Parent Cluster Ion Research Articles

Cluster ion polymerization of serine and tryptophan, the water loss channel

We present an experimental study on the stability of cluster cations formed by the assembly of tryptophan or serine moelcules inside charged helium droplets and subsequent droplet evaporation. The stability is probed via collision induced dissociation and mass spectrometry. We focus on the neutral loss of 18 Da that was previously proposed to result in the formation of an amide bond in amino acid clusters. We show that the cluster cations formed by the neutral loss of 18 Da from protonated parent cluster ion precursors are softly, presumably van der Waals, bound similar to the parent and protonated parent cluster cations. Cluster cations formed after neutral loss of 18 Da from the parent cluster cations are strongly bound, indicating the formation of new covalent bonds. Even though we observe a signal at m/z corresponding to dipeptide cations, their collision induced dissociation fragmentation does not allow unambiguous identification of their structure, particularly the amide bond. Our study at cryogenic conditions of He droplets demonstrates that peptide formation by the cluster ion polymerization is not a barrierless process and the formation of amide bonds may depend on activation methods and available catalysts.Graphical abstract

Proton Migration in Clusters Consisting of Protonated Pyridine Solvated by Water Molecules.

Proton transfer (PT) from protonated pyridine to water molecules is observed after excitation of microhydrated protonated pyridine (Py) clusters PyH(+) (H2 O)n (n=0-5) is induced by a single collision with an Ar atom at high incident velocity (95×10(3) m s(-1) ). Besides the fragmentation channel associated with the evaporation of water molecules, the charged-fragment mass spectrum shows competition between the production of the PyH(+) ion (or its corresponding charged fragments) and the production of H(+) (H2 O) or H(+) (H2 O)2 ions. The increase in the production of protonated water fragments as a function of the number of H2 O molecules in the parent cluster ion as well sd the observation of a stable H(+) (H2 O)2 fragment, even in the case of the dissociation of PyH(+) (H2 O)2 , are evidence of the crucial role of PT in the relaxation process, even for a small number of solvating water molecules.

Collisional Activation of N2O Decomposition and CO Oxidation Reactions on Isolated Rhodium Clusters

The reactions of nitrous oxide decorated rhodium clusters, RhnN2O(+) (n = 5, 6), have been studied by Fourier transform ion cyclotron resonance mass spectrometry. Collision induced dissociation with Ar is shown to lead to one of two processes; desorption of the intact N2O moiety (indicating molecular adsorption in the parent cluster) or N2O decomposition liberating molecular nitrogen with the latter becoming increasingly dominant at higher collision energies. Consistent with the results of earlier studies, which employed infrared excitation [Hermes, A. C.; et al. J. Phys. Chem. Lett. 2011, 2, 3053], Rh5ON2O(+) is observed to behave qualitatively differently to Rh5N2O(+) with decomposition of the nitrous oxide dominating the chemistry of the former. In other experiments, the reactivity of RhnN2O(+) clusters with CO has been studied. Chemisorption of (13)CO is calculated to deposit ca. 2 eV into the parent cluster, initiating a range of chemical processes on the cluster surface, which are fit to a simple reaction mechanism. Clear differences are again observed in the reaction branching ratios for Rh5N2O(+) and Rh6N2O(+) parent cluster ions. For the n = 5 cluster, the combined N2O reduction/CO oxidation is the most significant reaction channel, while the n = 6 cluster preferentially is oxidized to Rh6O(+) with loss of N2 and CO. Even larger differences are observed in the reactions of the N2O decorated cluster oxides, RhnON2O(+), for which more reaction possibilities arise. The results of all studies are discussed in relation to infrared driven processes on the same parent cluster species [Hamilton, S. M.; et al. J. Am. Chem. Soc. 2010, 132, 1448; J. Phys. Chem. A, 2011, 115, 2489].

X-ray absorption spectroscopy of mass-selected transition metal clusters using a cyclotron ion trap: An experimental setup for measuring XMCD spectra of free clusters

A Fourier-Transform-Ion-Cyclotron-Resonance mass spectrometer has been installed at the BESSY II storage ring. This unique setup enables to record soft X-ray absorption and magnetic circular dichroism spectra (XMCD) of mass-selected clusters in the photon energy range 200–1200 eV. The photoabsorption signal is obtained by recording the fragmentation pattern as produced after X-ray absorption and subsequent Auger decay of the core-excited parent cluster ion. The experimental setup and first X-ray absorption spectra on V 13 + and Nb 13 + are discussed. The feasibility of recording X-ray induced XMCD spectra is demonstrated for Co 22 +. Improvements that allow for quantitative determination of spin and orbital momenta in the future are briefly outlined.

Reactions within p-difluorobenzene/methanol heterocluster ions: a detailed experimental and theoretical investigation.

The reactivity of p-difluorobenzene/methanol cluster ions has been investigated by using triple quadrupole mass spectrometry and DFT calculations. The present study was performed in light of a recent investigation of p-difluorobenzene/methanol (P = F-C(6)H(4)-F and M = CH(3)OH) heterocluster ions where the solvent-catalyzed formation of p-fluoroanisole (A = CH(3)O-C(6)H(4)-F) was observed in P(M)(2)(+) clusters and not in PM(+) clusters. The results of our mass selected cluster ion study and theoretical calculations confirm that a single extra molecule of methanol can lower the reaction activation energy barrier in agreement with previous work for smaller clusters (PM(+) and P(M)(2)(+)). However, we also observe that P(M)(3)(+) and P(M)(4)(+) clusters undergo evaporative loss of neutral methanol to establish the P(M)(2)(+) cluster before reacting. P(M)(n>4)(+) clusters are capable of reacting through multiple pathways, in some cases generating a 1,4-dimethoxybenzene (B = CH(3)O-C(6)H(4)-OCH(3)) product via two separate substitution reactions within the same cluster ion. DFT calculations were employed to model the structures of the parent cluster ions, and transition state calculations were used to evaluate the activation energy for the p-fluoroanisole-forming substitution reaction. The calculations suggest that the reaction proceeds through a transition state containing a six-member hydrogen-bonded ring involving a reacting methanol and a second methanol that significantly lowers the activation energy.

Infrared spectra of HC2−∙(C2H2)n and O2−∙(C2H2)n clusters (n=2–5)

We study the solvation of HC2- and O2- with acetylene ligands by means of midinfrared photodissociation spectroscopy in the CH stretching region, monitoring C2H2 evaporation upon infrared photon absorption by the parent cluster ions. Our findings are interpreted with the help of density functional theory. The infrared spectra indicate that while the binding generally occurs through ionic H bonds, there are two different classes of ligands which differ in their binding strength. This holds true for both core ions, even though their electronic structures and charge distributions are very different.

Mass Spectrometric Investigation of Anions Formed upon Free Electron Attachment to Nucleobase Molecules and Clusters Embedded in Superfluid Helium Droplets

Here we report the first mass spectrometric study of negative ions formed via free electron attachment (EA) to nucleobases (NBs) embedded in helium clusters. Pure and mixed clusters of adenine and thymine have been formed by pickup of isolated NB molecules by cold helium droplets. In contrast to EA of isolated molecules in the gas phase we observe a long-lived parent anion NB- and, in addition, parent cluster ions NB-n up to size n=6. Moreover, we show that a low energy electron penetrating into a doped helium droplet causes efficient damage of the embedded nucleobases via resonant, site selective, dissociative electron attachment.

Electron impact ionization mass spectrometry of aliphatic alcohol clusters in helium nanodroplets

Electron impact ionization mass spectrometry has been applied for the first time to a range of molecular clusters inside helium nanodroplets. The clusters chosen for investigation are clusters of aliphatic alcohols. Fragmentation channels for the cluster cations in helium droplets differ significantly from previous findings in the gas phase. In addition to formation of abundant (ROH) n H + ions, as already reported in gas phase studies, there are also important contributions from (ROH) n + and (ROH) n−1 RO +. The observation of substantial quantities of parent cluster ions, (ROH) n +, has not been reported previously and shows that helium nanodroplets, in combination with electron impact ionization, form a potentially useful source of metastable ions in the gas phase. The survival of these intact alcohol cluster ions is attributed to rapid evaporative cooling of the ions by the helium before they can react to produce (ROH) n−1 H + + RO. The major enhancement of the (ROH) n−1 RO + fragment channel when compared with gas phase cluster studies is attributed to a cage effect by the surrounding helium atoms. This favours the loss of a hydrogen atom by α-cleavage over the loss of a larger hydrocarbon fragment, although both channels are observed.

Polymerization of Ethylene Molecules Chemisorbed on CrOH+ as a Model System of Chromium-Containing Catalyst

The reaction process of the production of CrOH(C2H4)2(+) was studied in connection with the ethylene polymerization on a silica-supported chromium oxide catalyst (the Phillips catalyst). Cluster ions CrOH(C2H4)2(+) and CrOH(C4H8)+ were produced by the reactions of CrOH+ with C2H4 (ethylene) and C4H8 (1-butene), respectively, and were allowed to collide with a Xe atom under single collision conditions. The cross section for dissociation of each parent cluster ion was measured as a function of the collision energy (collision-induced dissociation, or CID). It was found that (i) the CID cross section for the production of CrOH+ from CrOH(C2H4)2(+) increases sharply at the threshold energy of 3.16 +/- 0.22 eV and (ii) the CID cross section for the production of CrOH+ and C4H8 from CrOH(C4H8)+ also increases sharply at the threshold energy of 3.26 +/- 0.21 eV. In comparison with the calculations based on a B3LYP hybrid density functional method, it is concluded that two ethylene molecules in CrOH(C2H4)2(+) are polymerized to become 1-butene. The calculation also shows that the dimerization proceeds via CrOH(C2H4)+ (ethylene complex) and CrOH(C2H4)2(+) (ethylene complex), in which the ethylene molecules bind with CrOH+ through a pi-bonding.

Collision energy transfer in collision of NH4+(NH3)n−1 (n=3–9) with ND3

An incorporation of ND(3) into protonated ammonia cluster ions NH(4)(+)(NH(3))(n-1) (n=3-9), together with a dissociation of the cluster ions, was observed in the collision of the cluster with ND(3) at collision energies ranging from 0.04 to 1.4 eV in the center-of-mass frame. The branching fractions of the cluster ion species produced in the reactions were obtained as a function of the collision energy. The branching fractions of the incorporation products were successfully explained in terms of the Rice-Ramsperger-Kassel (RRK) theory at collision energies lower than the binding energy of the cluster ion. In addition, the internal energy distributions of the parent cluster ions were determined, and found to be in good agreement with those predicted using the evaporative ensemble model. In incorporations at collision energies lower than the binding energy of the cluster ion, all of the collision energy was transferred to the internal energy of the cluster ions; subsequently, an evaporation of ammonia molecules occurred in an equilibrium process after a complete energy redistribution in the clusters. In contrast, at collision energies higher than the binding energy of the cluster ion, a release of an ammonia molecule from the incorporation products occurred in a nonequilibrium process. The transition from the complex mode to the direct mode in the incorporation was observed at collision energies approximately equal to the binding energy. On the other hand, the collision energy dependence of the cross sections for the dissociation and for a nonreactive collision were estimated by a RRK simulation in which the collision energy transfer was interpreted by using the classical hard-sphere collision model. A relationship between reactivity and reaction modes in the collision of NH(4)(+)(NH(3))(4) with ND(3) is discussed via a comparison of the experimental results with the RRK simulation.

Hyperthermal surface-collisions of water cluster cations

Size-selected, protonated water cluster cations (H2O)nH+, 4 ≤ n ≤ 32, are scattered at normal incidence from the surface of a diamond-coated silicon wafer at collision energies 0 ≤ Ecoll ≤ 500 eV. The size distribution of collision-induced fragment-ions and the ion yield of scattered particles are analyzed, using a secondary time-of-flight mass spectrometer, as a function of the cluster size, n, and the collision energy, Ecoll. Even at low impact energies only very small fragment-ions can be detected, with a maximum fragment size of ∼35% of the colliding parent cluster ions. For clusters consisting of more than 10 molecules, the protonated water dimer (H2O)2H+ becomes the predominant fragment-ion. The total charge survival yield obeys a nonlinear increase with cluster size; for the largest clusters investigated, more than 35% of the impacting ions survive the surface collision in the cationic charge state.

Reactions of size-selected protonated water clusters H +(H 2O) n ( n = 2–6) with an acetone molecule in a guided ion beam apparatus

The reactions of size-selected protonated water clusters, H +(H 2O) n ( n=2–6), with an acetone molecule were studied in a guided ion beam (GIB) apparatus. The collision energy dependence of the reaction cross-sections was investigated over the range of 0.05–2.0 eV in the center-of-mass frame. Water cluster beams with very narrow kinetic energy spreads, 0.02–0.04 eV, were generated to allow accurate measurement of the reaction cross-sections. We observed reaction products of incorporation and dissociation: the incorporation products are cluster ions having acetone whereas the dissociation products are protonated water clusters resulting from the evaporation of water molecule(s) from the parent cluster ions. Our results demonstrate that the incorporation process is dominant at low collision energies and that the proportion of the dissociation process increases as the collision energy increases. The incorporation cross-section at n=5 is found to be the largest among the cluster sizes examined at the collision energy as low as 0.1 eV, which may be due to the water molecule in the second shell of the cluster. We also find that when the collision energy is larger than 0.5 eV, the dissociation cross-section has a maximum at n=5, which is also probably due to the water molecule in the second shell of the cluster, and a minimum at n=3, which may occur because the cluster ion core of the trimer, H 3O +, has one H-site that is not occupied by water molecules.

Spectroscopy and reactivity of size-selected Mg+–methanol clusters

This work presents photodissociation spectra of Mg+(CH3OD)n (n=1–5). Mass spectrometry of the parent cluster ions shows that C–H bond cleavage occurs in the ground electronic state. The branching ratios for products of photodissociation show strong selectivity; methyl loss is the exclusive quenching channel in n=1, while a unique pathway that eliminates CH3D occurs in n=2. Methyl loss does not occur for clusters with more than three solvent molecules. The maximum of spectral intensity shifts to the red with increasing cluster size until halting at n=3. These data signal the formation of a solvent shell at a relatively small size. As the cluster size increases, ligand loss and D atom loss become overwhelmingly favored. We address the selectivity in the products in terms of the initial photoexcited state, nonadiabatic coupling to the ground state potential energy surface, and subsequent dissociation and product formation dictated by dynamics on the ground state surface.

Photofragmentation studies of small selenium cluster cations Sen+ (n=3–8)

Selenium cluster cations are produced by the combination of laser vaporization and supersonic expansion techniques. Each small cluster cation Sen+ (n=3–8) is mass selected separately and subjected to one-photon laser photodissociation processes. The parent and daughter cluster ions are detected using a reflectron time-of-flight mass spectrometer. The appearance potentials of all the observed cluster fragment ions are estimated from their yield curves as a function of the laser wavelength. The neutral dimer evaporation is found to be the lowest energy photodissociation channel. In general, the odd-numbered cluster cations have much larger dissociation thresholds than those of the even-numbered cluster cations. In addition, the dissociation thresholds of the odd-numbered cations decrease with the increasing cluster size, while those of the even-numbered clusters increase with the increasing cluster size. A sequential neutral dimer evaporation mechanism is demonstrated in the photodissociation of some cluster cations at high photon energies.

The formation of ArCO+ ions by dissociative ionization of argon/carbonmonoxide clusters

The formation of ArCO+ ions has been investigated in a photoelectron photoion coincidence experiment by dissociative ionization of Ar2CO clusters. The kinetic energy released (KER) in this dissociation has been measured as a function of the internal energy of the parent cluster ion and compared to the KER expected on the basis of statistical theory of unimolecular reaction, e.g., phase space theory. The latter calculations are based on high level ab initio calculations of the ArCO+ ion and the Ar2CO+ ion. The ab initio calculations show a nonlinear equilibrium geometry of the ArCO+ ion. The comparison of experimental and theoretical KER leads to an adiabatic ionization potential of the ArCO of 13.03 eV and a dissociation energy of the ArCO+ ion of 1.00 eV.

Collisional reactions of Nan+ (n=3–9) with N2O

A collisional reaction of a sodium cluster ion, Nan+(n=3–9), with a nitrous oxide molecule, N2O, was investigated by using a tandem mass-spectrometer equipped with four octopole ion guides. Two types of ions, NakO+ and Nap+ (p<n), were produced by the reaction; NakO+ and Nap+ result from oxidation and direct collisional dissociation of the parent cluster ion, Nan+, respectively. The absolute total cross section and partial cross sections for the NakO+ (oxidation) and Nap+ (direct dissociation) production were measured as a function of the collision energy of a given parent cluster ion, Nan+, in the energy range up to 5 eV (center-of-mass frame). The cross sections and their collision-energy dependence show that the oxidation proceeds when electron harpooning from Nan+ to N2O occurs and otherwise the direct collisional dissociation proceeds.

Metastable Decomposition of {ROH}nH+ Cluster Ions (Where R = CH3 or CH3CH2)

We have studied the metastable decomposition of protonated alcohol cluster ions employing a reflectron time-of-flight mass spectrometer. Loss of a single alcohol molecule is observed as the dominant decomposition channel in all of these cluster ions. From the daughter ion spectra, we find experimental evidence that the {ROH}6H+ cluster ions possess a unique stability relative to the parent cluster ion. Theoretical calculations were performed to interpret the cause for this special stability and to indicate several possible isomeric structures for this cluster ion. The energetic ordering of the isomers can be interpreted in terms of hydrogen bonding and delocalization of charge.

Reaction of Fe 2S 2+ with C 2H 2 studied with a radiofrequency ion trap/time-of-flight mass spectrometer

The reactivity of iron sulfide cluster ions with acetylene has been studied in a radiofrequency ion trap with a time-of-flight mass spectrometer. A newly built set-up is described in detail. The cluster ions are produced by laser ablation and stored in the ion trap for about one second to react with acetylene. The product ions as well as the remaining parent cluster ions are analyzed with the time-of-flight mass spectrometer. The Fe 2S 2 + cluster ion reacts efficiently with C 2H 2 and forms Fe 2S 2(C 2H 2) n + ( n⪕4). The structure of Fe 2S 2 + is discussed on the basis of the favorable attachment of two or four C 2H 2 molecules.

Photochemistry of Naphthalene Cluster Ions (C10H8)n+, n = 2-7: Formation of C11H9+ Ion via Intervalence Band Excitation

The photodissociation dynamics of naphthalene cluster ions, (C{sub 10}H{sub g}){sub n}{sup +} with n = 2-7, in the visible and near-IR wavelength regions have been probed by photoionization-photofragmentation time-of-flight mass spectroscopy. An intracluster reaction leading to production of a C{sub 11}H{sub 9}{sup +} ion is observed when the parent cluster ions are excited with near-IR photons, while only intramolecular fragmentation pathways occur following visible band excitation. The formation of C{sub 11}H{sub 9}{sup +}, which is assumed to possess the benzotropylium structure, is ascribed to a photochemical rearrangement of the dimer ion core produced in the parent clusters upon multiphoton excitation via the intervalence transition band. The intracluster reaction is observed only for parent cluster sizes n {>=} 4, which suggests that the dimer ion core must be surrounded by a solvent cage involving at least two neutral molecules. We propose that the existence of a dimer core structure plays an important role in the photochemistry that occurs subsequent to excitation into the intervalence transition band. 33 refs., 7 figs.

Dissociation dynamics of Ar+n (n=3–16) in collision with He and Ne

Dissociation dynamics of Ar cluster ions, Ar+n (n=3–16), in collision with He and Ne atoms was investigated by measuring the absolute dissociation cross sections and the branching fractions of the product ions at the collision energies of 0.1–10 eV in the center-of-mass frame. The cross sections and the branching fractions were successfully explained in terms of collisional excitation of the parent cluster ion and subsequent dissociation of it; the collisional excitation was treated by a hard sphere collision model with the aid of the molecular dynamics (MD) and the diatomics-in-molecules (DIM) calculations reported previously, and the subsequent dissociation was explained by the Rice–Ramsperger–Kassel (RRK) theory.