Stability Of Cluster Ions Research Articles

Homochiral or Heterochiral: A Systematic Study of Threonine Clusters Using a FT ICR Mass Spectrometer

The strong chiral preferences of some magic clusters of amino acids have attracted continually increasing interests due to their unique structures, properties and possible roles in homochirogenesis. However, how chirality can influence the generation and stability of cluster ions in a wild range of cluster sizes is still unknown for most amino acids. In this study, the preference for threonine clusters to form homochiral and heterochiral complex ions has been investigated by electrospray ionization (ESI) mass spectrometry. Abundant cluster [Thrn+mH]m+ ions (7 ≤ n ≤ 78, 1 ≤ m ≤ 5) have been observed for both samples of enantiopure (100% L) and racemic (50:50 L:D) threonine solutions. Further analyses of the spectra show that the [Thr14+2H]2+ ion is characterized by its most outstanding homochiral preference, and [Thr7+H]+ and [Thr8+H]+ ions also clearly exhibit their homochiral preferences. Although most of the triply charged clusters (20 ≤ n ≤ 36) are characterized by heterochiral preferences, the quadruply charged [Thrn+4H]4+ ions (40 ≤ n ≤ 59) have no obvious chiral preference in general. On the other hand, a weak homochiral preference exists for most of the quintuply charged ions observed in the experiment.

Theoretical and Experimental Study of Negative LiF Clusters Produced by Fast Ion Impact on a Polycrystalline 7LiF Target

The emission of negative cluster ions produced by the impact of approximately 60 MeV (252)Cf fission fragments on a (7)LiF polycrystalline target is analyzed. The negative ion mass spectrum is dominated by the ((7)LiF)(n)F(-) series, n = 1 to approximately 30. The desorption yield distribution of the ((7)LiF)(n)F(-) members has a maximum at n = 2 and then decreases as the sum of two exponentials whose decay parameters are k(Fast) = 0.9 and k(Slow) = 0.08. These k values are the same as those observed for the positive series and close to others obtained for condensed gas targets. Relative cluster ion stabilities, deduced from the experimental ion abundances for the (LiF)(n)F(-) series, are proposed to be correlated with theoretical structures according to their internal energy by using the deviation plot (D-plot) methodology. A pool of candidate cluster structures was generated using a genetic algorithm and further analyzed and optimized using density functional theory (DFT) with the hybrid functional B3LYP (DFT/B3LYP) and Moller-Plesset perturbation theory (MP2). For the small clusters (n = 1 to 2), the most stable structures are found to be linear, whereas the larger clusters (n = 4 to 6) present cubic or polyhedral structures. Fragmentation energies, ionization potentials, and relative stabilities are reported for the most abundant families of the (LiF)(n)F(-) and (LiF)(n)(-) series.

Generation and collision‐induced dissociation of ammonium tetrafluoroborate cluster ions

Singly and doubly charged cluster ions of ammonium tetrafluoroborate (NH4BF4) with general formula [(NH4BF4)nNH4]+ and [(NH4BF4)m(NH4)2]2+, respectively, were generated by electrospray ionization (ESI) and their fragmentation examined using collision-induced dissociation (CID) and ion-trap tandem mass spectrometry. CID of [(NH4BF4)nNH4]+ caused the loss of one or more neutral NH4BF4 units. The n = 2 cluster, [(NH4BF4)2NH4]+, was unique in that it also exhibited a dissociation pathway in which HBF4 was eliminated to create [(NH4BF4)(NH3)NH4]+. Dissociation of [(NH4BF4)m(NH4)2]2+ occurred through two general pathways: (a) 'fission' to produce singly charged cluster ions and (b) elimination of one or more neutral NH4BF4 units to leave doubly charged product ions. CID profiles, and measurements of changing precursor and product ion signal intensity as a function of applied collision voltage, were collected for [(NH4BF4)nNH4]+ and compared with those for analogous [(NaBF4)nNa]+ and [(KBF4)nK]+ ions to determine the influence of the cation on the relative stability of cluster ions. In general, the [(NH4BF4)nNH4]+ clusters were found to be easier to dissociate than both the sodium and potassium clusters of comparable size, with [(KBF4)nK]+ ions the most difficult to dissociate.

Thermochemical stabilities of the gas-phase cluster ions of halide ions with rare gas atoms

Gas-phase clustering reactions of halide ions (X −) with rare gas atoms (Rg) have been investigated using a pulsed-electron beam high-pressure mass spectrometer. The thermochemical stability of the cluster ions X −(Rg) n has been determined. To evaluate bond energies that are difficult to obtain experimentally, ab initio calculations were also carried out. Among the cluster ions investigated, the interaction between the hardest F − ion and the softest Xe atom was found to be strongest (6.3 kcal/mol).

The structures and stabilities of mixed inert gas cluster ions: NeHe+ n and ArHe+ n

DIM-type matrices for RgHe+ n clusters have been established from the results of ab initio calculations on RgHe and RgHe+ (Rg = Ne and Ar). The method has been tested against ab initio calculations on linear and T shaped RgHe+ 2 and found to be satisfactory. Rotational invariance has been established for larger clusters, and the geometries and energies of clusters up to n = 16 have been determined. Ne cluster ions are more stable than Ar cluster ions because of the greater contribution from charge transfer, and are structurally different. The relative stabilities of these cluster ions are consistent with the available experimental data.

Solvation Energy and Gas-Phase Stability Influences on Alkali Metal Cluster Ion Formation in Electrospray Ionization Mass Spectrometry

The ability to observe abundant gas-phase metal cluster ions in electrospray ionization mass spectrometry (ESI-MS) is highly dependent on experimental conditions. Alkali halides (MX) and other alkali metal salts were used to investigate the formation of cluster ions in ESI-MS. All compounds were found to give cluster ions of the form (M(n)(+1)X(n))(+) and (M(n)X(n+1))(-), with only two alkali salts yielding doubly charged cluster ions. In homologous alkali halide series, the relative abundances of cluster ions increased with increasing size of either the cation (positive ion mode) or the anion (negative ion mode). Calculations using an electrostatic model show that the gas-phase stability of cluster ions is greater for smaller cations or anions when a fixed counterion is employed. This stability calculation goes in a direction just opposite to the trend in cluster ion abundances observed in ESI-MS. Studies of equimolar mixtures consisting of two alkali halides reveal two distinct trends. When the equimolar mixture was composed of differing ions that participate in the droplet charge excess with the same counterion, the less solvated ions were found to form more abundant cluster ions. When the ions participating in the charge excess were fixed, the preferred counterion in observed clusters was the one that is more solvated in solution and forms more stable clusters in the gas phase. These observations can be rationalized by an extended form of the charged residue model where the weakly solvated ions that are part of the charge excess are preferentially enriched in offspring droplets during uneven fission. By contrast, transfer of a particular counterion located in the bulk of the droplets to the offspring droplets is not disfavored when this counterion is strongly solvated.

Gas-phase thermochemical stabilities of cluster ions [(N2)m(Ar)n]+ with (m+n)=1–5

Thermochemical stabilities of nitrogen-argon cluster ions [(N2)m(Ar)n]+ were measured using a pulsed-electron beam mass spectrometer. The thermochemical data obtained for the exchange reactions N4++Ar=N2Ar++N2 (ΔH0=−1.0 kcal/mol) and Ar2++N2=N2Ar++Ar (ΔH0=−2.1 kcal/mol) lead to the determination of the bond dissociation energies (D), D(N2Ar+→N2++Ar)=26.8, D(N2Ar+→Ar++N2)=30.5, and D(Ar2+→Ar++Ar)=28.4 kcal/mol. For the mixed cluster ions [(N2)m(Ar)n]+, the irregular decreases in bond dissociation energies are found with (m+n)=3→4. This fall-off indicates that the core ions in the mixed cluster ions are trimer cations, [(N2)m(Ar)n]+ with (m+n)=3 in agreement with the experimental results by Magnera and co-workers [Chem. Phys. Lett. 192, 99 (1992); J. Chem. Soc. Faraday Trans. 86, 2427 (1990)]. The most stable cluster ions of [(N2)m(Ar)n]+ are found to be those composed of the core ion N2Ar+N2 solvated by further N2 ligands. The rate of exchange reaction (N2)m++Ar=N2Ar+N2(N2)m−3+N2 was found to become slower with decrease of temperature due to the existence of energy barrier. Ab initio [G2(MP2)] calculations on some cluster ions [(N2)m(Ar)n]+ were performed. Trimer cations have linear geometries, and the fourth neutral molecule is coordinated in the bridged (bidentate) form to the trimer cation.

Reactions and properties of clusters

The elucidation from a molecular point of view of the differences and similarities in the properties and reactivity of matter in the gaseous compared to the condensed state is a subject of considerable current interest. One of the promising approaches to this problem is to utilize mass spectrometry in conjunction with laser spectroscopy and fast-flow reaction devices to investigate the changing properties, structure and reactivity of clusters as a function of the degree of solvation under well-controlled conditions. In this regard, an investigation of molecular cluster ions has provided considerable new insight into the basic mechanisms of ion reactions within a cluster, and this paper reviews some of the recent advances in cluster production, the origin of magic numbers and relationship to cluster ion stabilities, and solvation effects on reactions. There have been some notable advances in the production of large cluster ions under thermal reaction conditions, enabling a systematic study of the influence of solvation on reactions to be carried out. These and other new studies of magic numbers have traced their origin to the thermochemical stability of cluster ions. There are several classes of reaction where solvation has a notable influence on reactivity. A particularly interesting example comes from recent studies of the reactions of the hydroxyl anion with CO 2 and SO 2, studied as a function of the degree of hydration of OH −. Both reactions are highly exothermic, yet the differences in reactivity are dramatic. In the case of SO 2, the reaction occurs at near the collision rate. By contrast, CO 2 reactivity plummets dramatically for clusters having more than four water molecules. The slow rate is in accord with observations in the liquid phase.

Investigation of the stabilities of neutral and ionic lead and lead-antimony clusters under single and multiphoton ionization conditions

The fluence dependence of lead cluster ion distributions at 222 nm and 308 nm reveal markedly different behavior. Results obtained at 308 nm display a simple uniform increase in intensity with higher laser fluence with little change in relative intensities. At 222 nm, however, a significant transformation is found from a markedly different low fluence distribution to a high fluence pattern, which is essentially indistinguishable from that observed at 308 nm. It is concluded that mass spectra obtained at 308 nm, regardless of fluence, or at 222 nm and high fluence contain appreciable contributions from fragmentation. Hence, under these conditions the mass spectra are found to be dominated by cluster ion stabilities. Magic numbers observed at both high and low fluence correspond well to those obtained using electron-impact ionization, and in many instances parallel the magic numbers characteristic of rare-gas clusters. This suggests the stabilities of both neutral and monovalent cationic lead clusters are largely determined by close-packing considerations, and are not appreciably influenced by electronic structure. Similar preferences for close-packed structures are also found for mixed lead-antimony clusters containing one or two antimony atoms that are ionized using high fluence 308 nm excitation.

Complexation between Saccharide and Ammonium Ion in Gas Phase. Regarding Stereochemical Effects of Methoxy Groups at Permethylated Monosaccharides on Stabilities of Cluster Ions in FAB Mass Spectrometry

Abstract The relative FABMS intensities of the cluster ions between permethylated monosaccharides and glycine methyl ester hydrochloride strongly suggest the configuration effect of the OCH3 groups, reflecting the relative stabilities of the cluster ions in gas phase.

Gas phase stabilities of cluster ions H +(CO) 2(CO) n, H +(N 2) 2(N 2) n and H +(O 2) 2(O 2) n with n=1–14

Thermochemical stabilities of H + (CO) 2(CO) n , H + (N 2) 2(N 2) n and H + (O 2) 2(O 2) n with n electron-beam high-pressure mass spectrometer. The H + (CO) 2(CO) n cluster with n=4 has been found to form a rather solid first shell indicating the participation of covalent bond in the shell. For the H + (N 2) 2(N 2) n clusters the shell formation with and 7 is suggested. The H + (O 2) 2(O 2) n cluster with n=1 has been found to be much more stable than those with n⩾2. The ion may be represented as H +(O 2) 3(O 2) 2(O 2) n . The ring structure for the core ion H +(H 2) 3 has been proposed.

Observation of doubly-charged dimer ion emission in liquid-metal-ion sources

Liquid-metal-ion sources (LMIS's) emit atomic and cluster ions. In the mass analysis of LMIS's using Au-Sb and Pt-Sb alloys, the doubly-charged dimer ions Sb 2+ 2 are observed as well as the doubly-charged diatomic ions AuSb 2+ and PtSb 2+. The flux abundance of Sb 2+ 2 in Sb + +Sb 2+ 2 for a Au-Sb alloy is approximately 20%, which is larger by one order of magnitude than that for a Pt-Au alloy. It is predicted that the stability of cluster ions is strongly affected by the electrostatic field near the ion emitter. The formation mechanism of M 2+ 2 ions is discussed using mass spectra and energy distribution curves.

Abundance of Na cluster ions ejected from a liquid metal ion source

Sodium cluster ions Na+ n withn ranging up to 25 have been observed from a liquid sodium ion source by using a magnetic mass analyzer. Ion intensity as a function of cluster size showed distinct steps and local maxima atn=3, 5, 11, 13 and 19 (magic numbers), and a pronounced odd-even alternation. The features in the ion abundance curve are attributed to the relative stability of cluster ions. The observed magic numbers are only partially explained by the electronic shell model, indicating need to include a consideration of atomic structure in a cluster.

Production and stability of neon cluster ions up to Ne +90

Clusters of Ne n atoms (up to n = 90) formed in a supersonic nozzle expansion have been studied by electron impact ionization mass spectrometery. The Ne cluster mass spectra show the special stability of cluster ions with icosahedral structure ( n = 13 and 55) confirming - despite previous discrepancies - the general validity of the sphere-packing principle. Neon cluster ions exhibit other magic numbers, e.g. at n = 21 and 75, not common to all of the other rare gases.

Thermospray Mass Spectrometry , a New Technique for Studying the Relative Stability of Cluster Ions of Salts

Thermospray (TSP) mass spectrometry provides a means of studying stability effects in the frequency distribution of cluster ions of salts under conditions of heat transfer to the cluster ions via thermal collisions in a gas. This is demonstrated for the cluster ions of ammonium chloride. The TSP mass spectrum is compared with that obtained by sputtering of the salt in secondary ion mass spectrometry.

Mass spectrometric investigation of NH 3 and CO 2 cluster ions: appearance potentials and stability

Electron impact ionization of (CO 2) x and (NH 3) y (x up to 5, y up to 10) was investigated in a molecular beam experiment coupled with a double focusing mass spectrometer. A special ion optical system and operating mode (deflection method) was used to discriminate between beam species and background species. For both neat and seeded (He, Ar, Kr) beams we have studied neutral cluster beam properties. The following cluster ion appearance potentials were obtained: (CO 2) 2+e→(CO 2) + 2 AP=13.1±0.2 eV (NH 3) 2+e→NH 4 + AP=9.95±0.3 eV (NH 3) 3+e→(NH 3) 2H + AP=9.50±0.3 eV The exothermicity of the reaction of CO + 2 with CO 2 to form (CO 2) + 2 was found to be 17.1±5 kcal/mole in good agreement with previous high pressure mass spectrometry determination. The dissociation energy for the proton bound dimer to NH 4 ++NH 3 was found to be 0.60±0.2 eV in good agreement with a previous photoionization result of 0.60 eV, in contrast to a number of recent ion molecule studies supporting a value of about 1.1 eV. In addition, metastable transitions occurring in the mass spectrum of C 3H 8, N 2O and (CO 2) x, (NH 3) y were investigated by means of decoupling the acceleration and deflection fields of the mass spectrometer, yielding the first results on the stability of cluster ions.

Proton affinities and cluster ion stabilities in CO2 and CS2. Applications in Martian ionospheric chemistry

Thermodynamic values for the reaction CO2H++CH4?CH5+ +CO2 were measured by pulsed high pressure mass spectrometry as ΔH°=−2.8 kcal/mole, ΔS°=−2.4 e.u.; for CS2H+ +H2O?H3O++CS2 as ΔH°=−2.3 kcal/mole, ΔS°=−7.7 e.u.; and for CS2H++H2S?H3S++CS2 as ΔH°=−7.6 kcal/mole, ΔS°=−9.6 e.u. The proton affinities of CO2 and CS2 are found to be 124.0 and 166.6 kcal/mole, respectively. The large entropy changes in the reactions involving CS2 are explained by a nonlinear CS2H+ ion. Stabilities of cluster ions, i.e., −ΔH° for the ion–molecule association reactions leading to the cluster ions were measured (in kcal/mole) as follows: For radical ions: CO2+⋅CO2, 16.2; CO2+⋅2CO2, 6.0; CS2+⋅CS2, 21.9; S2+⋅CS2, 21.9. For protonated, even-electron ions: CO2H+⋅CO2, 20.1; CS2H+CS2, 11.1; H3O+⋅CO2, 14.4. The stability of CO2H+⋅CO2 is unusually high for a small protonated cluster ion and indicates significant hydrogen bonding in this ion. Entropies of association were found in the range −17 to −26 e.u. Limits for the stabilities of O2+⋅nCO2 (n=1–3) were also established. The application of these results in Martian ionospheric chemistry is discussed.