Strongly cluster size dependent reaction behavior of CO with O2 on free silver cluster anions

Reactions of free silver anions Agn- (n = 1 - 13) with O2, CO, and their mixtures are investigated in a temperature controlled radio frequency ion trap setup. Cluster anions Agn- (n = 1 - 11) readily react with molecular oxygen to yield AgnOm- (m = 2, 4, or 6) oxide products. In contrast, no reaction of the silver cluster anions with carbon monoxide is detected. However, if silver cluster anions are exposed to the mixture of O2 and CO, new reaction products and a pronounced, discontinuous size dependence in the reaction behavior is observed. In particular, coadsorption complexes Agn(CO)O2- are detected for cluster sizes with n = 4 and 6 and, the most striking observation, in the case of the larger odd atom number clusters Ag7-, Ag9-, and Ag11-, the oxide product concentration decreases while a reappearance of the bare metal cluster signal is observed. This leads to the conclusion that carbon monoxide reacts with the activated oxygen on these silver clusters and indicates the prevalence of a catalytic reaction cycle.     

Electron solvation in water clusters following charge transfer from iodide

The dynamics following charge transfer to solvent from iodide to a water cluster are studied using time-resolved photoelectron imaging of I-(H2O)n and I-(D2O)n clusters with n< or =28. The results show spontaneous conversion, on a time scale of approximately 1 ps, from water cluster anions with surface-bound electrons to structures in which the excess electron is more strongly bound and possibly more internalized within the solvent network. The resulting dynamics provide valuable insight into the electron solvation dynamics in water clusters and the relative stabilities between recently observed isomers of water cluster anions.     

Electron interaction with nitromethane embedded in helium droplets: attachment and ionization measurements

Results of a detailed study on electron interactions with nitromethane (CH(3)NO(2)) embedded in helium nanodroplets are reported. Anionic and cationic products formed are analysed by mass spectrometry. When the doped helium droplets are irradiated with low-energy electrons of about 2 eV kinetic energy, exclusively parent cluster anions (CH(3)NO(2))(n)(-) are formed. At 8.5 eV, three anion cluster series are observed, i.e., (CH(3)NO(2))(n)(-), [(CH(3)NO(2))(n)-H](-), and (CH(3)NO(2))(n)NO(2)(-), the latter being the most abundant. The results obtained for anions are compared with previous electron attachment studies with bare nitromethane and nitromethane condensed on a surface. The cation chemistry (induced by electron ionization of the helium matrix at 70 eV and subsequent charge transfer from He(+) to the dopant cluster) is dominated by production of methylated and protonated nitromethane clusters, (CH(3)NO(2))(n)CH(3)(+) and (CH(3)NO(2))(n)H(+).     

Thermal effects on energetics and dynamics in water cluster anions (H2O)n(-)

The electron binding energies and relaxation dynamics of water cluster anions (H(2)O)(n)(-) (11 ≤ n ≤ 80) formed in co-expansions with neon were investigated using one-photon and time-resolved photoelectron imaging. Unlike previous experiments with argon, water cluster anions exhibit only one isomer class, the tightly bound isomer I with approximately the same binding energy as clusters formed in argon. This result, along with a decrease in the internal conversion lifetime of excited (H(2)O)(n)(-) (25 ≤ n ≤ 40), indicates that clusters are vibrationally warmer when formed in neon. Over the ranges studied, the vertical detachment energies and lifetimes appear to converge to previously reported values.     

Electronic relaxation dynamics of water cluster anions

The electronic relaxation dynamics of water cluster anions, (H(2)O)(n)(-), have been studied with time-resolved photoelectron imaging. In this investigation, the excess electron was excited through the p<--s transition with an ultrafast laser pulse, with subsequent electronic evolution monitored by photodetachment. All excited-state lifetimes exhibit a significant isotope effect (tau(D)2(O)/tau(H)2(O) approximately 2). Additionally, marked dynamical differences are found for two classes of water cluster anions, isomers I and II, previously assigned as clusters with internally solvated and surface-bound electrons, respectively. Isomer I clusters with n > or = 25 decay exclusively by internal conversion, with relaxation times that extrapolate linearly with 1/n toward an internal conversion lifetime of 50 fs in bulk water. Smaller isomer I clusters (13 < or = n < or = 25) decay through a combination of excited-state autodetachment and internal conversion. The relaxation of isomer II clusters shows no significant size dependence over the range of n = 60-100, with autodetachment an important decay channel following excitation of these clusters. Photoelectron angular distributions (PADs) were measured for isomer I and isomer II clusters. The large differences in dynamical trends, relaxation mechanisms, and PADs between large isomer I and isomer II clusters are consistent with their assignment to very different electron binding motifs.     

The solvation of iodine anions in water clusters: PES studies

We have measured the photoelectron-spectra of I^? (H₂O)_n clusters in the size range n=1–60. We have found that the first six water molecules form a solvation layer with an average 0.35 eV electrostatic stabilization of the anion. At larger cluster sizes the electrostatic stabilization of water does not fit a continuous dielectric solvent. The most stable structures of the clusters consist of internally solvated anions. In the size range n=34–40 we have found evidence for existence of cluster structures with surface solvated anions.     

Theoretical study of the geometries and dissociation energies of molecular water on neutral aluminum clusters Al(n) (n = 2-25)

Geometries and dissociation energies of water molecules on Al(n) (n = 2-25) clusters were investigated using density functional theory with all electron relativistic spin-polarized calculations under the generalized gradient approximation. An extensive structure search was performed to identify the low-energy conformations of Al(n)H(2)O complexes for each size. Optimal adsorption sites were assigned for low-energy isomers of the clusters. Size and site specific dependences were studied for the Al(n)H(2)O complexes in stabilities, geometries, adsorption energies, dissociation energies, Al-O bond lengths, and other characteristic quantities. The stabilities and geometries revealed that H atom in H(2)O is not inclined to bond with Al atoms. The most stable Al(n)H(2)O configurations for each size tend to correspond to the most stable bare Al(n) cluster except of Al(6) and Al(24) clusters. The HO bond lengths increase generally 0.01 ? with respect to the isolated H(2)O in all of the adsorption complexes. The dissociation energy of an isolated H(2)O into HO and H was 5.39 eV, which decreased about two-thirds to the energy range of 0.83-2.12 eV with the help of Al(n) clusters. In spite of the fluctuations, the dissociation energies of Al(n)H(2)O complexes rise with the size increasing as a whole. In addition, we also found that the bare Al(n) clusters with high vertical ionization potentials usually have high dissociation energies of H(2)O in the corresponding adsorption models. The energetically preferred spin-multiplicity of all the odd-n Al(n)H(2)O complexes is doublet, and it is singlet for all the even-n complexes with exception of Al(2)H(2)O which is triplet.     

Infrared spectroscopy of water cluster anions, (H2O)n=3-24-)in the HOH bending region: persistence of the double H-bond acceptor (AA) water molecule in the excess electron binding site of the class I isomers

We report vibrational predissociation spectra of water cluster anions, (H(2)O)(n=)()(3)(-)(24)(-) in the HOH bending region to explore whether the characteristic red-shifted feature associated with electron binding onto a double H-bond acceptor (AA) water molecule survives into the intermediate cluster size regime. The spectra of the "tagged" (H(2)O)(n)()(-).Ar clusters indeed exhibit the signature AA band, but assignment of this motif to a particular isomer is complicated by the fact that argon attachment produces significant population of three isomeric forms (as evidenced by their photoelectron spectra). We therefore also investigated the bare clusters since they can be prepared exclusively in the high binding (isomer class I) form. Because the energy required to dissociate a water molecule from the bare complexes is much larger than the transition energies in the bending region, the resulting (linear) action spectroscopy selectively explores the properties of clusters with most internal energy content. The (H(2)O)(15)(-) predissociation spectrum obtained under these conditions displays a more intense AA feature than was found in the spectra of the Ar tagged species. This observation implies that not only is the AA motif present in the class I isomer, but also that it persists when the clusters contain considerable internal energy.     

Electron impact ionization of water-doped superfluid helium nanodroplets: observation of He(H(2)O)(n)(+) clusters

Electron impact mass spectra have been recorded for helium nanodroplets containing water clusters. In addition to identification of both H(+)(H(2)O)(n) and (H(2)O)(n)(+) ions in the gas phase, additional peaks are observed which are assigned to He(H(2)O)(n)(+) clusters for up to n=27. No clusters are detected with more than one helium atom attached. The interpretation of these findings is that quenching of (H(2)O)(n)(+) by the surrounding helium can cool the cluster to the point where not only is fragmentation to H(+)(H(2)O)(m) (where m < or = n-1) avoided, but also, in some cases, a helium atom can remain attached to the cluster ion as it escapes into the gas phase. Ab initio calculations suggest that the first step after ionization is the rapid formation of distinct H(3)O(+) and OH units within the (H(2)O)(n)(+) cluster. To explain the formation and survival of He(H(2)O)(n)(+) clusters through to detection, the H(3)O(+) is assumed to be located at the surface of the cluster with a dangling O-H bond to which a single helium atom can attach via a charge-induced dipole interaction. This study suggests that, like H(+)(H(2)O)(n) ions, the preferential location for the positive charge in large (H(2)O)(n)(+) clusters is on the surface rather than as a solvated ion in the interior of the cluster.     

Fragmentation cross sections of protonated water clusters

We have measured fragmentation cross sections of protonated water cluster cations (H(2)O)(n=30-50)H(+) by collision with water molecules. The clusters have well-defined sizes and internal energies. The collision energy has been varied from 0.5 to 300 eV. We also performed the same measurements on deuterated water clusters (D(2)O)(n=5-45)D(+) colliding with deuterated water molecules. The main fragmentation channel is shown to be a sequential thermal evaporation of single molecules following an initial transfer of relative kinetic energy into internal energy of the cluster. Unexpectedly, that initial transfer is very low on average, of the order of 1% of collision energy. We evaluate that for direct collisions (i.e., within the hard sphere radius), the probability for observing no fragmentation at all is more than 35%, independently of cluster size and collision energy, over our range of study. Such an effect is well known at higher energies, where it is attributed to electronic effects, but has been reported only in a theoretical study of the collision of helium atoms with sodium clusters in that energy range, where only vibrational excitation occurs.     

Probing the electronic structure and band gap evolution of titanium oxide clusters (TiO(2))(n)(-) (n = 1-10) using photoelectron spectroscopy

TiO2 is a wide-band-gap semiconductor, and it is an important material for photocatalysis. Here we report an experimental investigation of the electronic structure of (TiO2)n clusters and how their band gap evolves as a function of size using anion photoelectron spectroscopy (PES). PES spectra of (TiO2)n- clusters for n = 1-10 have been obtained at 193 nm (6.424 eV) and 157 nm (7.866 eV). The high photon energy at 157 nm allows the band gap of the TiO2 clusters to be clearly revealed up to n = 10. The band gap is observed to be strongly size-dependent for n < 7, but it rapidly approaches the bulk limit at n = 7 and remains constant up to n = 10. All PES features are observed to be very broad, suggesting large geometry changes between the anions and the neutral clusters due to the localized nature of the extra electron in the anions. The measured electron affinities and the energy gaps are compared with available theoretical calculations. The extra electron in the (TiO2)n- clusters for n > 1 appears to be localized in a tricoordinated Ti atom, creating a single Ti3+ site and making these clusters ideal molecular models for mechanistic understanding of TiO2 surface defects and photocatalytic properties.     

Hypoiodous acid as guest molecule in protonated water clusters: a combined FT-ICR/DFT study of I(H2O)n+.

Cationic water clusters containing iodine, of the composition I(H2O)n+, n = 0-25, are generated in a laser vaporization source and investigated by FT-ICR mass spectrometry. An investigation of blackbody radiation-induced fragmentation of size-selected clusters I(H2O)n+, n = 3-15, under collision-free conditions revealed an overall linear increase of the unimolecular rate constant with cluster size, similar to what has been observed previously for other hydrated ions. Above a certain critical size, I(H2O)n+, n greater than or approx. 13, reacts with HCl by formation of the interhalide ICl and a protonated water cluster, which is the reverse of a known solution-phase reaction. Accompanying density functional calculations illustrate the conceptual differences between cationic and anionic iodine-water clusters I(H2O)n+/-. While I-(H2O)n is genuinely a hydrated iodide ion, the cationic closed-shell species I(H2O)n+ may be best viewed as a protonated water cluster, in which one water molecule is replaced by hypoiodous acid. In the strongly acidic environment, HOI is protonated because of its high proton affinity. However, similar to the well-known H3O+/H5O2+ controversy in protonated water clusters, a smooth transition between H2IO+ and H4IO2+ as core ions is observed for different cluster sizes.     

Solvation-induced cluster anion core switching from NNO2(-)(N2O)n-1 to O(-)(N2O)n

We report a photoelectron imaging study of the [O(N(2)O)(n)](-), 0or=4 (and up to at least n=9) signatures of an O(-) core are predominantly observed. Photofragmentation studies at 355 nm support these results.     

Photoelectron spectroscopy of the [glycine x (H2O)(1,2)]- clusters: sequential hydration shifts and observation of isomers

The electron binding energies of the small hydrated amino acid anions, [glycine x (H2O)(1,2)]-, are determined using photoelectron spectroscopy. The vertical electron detachment energies (VDEs) are found to increase by approximately 0.12 eV with each additional water molecule such that the higher electron binding isomer of the dihydrate is rather robust, with a VDE value of 0.33 eV. A weak binding isomer of the dihydrate is also recovered, however, with a VDE value (0.14 eV) lower than that of the monohydrate. Unlike the situation in the smaller (n < or = 13) water cluster anions, the [Gly x (H2O)(n > or = 6)]- clusters are observed to photodissociate via water monomer evaporation upon photoexcitation in the O-H stretching region. We discuss this observation in the context of the mechanism responsible for the previously observed [S. Xu, M. Nilles, and K. H. Bowen, Jr., J. Chem. Phys. 119, 10696 (2003)] sudden onset in the cluster formation at [Gly x (H2O)5]-.     

Chemical stability and reactivity of deprotonated oligonucleotides (DNA) in the gas phase: protonation and solvation with hydrogen bromide

Selected deprotonated oligodeoxynucleotides generated by electrospray ionization were exposed to a variety of neutral molecules in the gas phase at room temperature in flowing helium gas at 0.35 Torr. Single-stranded [AGTCTG-nH]n- and single- and double-stranded [GCATGC-nH]n- anions were found to be remarkably unreactive with strong oxidants (O3, O2, N2O) and potential intercalators (benzene, pyridine, toluene, and quinoxaline). Hydration also was observed to be inefficient. However, [AGTCTG-nH]n- anions with n=2, 3, 4, and 5 were seen to be sequentially protonated and/or hydrobrominated with HBr (but not damaged) and displayed an interesting "end effect" against protonation. Measurements are provided for the rate coefficients of reaction and the efficiencies of protonation. These experimental results point toward the exciting prospect of measuring the intrinsic chemistry of other bare DNA-like anions, including double-stranded oligonucleotide anions in the gas phase at room temperature.     

Microhydration of X2 gas (X = Cl, Br, and I): a theoretical study on X2.nH2O clusters (n = 1-8)

Structure and properties of hydrated clusters of halogen gas, X2.nH2O (X = Cl, Br, and I; n = 1-8) are presented following first principle based electronic structure theory, namely, BHHLYP density functional and second-order Moller-Plesset perturbation (MP2) methods. Several geometrical arrangements are considered as initial guess structures to look for the minimum energy equilibrium structures by applying the 6-311++G(d,p) set of the basis function. Results on X2-water clusters (X = Br and I) suggest that X2 exists as a charge separated ion pair, X+delta-X-delta in the hydrated clusters, X2.nH2O (n > or = 2). Though the optimized structures of Cl2.nH2O clusters look like X2.nH2O (X = Br and I) clusters, Cl2 does not exist as a charge separated ion pair in the presence of solvent water molecules. The calculated interaction energy between X2 and solvent water cluster increases from Cl2.nH2O to I2.nH2O clusters, suggesting solubility of gas-phase I2 in water to be a maximum among these three systems. Static and dynamic polarizabilities of hydrated X2 clusters, X2.nH2O, are calculated and observed to vary linearly with the size (n) of these water clusters with correlation coefficient >0.999. This suggests that the polarizability of the larger size hydrated clusters can be reliably predicted. Static and dynamic polarizabilities of these hydrated clusters grow exponentially with the frequency of an external applied field for a particular size (n) of hydrated cluster.     

Comparison of Adsorption Probabilities of O(2) and CO on Copper Cluster Cations and Anions

Reactions of size-selected copper cluster cations and anions, Cu(n)(±), with O(2) and CO have been systematically investigated under single collision conditions by using a tandem-mass spectrometer. In the reactions of Cu(n)(±) (n = 3-25) with O(2), oxidation of the cluster is prominently observed with and without releasing Cu atoms at the collision energy of 0.2 eV. The reactivity of Cu(n)(+) is governed to some extent by the electronic shell structure; the relatively small reaction cross sections observed at n = 9 and 21 correspond to the electronic shell closings, and those at odd sizes in n ≤ 16 match with the clusters having no unpaired electron. On the other hand, the reactivity of Cu(n)(-) exhibits no remarkable decrease by the electronic shell closings and the even-numbered electrons. These behaviors may be due to an influence of the electron detachment of the reaction intermediate, Cu(n)O(2)(-). Both the cations and anions show the dominant formation of Cu(n-1)O(2)(±) in n ≤ 16 and Cu(n)O(2)(±) in n ≥ 17 in the experimental time window. By contrast, Cu(n)(-) (n = 3-11) do not react with CO at the collision energy of 0.2 eV, while Cu(n)(+) (n = 3-19) adsorb CO though the cross sections are relatively small. The difference in the reactivity between the charge states can be understood in terms of the frontier orbitals of the Cu cluster and O(2) or CO.     

Nonergodicity in electron capture dissociation investigated using hydrated ion nanocalorimetry

Hydrated divalent magnesium and calcium clusters are used as nanocalorimeters to measure the internal energy deposited into size-selected clusters upon capture of a thermally generated electron. The infrared radiation emitted from the cell and vacuum chamber surfaces as well as from the heated cathode results in some activation of these clusters, but this activation is minimal. No measurable excitation due to inelastic collisions occurs with the low-energy electrons used under these conditions. Two different dissociation pathways are observed for the divalent clusters that capture an electron: loss of water molecules (Pathway I) and loss of an H atom and water molecules (Pathway II). For Ca(H(2)O)(n)(2+), Pathway I occurs exclusively for n >or= 30 whereas Pathway II occurs exclusively for n 相似文献   

Quantum-classical simulation of electron localization in negatively charged methanol clusters

A series of quantum molecular dynamics simulations have been performed to investigate the energetic, structural, dynamic, and spectroscopic properties of methanol cluster anions, [(CH(3)OH)(n)](-), (n = 50-500). Consistent with the inference from photo-electron imaging experiments, we find two main localization modes of the excess electron in equilibrated methanol clusters at ～200 K. The two different localization patterns have strikingly different physical properties, consistent with experimental observations, and are manifest in comparable cluster sizes to those observed. Smaller clusters (n ≤ 128) tend to localize the electron in very weakly bound, diffuse electronic states on the surface of the cluster, while in larger ones the electron is stabilized in solvent cavities, in compact interior-bound states. The interior states exhibit properties that largely resemble and smoothly extrapolate to those simulated for a solvated electron in bulk methanol. The surface electronic states of methanol cluster anions are significantly more weakly bound than the surface states of the anionic water clusters. The key source of the difference is the lack of stabilizing free hydroxyl groups on a relaxed methanol cluster surface. We also provide a mechanistic picture that illustrates the essential role of the interactions of the excess electron with the hydroxyl groups in the dynamic process of the transition of the electron from surface-bound states to interior-bound states.     

Sodium microsolvation in ethanol: common features of Na(HO-R)n (R=H, CH3, C2H5) clusters

Ethanol clusters are generated in a continuous He seeded supersonic expansion and doped with sodium atoms in a pick-up cell. By this method clusters of the type Na(C(2)H(5)OH)(n) are formed and characterized by determining size selectively their ionization potentials (IPs) for n = 2-40 in photoionization experiments. A continuous decrease to 3.1 eV is found from n = 2 to 6 and a constant value of 3.07 ± 0.06 eV for n = 10-40. This IP evolution is similar to the sodium-water and the sodium-methanol system. Quantum chemical calculations (B3LYP and MP2) of the IPs indicate adiabatic contributions to the photoionization process for the cluster sizes n = 4 and 5, which is similar to the sodium-methanol case. The results of the extrapolated IPs and the vertical binding energies (VEBs) of cluster anions are compared with the recently reported VEBs of solvated electrons in liquid water, methanol, and ethanol solutions in the range of 3.1-3.4 eV. The new results imply that the extrapolated VBEs of solvated electrons in anionic clusters match the VBE in liquid water, while they are about 0.5 eV too low for methanol. The influence of the presence of counterions on these findings is discussed.