introduction to the gas phase. In the case of C222 with a three-dimensional cavity, due to the existence of an inclusive site, an interesting behavior was observed. In C222–H3O+ and C222–CH3NH3+ systems, only proton exchange reaction and an inclusion complex formation (C222.H+) are found, while for C222–NH4+ happens inclusive ammonium complex C222·NH4+ in addition to proton exchange reaction and C222.H+ formation. Encapsulation of ammonium ion in the cage of C222 was attributed to size match between ammonium and host cavity that depends on temperature. The exchange constant, enthalpy and entropy of C222·NH4+ complex formation in the gas phase have been determined.

Potentiality of carbon nanotube to encapsulate some alkylating agent anticancer drugs: a molecular simulation study

Encapsulation of chlorambucil, cyclophosphamide, and melphalan with (9,9) carbon nanotube was investigated in aqueous solution. The structures were modeled in gas phase applying density functional calculations. Then, solvation free energies and association free energies of considered structures in water were studied by Monte Carlo simulation and perturbation method. Outcomes of gas phase study revealed that all three drugs can encapsulate into carbon nanotube. Monte Carlo simulation results showed that electrostatic interactions play a crucial role in the intermolecular energies after encapsulation in aqueous media. It is found that among three drugs, solvation free energy of chlorambucil was increased after encapsulation. Moreover, solvation of carbon nanotube was enhanced after encapsulation of drugs which improve the pharmaceutical applications. Calculated association free energies indicated that chlorambucil–carbon nanotube was the only stable encapsulated drug in aqueous solution.

Kinetic and gas-phase study of the chemical vapor deposition of silicon carbide from C2H3SiCl3/H2

The chemical vapor deposition (CVD) of silicon carbide from vinyltrichlorosilane (VTS) was studied to identify a range of conditions leading to pure crystalline SiC. The deposition rate was recorded to evidence the various deposition regimes. Gas phase, elemental analyses and infiltration tests were also performed. Three distinct chemical reaction regimes were identified. In CVD conditions, carbon is co-deposited at low temperature while VTS is only partially decomposed. In infiltration conditions, the composition switches to pure SiC inside the porous substrate because of a depletion of reactive hydrocarbon species. Competing heterogeneous reactions are responsible for a hysteresis versus temperature, in both deposition rate and composition of the deposit. The high temperature domain is the most suitable to deposit pure crystalline SiC in CVD conditions. Hydrogen dilution strongly accelerates the homogeneous decomposition of VTS as compared to argon. Assumptions on the reaction mechanism were proposed describing the chemistry of this precursor.

Catalytic CO Oxidation by O2 Mediated with Single Gold Atom Doped Titanium Oxide Cluster Anions AuTi2 O4-6.

Gas-phase studies on catalytic CO oxidation by O2 mediated with gold-containing heteronuclear metal oxide clusters are vital to obtain the structure-reactivity relationship of supported gold catalysts, while it is challenging to trigger the reactivity of clusters with closed-shell electronic structure in O2 activation. Herein, we identified that CO oxidation by O2 can be catalyzed by the AuTi2 O4-6- clusters, among which AuTi2 O4- with closed-shell electronic structure can effectively activate O2 . The reactions were characterized by mass spectrometry and quantum chemical calculations. Theoretical calculations showed that in the initial stage of O2 activation, the Ti2 O4 moiety in AuTi2 O4- contributes dominantly to activate O2 into superoxide (O2- ⋅) without participation of the Au atom. In subsequent steps, the intimate charge transfer interaction between Au and the Ti2 O4 moiety drives the direct dissociation of the O2- ⋅ unit.

ORGANOMETALLIC GAS-PHASE ION CHEMISTRY AND CATALYSIS: INSIGHTS INTO THE USE OF METAL CATALYSTS TO PROMOTE SELECTIVITY IN THE REACTIONS OF CARBOXYLIC ACIDS AND THEIR DERIVATIVES.

Carboxylic acids are valuable organic substrates as they are widely available, easy to handle, and exhibit structural and functional variety. While they are used in many standard synthetic protocols, over the past two decades numerous studies have explored new modes of metal-mediated reactivity of carboxylic acids and their derivatives. Mass spectrometry-based studies can provide fundamental mechanistic insights into these new modes of reactivity. Here gas-phase models for the following catalytic transformations of carboxylic acids and their derivatives are reviewed: protodecarboxylation; dehydration; decarbonylation; reaction as coordinated bases in C-H bond activation; remote functionalization and decarboxylative C-C bond coupling. In each case the catalytic problem is defined, insights from gas-phase studies are highlighted, comparisons with condensed-phase systems are made and perspectives are reached. Finally, the potential role for mechanistic studies that integrate both gas- and condensed-phase studies is highlighted by recent studies on the discovery of new catalysts for the selective decomposition of formic acid and the invention of the new extrusion-insertion class of reactions for the synthesis of amides, thioamides, and amidines. © 2020 John Wiley & Sons Ltd. Mass Spec Rev.

A systematic approach to methyl cinnamate photodynamics

ABSTRACT Methyl cinnamate (MC) is an ester within the cinnamate family. Recent time-resolved gas-phase studies have suggested that upon excitation to its first singlet ππ* (11ππ*) state, there is initial decay to the lowest lying singlet 1nπ* (11nπ*) state within 10 ps, en route to trans–cis isomerisation. In the present study, we have implemented time-resolved ion yield (TR-IY) and time-resolved photoelectron spectroscopy (TR-PES) experiments in the gas-phase to precisely determine the lifetime of the 11ππ* state. We found this lifetime to be ∼ 4.5 ps using both TR-IY and TR-PES. MC was also studied in a more complex cyclohexane solution environment, using transient electronic absorption spectroscopy. Along with complementary steady-state irradiation and 1H NMR studies, these studies demonstrate that trans–cis isomerisation is preserved in the more complex, cyclohexane solution environment.

Gas‐Phase Transformation of Fluorinated Benzoporphyrins to Porphyrin‐Embedded Conical Nanocarbons

Geodesic nitrogen‐containing graphene fragments are interesting candidates for various material applications, but the available synthetic protocols, which need to overcome intrinsic strain energy during the formation of the bowl‐shaped skeletons, are often incompatible with heteroatom‐embedded structures. Through this mass spectrometry‐based gas‐phase study, we show by means of collision‐induced dissociation experiments and supported by density functional theory calculations, the first evidence for the formation of a porphyrin‐embedded conical nanocarbon. The influences of metalation and functionalization of the used tetrabenzoporphyrins have been investigated, which revealed different cyclization efficiencies, different ionization possibilities, and a variation of the dissociation pathway. Our results suggest a stepwise process for HF elimination from the fjord region, which supports a selective pathway towards bent nitrogen‐containing graphene fragments.

Molecular dynamics and density functional theory studies of γ-butyrolactone (GBL) + ethanol and γ-valerolactone (GVL) + ethanol liquid mixtures

γ-Butyrolactone (GBL) and γ-valerolactone (GVL) are two of the lactones with wide application in industry. The interactions of these two lactones with alcohols have been a subject of experimental research. This article reports the theoretical investigation of GBL/ethanol and GVL/ethanol mixtures throughout the entire composition range using molecular dynamics and ab initio calculations. Molecular dynamics simulation using the optimized potentials for liquid simulations force field reproduces the experimental thermodynamics results quite well. Investigation of the liquid structure show that ethanol interacts with GBL and GVL molecules at specific sites, namely, almost collinearly with the lactone CO bond, below and above the lactone ring plane, and between the CC ring bonds of the lactone. While ab initio calculations largely support the interaction of ethanol with the lactones via O-H---O hydrogen bonds, hydrogen boding via lactone CH and ethanol O clearly have a significant role as well. The O-H---O interactions are dominated by the carbonyl oxygen. Dispersion interactions seem to play a more important role in stabilizing those interactions in which the hydrogen bonding is via the lactone ring oxygen. Generally, the two lactones behave very similarly in both liquid and gas phase studies, with minor differences attributed to more steric hindrance due the methyl group in γ-valerolactone.

Optical Absorption of Atomically-Precise Sn14 Nanoclusters: The Antagonistic Interplay of Ligand Stabilization, Molecular Symmetry, and Solvatochromism.

The synthesis of atomically precise clusters is nowadays well established. The study of isolated clusters in the gas phase has also become an approved field of research. Although both approaches examine the same research objects, namely nanoclusters, little is known about to what extent results from gas phase studies can be transferred to colloidal systems and vice versa. In particular, it is not yet sufficiently understood how ligands influence the geometric and electronic structure of clusters from an experimental point of view. By comparing a ligand-stabilized tin nanocluster in solution with an isolated species in the gas phase and considering different geometric arrangements with the same number of tin atoms, the impacts of ligand stabilization, molecular symmetry, and solvatochromism on the optical behavior are thoroughly worked out for the first time.

Comparative study of gas-phase “dehydration” of alkyl lactates and lactic acid for acrylic acid production over hydroxyapatite catalysts

Gas-phase “dehydration” of methyl (ML) and ethyl lactates (EL) for acrylic acid (AA) production was studied and compared with the dehydration of lactic acid (LA) over serial hydroxyapatite catalysts of varying compositions (HAPm-360, molar Ca/P ratio m = 1.58–1.68, calcination temperature: 360 °C). The product distribution from both lactic esters was found very similar to that from LA; AA but not methyl (MA) or ethyl acrylate (EA) also appeared as the prevailing product (> 50 mol% by selectivity) from both ML and EL. Compared with the reaction of LA, both esters showed much lower reactivity but higher AA selectivity. Among the two esters EL was less reactive but produced a higher selectivity for AA formation. Correlating the AA selectivity or yield with the catalyst surface acid-base property uncovers that a suitable balance between the surface acidity and basicity (or acidity/basicity ratio) is the key to the lactic ester-to-AA reactions. However, the optimum surface acidity/basicity ratio for producing the highest AA selectivity from both esters was higher than that from LA. These results suggest that the lactic ester-to-AA reactions proceeded by catalytic tandem processes involving a formation of LA (or adsorbed lactate) as the key intermediate at the catalyst surface. Acid-base catalysis responsible for these observations is discussed.

Highly Selective Binding and Inhibition of Pyr-His-Pro-NH2 (TRH) Function using a Polypyridinyl Macrocyclic Receptor with an Amphiphilic Cavity.

Macrocycle, cyclo[4] [(1,3-(4,6)-dimethylbezene)[4](2,6-(3,5)-dimethylpyridine (B4P4), shows highly selective binding affinity with protirelin (Pyr-His-Pro-NH2 ; TRH) among the tested 26 drug or drug adductive substrates. The stable complexation in a 1:1 manner was fully characterized in solution, gas phase, and solid state study. Furthermore, B4P4 acts as an efficient TRH inhibitor even at [macrocycle]:[drug] <1:300, both in membrane transport and cellar incubation. The current work provides an unprecedented strategy for macrocycles to be efficiently used in drug target therapy.