Observation of methyl iodide clusters in gas phase by infrared cavity ring-down spectroscopy

Infrared spectra of methyl iodide clusters produced in a supersonic jet have been observed in the C–H stretching region by cavity ring-down spectroscopy. The dependence of the spectra on the mixing ratio of CH₃I versus He and on the stagnation pressure has led to a tentative assignment of the absorption peaks to trimer up to pentamer, based on our previous study with matrix isolation technique (Chem. Phys. Lett. 343 (2001) 185). Ab initio calculations at the MP2 level for the trimer and tetramer have shown that two stable isomers exist for the tetramer whereas only one isomer is found to be stable for the trimer. The tentative assignment of the observed spectra has been in qualitative agreement with the results of the calculations. The structure of each isomer and its photochemical relevance are discussed.     

Stable clusters in the condensed state and some possibilities for gas phase clusters

The classical naked cluster ions of the post-transition elements that are stable in solid compounds and their lower charged analogues observed in mixed metal beams reflect the reduced number of good bonding orbitals. New cluster ions of indium that are hypoelectronic (fewer than 2n+2 skeletal bonding electrons) because of distortions or the bonding of heterometal atoms within the clusters are described. A large family of new, orbital-rich clusters of the group III and IV transition metals sheathed by halide are all centered by a wide variety of heteroatoms. Factors in their stability, possible analogous naked cluster targets, and some calculations are considered.     

Novel cage clusters of MoS2 in the gas phase

Laser evaporation of MoS(2) nanoflakes gives negatively charged magic number clusters of compositions Mo(13)S(25) and Mo(13)S(28), which are shown to have closed-cage structures. The clusters are stable and do not show fragmentation in the post-source decay analysis even at the highest laser powers. Computations suggest that Mo(13)S(25) has a central cavity with a diameter of 4.5 A. The nanosheets of MoS(2) could curl upon laser irradiation, explaining the cluster formation.     

Formation of mixed Fe-Mo oxo clusters in the gas phase

Reactions of oxygen-containing molybdenum clusters Mo_xO_y (x = 1–3, y = 1–9) with iron carbonyl ions Fe(CO) _n ⁺ (n = 1–3) were studied by the ion cyclotron resonance technique. The reactions were found to yield mixed Fe-Mo oxo clusters Mo_xO_yFe⁺ (x = 2, 3; y = 5, 6, 8, 9).     

Observation of poly(ethylene glycol) clusters with the chlorine anion in the gas phase under electrospray conditions

It is demonstrated herein that poly(ethylene glycol) (PEG) oligomers can form stable complexes with the chlorine anion in the gas phase as evidenced by results from electrospray ionization mass spectrometry (ESI‐MS) and molecular dynamics simulation. While the formation of crown‐ether‐like structures by acyclic polyethers in their complexes with alkali metal cations coordinated by the ether oxygen atoms has been extensively studied, the possibility of forming ‘inversed’ quasi‐cyclic structures able to bind a monoatomic anion has not been proved till now. We have observed the formation of stable gas‐phase complexes of oligomers of PEG‐400 with the Cl^? anion experimentally by ESI‐MS for the first time. It is suggested that a necessary precondition for obtaining the polyether‐chlorine anion clusters is the prevention of the formation of neutral ion pairs. Molecular dynamics simulation has demonstrated the wrapping of the Cl^? anion by the PEG chain, to stabilize the PEG_n?Cl^? clusters in the gas phase. The conformation of the polyether chain in such quasi‐cyclic or quasi‐helical complexes is ‘inversed’ compared with that in the complexes with cations: that is its hydrogen atoms are turned towards the central anion. Awareness of the possibility of the Cl^? anion being trapped in quasi‐cyclic PEG structures may be of practical importance when considering the intermolecular interactions of PEGs. Copyright © 2011 John Wiley & Sons, Ltd.     

Multiply protonated betaine clusters are stable in the gas phase

Multiply protonated clusters of betaine are formed via electrospray ionisation and fragment via competitive betaine neutral loss and charge separation; theoretical calculations suggest the [M(n)+2H](2+) ions consist of a [M(2)+2H](2+) core based on the hydrogen bonded carboxylic acid dimer.     

Polymerization in clusters and in the gas phase using metal ions

Consecutive addition and elimination reactions have been observed following the interaction of Ti⁺ with isobutylene beam expansion. Clusters provide a feasible and valuable approach for understanding the mechanism of ionic polymerization, and how the size of polymer chains is controlled in such a process.     

Laser photoionization and spectroscopy of gas phase silver clusters

Silver clusters containing up to 40–50 atoms are produced by laser vaporization in a pulsed-nozzle molecular beam source and studied with laser photoionization mass spectroscopy. A variety of Nd:YAG pumped dye laser and UV excimer laser wavelengths are used to achieve ionization. Ionization dynamics are studied by varying the laser wavelength and fluence. Bracketing experiments under single-photon ionization conditions are used to estimate ionization potentials as a function of cluster size. An even-odd ionization potential alternation is observed with odd-numbered clusters (N=3, 5, 7 ...) having lower ionization potentials than adjacent even-numbered species. Shell closings at clusters containing 2, 8 20 and 40 electrons are observed consistent with a one-electron shell model picture of cluster electronic structure. Resonance-enhanced ionization produces a vibrationally resolved spectrum for the trimer, Ag₃, yielding an electronic state assignment and excited state vibrational frequencies. Fragmentation in dimer ionization via theE state at 249 nm establishes the dissociation energy of Ag ₂ ⁺ to be <2.1 eV.     

Techniques used to study the chemistry of gas phase elemental clusters

Recent studies of the chemistry of elemental clusters (such as C _n , Si _n , and Fe _n ) in the gas phase have revealed a number of fascinating results. This review article discusses and compares the various techniques that have been used to investigate the chemical properties of gas phase elemental clusters. Examples are provided of the often complimentary information that can be obtained from the different methods.     

Multiply charged anionic metal clusters

The stability and decay channels of multiply charged anionic metal clusters are studied using the uniform jellium background model and the local density approximation, with self-interaction corrections when necessary. Shell effects are introduced using an adaptation of the nuclear Strutinsky method. Singly charged anions are stable for all sizes, but multiply charged negative ions are stable against spontaneous electron decay only above certain critical sizes. Below the border of stability, the anions are metastable against electron tunneling through a Coulombic barrier. Lifetimes for such decay processes are estimated. Fission channels may compete with the electron autodetachment and are studied for the case of doubly charged anions.     

Observation of Na(+)-bound oligomers of quercetin in the gas phase

While developing a liquid chromatography/tandem mass spectrometry method for the analysis of the flavonoid quercitin, it was observed that quercetin (3,3',4',5,7-pentahydroxyflavone) exhibited clustering in both the positive and negative ion mode. Two series of positive ion clusters were observed; the first series corresponds to singly charged [2M + Na](+) at m/z 627.2 to [13M + Na](+) at m/z 3947.5, while the second series corresponds to doubly charged [7M + 2Na](2+) at m/z 1080.4 to [25M + 2Na](2+) at m/z 3798.5. In the negative ion mode, the behavior of quercetin parallels that of apigenin (4',5,7-trihydroxyflavone) in that [M + NO(3)](-), [2M + NO(3)](-), and [3M + NO(3)](-) were observed at m/z 364.1, 666.0, and 968.9, respectively; in addition, quercitin clusters with chloride ions ([2M + Cl](-) at m/z 638.9 and [3M + Cl](-) at m/z 940. 9) were observed. The results of tandem mass spectrometric examination of several cluster ions are reported.     

Formation and reactions of clusters in the gas phase: small peptides and carboxylic acids

The cluster formation of seventeen small dipeptides with different primary structures and vanillic acid was investigated by means of a neutral laser desorption and supersonic beam expansion followed by multi photon ionization time of flight mass spectrometry. The structures of these clusters have been characterized by mass spectrometric methods as well as by DFT calculations. It is shown that the structure of the cluster from a dipeptide and vanillic acid is described by a hydrogen bond between the phenolic group of the vanillic acid and the N-terminal amino function of the dipeptide. The intensity of the cluster ion and the main fragmentation product, the protonated peptide ion, can be linked to the proton affinity of the peptide. Furthermore the fragmentation reactions of the protonated peptide are accompanied by extensive hydrogen rearrangements yielding both a and y fragments. The intensities of these fragments follow the proton affinity of the dipeptide.     

Reaction of carbon monoxide and hydrogen on neutral Nb8 clusters in the gas phase

Reactions of neutral V(n), Nb(n), and Ta(n) metal clusters (n< or =11) with CO+H(2) mixed gases and CH(3)OH in a flow tube reactor (1-50 Torr) are studied by time of flight mass spectroscopy and density functional theory calculations. Metal clusters are generated by laser ablation, and reactants and products are ionized by low fluence (approximately 200 microJ/cm(2)) 193 nm excimer laser light. Nb(n) clusters exhibit strong size dependent reactivity in reactions both with CO+H(2) and CH(3)OH compared with V(n) and Ta(n) clusters. A "magic number" (relatively intense) mass peak at Nb(8)COH(4) is observed in the reaction of Nb(n) clusters with CO+H(2), and CH(3)OH is suggested to be formed. This feature at Nb(8)COH(4) remains the most intense peak independent of the relative concentrations of CO and H(2) in the flow tube reactor. No other Nb(n), Ta(n), or V(n) feature behaves in this manner. In reactions of CH(3)OH with metal clusters M(n) (M=V, Nb, and Ta, n=3-11), nondehydrogenated products M(n)COH(4)/M(n)CH(3)OH are only observed on Nb(8) and Nb(10), whereas dehydrogenated products M(n)CO/CM(n)O are observed for all other clusters. These observations support the suggestion that CH(3)OH can be formed on Nb(8) in the reaction of Nb(n) with CO+H(2). A reaction mechanism is suggested based on the experimental results and theoretical calculations of this work and of those in the literature. Methanol formation from CO+H(2) on Nb(8) is overall barrierless and thermodynamically and kinetically favorable.     

Characterization of electronically excited states in anionic acetonitrile clusters

We have identified and examined the excited state of the cluster-solvated, valence-bound acetonitrile anion dimer, consistent with recent experimental findings, determining that the cluster excited state is of predominantly single-excitation character. Potential energy surface scans in coordinates specific to a "dissociative" normal mode common between the excited and ground states of the valence anion as well as the ground-state neutral dimer species shed light on the proposed vibrational autodetachment mechanism, with calculated excited-state lifetime consistent with experiment.     

New insight into the mechanisms of reactions between some anionic nucleophiles and phenyl acetate in the gas phase

Experiments carried out in an ion cyclotron resonance (ICR) drif cell, an ICR trapped ion cell and a flowing afterglow (FA) system show that in the gas phase phenyl acetate reacts with a variety of clustered and unclustered nucleophiles to yield mainly the product ion C₆H₅O^?.     

Cluster phase chemistry: gas-phase reactions of anionic sodium salts of dicarboxylic acid clusters with water molecules

A homologous series of anionic gas-phase clusters of dicarboxylic acids (oxalic acid, malonic acid, succinic acid, glutaric acid, and adipic acid) generated via electrospray ionization (ESI) are investigated using collision-induced dissociation (CID). Sodiated clusters with the composition (Na(+))(2)(n+1)(dicarboxylate(2-)(n+1) for singly charged anionic clusters, where n = 1-4, are observed as major gas-phase species. Isolation of the clusters followed by CID results mainly in sequential loss of disodium dicarboxylate moieties for the clusters of succinic acid, glutaric acid, and adipic acid (C4-C6). However, all oxalate (C2) and malonate (C3) clusters and dimers (n = 1) of succinate (C4) and glutarate (C5) exhibit more complex chemistry initiated by collision of the activated cluster with water molecules. For example, with water addition, malonate clusters dissociate to yield sodium acetate, carbon dioxide, and sodium hydroxide. More generally, water molecules serve as proton donors for reacting dicarboxylate anions in the cluster and introduce energetically favorable dissociation pathways not otherwise available. Density functional theory (DFT) calculations of the binding energy of the cluster correlate well with the cluster phase reactions of oxalate and malonate clusters. Clusters of larger dicarboxylate ions (C4-C6) are more weakly bound, facilitating the sequential loss of disodium dicarboxylate moieties. The more strongly bound small dicarboxylate anions (oxalate and malonate) preferentially react with water molecules rather than dissociate to lose disodium dicarboxylate monomers when collisionally activated. Implications of these results for the atmospheric aerosol chemistry of dicarboxylic acids are discussed.     

Gas-phase IR spectroscopy of anionic iron carbonyl clusters

The first gas-phase vibrational spectra are presented for several anionic iron carbonyl clusters, ranging in size from Fe(CO)4- to Fe5(CO)14- in the CO-stretching region (1600-2100 cm-1). The experimental spectra provide some immediate structural information about the clusters in the form of low-wavenumber (1750-1850 cm-1) bands marking the presence of bridging carbonyl ligands (mu2-COs). Supporting DFT calculations are presented for the smaller clusters (<3 Fe atoms) and give good agreement with the experimental data, allowing structural assignments for these cases. The Fe2(CO)7- spectrum suggests a structure lacking bridging carbonyl ligands, in agreement with the DFT results. For the case of Fe2(CO)8-, there are two possible structures based on the calculations, both with and without bridging carbonyls. The presence of a low-frequency band ( approximately 1770 cm-1) in the experimental spectrum conclusively demonstrates the existence of the bridged form. The ramifications of these data for metal-metal bonding in the clusters are also considered.