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1.
J Phys Chem A ; 2020 Mar 10.
Artigo em Inglês | MEDLINE | ID: mdl-32106678

RESUMO

Nucleation kinetics in gas phase remains an open issue with no general model. The derivation of the reaction constants assuming a canonical ensemble fails to describe anisotropic materials such as oxides. We have developed a general and versatile model using activated complex kinetics with a microcanonical approach. This approach handles the kinetics issue in cluster growth when the transient nature of the processes hinders the use of the canonical ensemble. The model efficiently reproduces experimental size distributions of alumina clusters formed by laser ablation with different buffer gas densities, including magic numbers. We show that the thermodynamic equilibrium is not reached during the growth. The bounding energy measured is 10 times lower than the one deduced from DFT calculation, but also the one expected from the bulk cohesive energy.

2.
Phys Chem Chem Phys ; 21(41): 23129-23135, 2019 Oct 24.
Artigo em Inglês | MEDLINE | ID: mdl-31609369

RESUMO

The resistivity to oxidation of small copper clusters, Cun+ (n ≤ 5), in the gas phase with a precise atomicity at the molecular level was investigated using a combination of thermal desorption spectrometry and mass spectrometry. Oxide clusters, CunOm+, with more O atoms than those present with a stoichiometry of n : m = 1 : 1 were produced at room temperature in the presence of O2, and the weakly bound excess oxygen atoms involved in the clusters were removed by post heating. Non-oxidized Cu2+ and Cu3+ clusters were formed in the range of 323-923 K, whereas partially oxidized clusters, Cu4O2+ and Cu5O2+, were generated for n = 4 and 5. Considering the fact that CunOm+ (m = n/2 + 1) tends to be generated for n ≥ 6, the small copper clusters were concluded to be resistive to oxidation. The possible reaction paths for the oxidation of Cu2+ and Cu4+ clusters were obtained by density functional calculations, which were consistent with the experimental findings. The oxidation states of the Cu atoms in the clusters were discussed based on the natural charges of the atoms.

3.
Phys Chem Chem Phys ; 20(11): 7781-7790, 2018 Mar 14.
Artigo em Inglês | MEDLINE | ID: mdl-29504007

RESUMO

The thermal decomposition of free cationic iron-sulfur clusters FexSy+ (x = 0-7, y = 0-9) is investigated by collisional post-heating in the temperature range between 300 and 1000 K. With increasing temperature the preferential formation of stoichiometric FexSy+ (y = x) or near stoichiometric FexSy+ (y = x ± 1) clusters is observed. In particular, Fe4S4+ represents the most abundant product up to 600 K, Fe3S3+ and Fe3S2+ are preferably formed between 600 K and 800 K, and Fe2S2+ clearly dominates the cluster distribution above 800 K. These temperature dependent fragment distributions suggest a sequential fragmentation mechanism, which involves the loss of sulfur and iron atoms as well as FeS units, and indicate the particular stability of Fe2S2+. The potential fragmentation pathways are discussed based on first principles calculations and a mechanism involving the isomerization of the cluster prior to fragmentation is proposed. The fragmentation behavior of the iron-sulfur clusters is in marked contrast to the previously reported thermal dissociation of analogous iron-oxide clusters, which resulted in the release of O2 molecules only, without loss of metal atoms and without any tendency to form particular prominent and stable FexOy+ clusters at high temperatures.

4.
J Synchrotron Radiat ; 24(Pt 5): 1086-1091, 2017 Sep 01.
Artigo em Inglês | MEDLINE | ID: mdl-28862633

RESUMO

X-ray free-electron lasers (XFELs) have opened new opportunities for time-resolved X-ray crystallography. Here a nanosecond optical-pump XFEL-probe device developed for time-resolved serial femtosecond crystallography (TR-SFX) studies of photo-induced reactions in proteins at the SPring-8 Angstrom Compact free-electron LAser (SACLA) is reported. The optical-fiber-based system is a good choice for a quick setup in a limited beam time and allows pump illumination from two directions to achieve high excitation efficiency of protein microcrystals. Two types of injectors are used: one for extruding highly viscous samples such as lipidic cubic phase (LCP) and the other for pulsed liquid droplets. Under standard sample flow conditions from the viscous-sample injector, delay times from nanoseconds to tens of milliseconds are accessible, typical time scales required to study large protein conformational changes. A first demonstration of a TR-SFX experiment on bacteriorhodopsin in bicelle using a setup with a droplet-type injector is also presented.

5.
Phys Chem Chem Phys ; 19(31): 20401-20411, 2017 Aug 09.
Artigo em Inglês | MEDLINE | ID: mdl-28730209

RESUMO

The geometric and electronic properties of silicon-atom-doped aluminum clusters, AlnSim (n = 7-30, m = 0-2), were investigated experimentally. The size dependences of the ionization energy and electron affinity of AlnSim show that the stability of AlnSim is governed by the total number of valence electrons in the clusters, where Al and Si atoms behave as trivalent and tetravalent atoms, respectively. Together with theoretical calculations, it has been revealed that neutral Al10Si and Al12Si have a cage-like geometry with central Si atom encapsulation and closed electronic structures of superatomic orbitals (SAOs), and also that they both exhibit geometric robustness against reductive and oxidative changes as cage-like binary superatoms of Si@Al10 and Si@Al12. As well as the single-atom-doped binary superatoms, the effect of symmetry lowering was examined by doping a second Si atom toward the electron SAO closing of 2P SAO, forming Al11Si2. The corresponding anion and cation clusters keep their geometry of the neutral intact, and the ionization energy is low compared to others, showing that Al11Si2 is characterized to be, Si@Al11Si as an alkaline-like binary superatom. For Al21Si2, a face-sharing bi-icosahedral structure was identified to be the most stable as dimeric superatom clusters.

6.
J Phys Chem Lett ; 8(10): 2143-2147, 2017 May 18.
Artigo em Inglês | MEDLINE | ID: mdl-28445054

RESUMO

The catalytic decomposition of NO by cationic platinum oxide cluster Pt3O4+ was investigated by mass spectrometry and thermal desorption spectrometry. Upon reaction with two NO molecules, molecular oxygen desorbed from the cluster at room temperature to form Pt3O4N2+. Then, at temperatures above 400 K, desorption of N2 from Pt3O4N2+ was observed. These processes were confirmed by isotope-labeling experiments, and the energetics of O2 and N2 release were determined by density functional calculations. The combination of these elementary steps resulted in the catalytic decomposition of NO by Pt3O4+.

7.
J Phys Chem A ; 120(43): 8599-8605, 2016 Nov 03.
Artigo em Inglês | MEDLINE | ID: mdl-27736071

RESUMO

Infrared multiple photon dissociation (IRMPD) spectra of Rh6Om+ (m = 4-10) are obtained in the 300-1000 cm-1 spectral range using the free electron laser for infrared experiments (FELIX) via dissociation of Rh6Om+ or Rh6Om+-Ar complexes. The spectra are compared with the calculated spectra of several stable geometries obtained by density functional theory (DFT) structural optimization. The spectrum for Rh6O4+ shows prominent bands at 620 and 690 cm-1 and is assigned to a capped-square pyramidal Rh atom geometry with three bridging O atoms and one O atom in a hollow site. Rh6O5+ displays bands at 460, 630, 690, and 860 cm-1 and has a prismatic Rh geometry with three bridging O atoms and two O atoms in a hollow site. Rh6O6+ shows three intense bands around 600-750 cm-1 and multiple weak bands in the range of 350-550 cm-1. This species has a prismatic Rh geometry with four bridging O atoms and two O atoms in a hollow site. Considering that Rh6Om+ (m ≤ 3) adopts tetragonal bipyramidal Rh6 structures, the change at m = 4 to capped bipyramidal and at m = 5 to prismatic geometries results in a reduction of the number of triangular hollow sites. Since NO preferentially binds on a triangular hollow site through the N atom, the geometry change lowers the possibility of NO dissociative adsorption.

8.
J Phys Chem A ; 120(39): 7624-7633, 2016 Oct 06.
Artigo em Inglês | MEDLINE | ID: mdl-27627215

RESUMO

The stability and reactivity of cationic gold-cerium oxide clusters, AumCenO2n+x+ (m ≤ 4, n ≤ 7, -1 ≤ x ≤ 2), were examined experimentally and computationally. These clusters were generated by simultaneous laser ablation of gold and cerium oxide targets and analyzed by time-of-flight mass spectrometry combined with gas-phase temperature-programmed desorption. Stable compositions of gold-cerium oxide clusters were identified as AumCenO2n+ and AumCenO2n+1+ for m ≥ 1, containing one oxygen atom more than the stable gold-free cerium oxide clusters CenO2n-1+ and CenO2n+. In either case, the stable clusters mainly consisted of Ce4+ and O2-, and the gold atoms had an oxidation state of +1. The reactivity of cerium oxide clusters toward CO was modified by gold atoms, which hindered CO oxidation while efficiently promoting its adsorption. According to density functional theory calculations, the oxygen-centered radical of cerium oxide clusters, considered to be the reactive site, was geometrically and electronically inactivated by gold atoms, which functioned as a CO adsorption site.

9.
Acta Crystallogr D Struct Biol ; 72(Pt 4): 520-3, 2016 Apr.
Artigo em Inglês | MEDLINE | ID: mdl-27050131

RESUMO

A liquid-droplet injector has been developed that delivers pristine microcrystals to an X-ray irradiation area for conducting serial femtosecond crystallography (SFX) with an X-ray free-electron laser (XFEL). By finely tuning the pulsed liquid droplets in time and space, a high hit rate of the XFEL pulses to microcrystals in the droplets was achieved for measurements using 5 µm tetragonal lysozyme crystals, which produced 4265 indexable diffraction images in about 30 min. The structure was determined at a resolution of 2.3 Å from <0.3 mg of protein. With further improvements such as reduction of the droplet size, liquid droplets have considerable potential as a crystal carrier for SFX with low sample consumption.


Assuntos
Cristalografia por Raios X/instrumentação , Cristalografia por Raios X/métodos
10.
J Phys Chem A ; 120(3): 356-63, 2016 Jan 28.
Artigo em Inglês | MEDLINE | ID: mdl-26730616

RESUMO

Gas-phase rhodium oxide clusters, RhnOm(+), were investigated by measuring the rate constants of oxidation and thermal desorption spectrometry. RhnOm(+) was suggested to be categorized into different states as m/n ≤ 1, 1 < m/n ≤ 1.5, and 1.5 < m/n in terms of energy and kinetics. For m/n ≤ 1, the O atoms readily adsorbed on the cluster with a large binding energy until RhO was formed. Under the O2-rich environment, oxidation proceeded until Rh2O3 was formed with a moderate binding energy. In addition, O2 molecules attached weakly to the cluster, and Rh2O3 formed RhnOm(+) (1.5 < m/n). The energetics and geometries of Rh6Om(+) (m = 6-12) were obtained using density functional theory calculations and were found to be consistent with the experimental results.

11.
J Phys Chem A ; 119(41): 10255-63, 2015 Oct 15.
Artigo em Inglês | MEDLINE | ID: mdl-26394781

RESUMO

The reactivity of cerium oxide cluster cations, CenO2n+x(+) (n = 2-9, x = -1 to +2), with NO was investigated using gas-phase temperature-programmed desorption (TPD) combined with mass spectrometry. Target clusters were prepared in the gas phase via the laser ablation of a cerium oxide rod in the presence of oxygen, which was diluted using helium as a carrier gas. NO adsorbed onto stoichiometric and oxygen-rich clusters of CenO2n+x(+) (x = 0-2), forming CenO2n+x(NO)(+) (x = 0-2) species. Gas-phase TPD was measured for the NO-adsorbed clusters, revealing that CenO2n(NO)(+) released NO2 at 600-900 K, forming CenO2n-1(+). Therefore, the overall reaction was the oxidation of NO by the CenO2n(+) clusters, which was explained in terms of a Langmuir-Hinshelwood type reaction. An activation barrier existed between the initial complex (CenO2n(NO)(+)) and the final oxidation products (CenO2n-1(+) + NO2). To determine the nature of the intermediates and the activation barrier, TPD was also performed on CenO2n-1(NO2)(+), which had been prepared through the adsorption of NO2 on CenO2n-1(+) for comparison. The activation barrier was associated with the release of NO2 from the intermediate complex (CenO2n-1(+)-NO2 → CenO2n-1(+) + NO2) rather than the structural rearrangement that formed NO2 in the other intermediate complex (CenO2n(+)-NO → CenO2n-1(+)-NO2).

12.
J Phys Chem A ; 119(37): 9573-80, 2015 Sep 17.
Artigo em Inglês | MEDLINE | ID: mdl-26317244

RESUMO

Adsorption of NO molecules on gas phase cobalt cluster ions, Con(+) (n = 4-9), was investigated in thermal equilibrium with He gas at 300 K. The Con(+) clusters, contrary to the isolated clusters in a vacuum, adsorbed NO without undergoing significant dissociation. Thermal desorption spectroscopy of Con(+)(NO)m indicated that Con(+) clusters with n = 4-6 and n = 7-9 can have four and six adatoms chemisorbed, respectively. Reduction of NO occurred, releasing N2 molecules, to form Con(+)Ok(NO)m-k (k = 2, 4, ...). The reaction mechanism involved the exchange of chemisorbed N atoms with the O atom in NO bound to the clusters. The reactivity of Con(+) (n = 4-9) exhibited periodic n dependence, and Co6(+) and Co9(+) was similar to the case of the isolated Co16(+) clusters holding up to eight adatoms reported by Anderson et al. ( J. Chem. Phys . 2009 , 130 , 10992 - 11000 ).

13.
J Phys Chem A ; 119(29): 8055-61, 2015 Jul 23.
Artigo em Inglês | MEDLINE | ID: mdl-26125658

RESUMO

Palladium oxide cluster ions, PdnOm(+), were prepared in the gas phase using laser ablation of a palladium rod in the presence of oxygen. The cluster ions were heated to 1000 K downstream from the cluster source (post heating), and the abundance of PdnOm(+) (n = 2-7) was examined using mass spectrometry. Temperature-programmed desorption experiments revealed that an oxygen molecule is released from oxygen-rich PdnOm(+), forming oxygen-deficient PdnOm-2(+). It was found that Pd6O4(+) was thermally stable up to 1000 K. The activation energy for oxygen molecule desorption has been obtained and compared with previous results by Lang et al.

14.
J Phys Chem A ; 119(31): 8461-8, 2015 Aug 06.
Artigo em Inglês | MEDLINE | ID: mdl-26158761

RESUMO

Cationic rhodium clusters, Rh(n)(+) (n = 4-8), were prepared in the gas phase by the laser ablation of a Rh rod. The Rh(n)(+) clusters were introduced into a reaction gas cell filled with nitric oxide (NO) diluted with He, where they were subjected to collisions with NO and He in a thermal equilibrium at 300 K. The NO molecules were found to adsorb sequentially on the Rh(n)(+) clusters forming Rh(n)(+)(NO)m. To examine the adsorption form and the reaction of NO, we heated Rh(n)(+)(NO)m in an extension tube located after the reaction gas cell and the thermal response of the clusters, desorption of the fragments, was recorded as a function of temperature (300-1000 K). The desorption of NO molecules was predominantly observed below 500 K, giving either Rh(n)(+)(NO)n+1 or Rh(n)(+)(NO)n+2, which indicates that there were NO molecules loosely adsorbed on the Rhn(+) clusters. Further desorption was found to proceed at higher temperatures (500-1000 K), whereby NO was released from the smaller clusters, Rh(n)(+) (n ≤ 5). In contrast, for the larger clusters (n ≥ 6), N2 release was clearly observed at high temperatures (>800 K). Thus, the reduction of NO occurred for larger clusters at higher temperatures.

15.
J Phys Chem A ; 119(31): 8433-42, 2015 Aug 06.
Artigo em Inglês | MEDLINE | ID: mdl-26153899

RESUMO

Temperature-programmed desorption (TPD) experiments were performed on gas-phase manganese oxide cluster ions, namely, Mn(n)O(m)(+) (n = 3-20) and Mn(n)O(m)(-) (n = 3-18). These cluster ions were prepared by laser ablation of a manganese rod in the presence of oxygen gas, and their composition was investigated using mass spectrometry. The composition of Mn(n)O(m)(±) distribution lies above the m = (4/3)n line. When the cluster ions were heated to 1000 K, Mn(n)O(m)(+) (m = (4/3)n + δ, with δ = -1, 0) and Mn(n)O(m)(-) (m = (4/3)n + δ, with δ = 0, 1) was found to be the predominant species, formed by thermal dissociation. These experimental findings indicate that the nascent manganese oxide clusters comprise robust Mn(n)O(m)(±) (m/n ≈ 4/3) and weakly bound excess oxygen atoms. On the basis of the TPD experiments, the oxygen-molecule release was identified as the main dissociation channel. The temperature dependence of O2 desorption was found to be similar among the clusters with the same oxygen excess or deficiency regardless of the number of Mn atoms. The threshold energy of O2 desorption was estimated for Mn4O(m)(+) (m = 6-11) and compared with bond dissociation energies calculated by density functional theory.

16.
J Phys Chem A ; 119(26): 6766-72, 2015 Jul 02.
Artigo em Inglês | MEDLINE | ID: mdl-26043808

RESUMO

Adsorption and desorption of hydrogen by gas-phase Pd clusters, Pdn(+), were investigated by thermal desorption spectroscopy (TDS) experiments and density functional theory (DFT) calculations. The desorption processes were examined by heating the clusters that had adsorbed hydrogen at room temperature. The clusters remaining after heating were monitored by mass spectrometry as a function of temperature up to 1000 K, and the temperature-programmed desorption (TPD) curve was obtained for each Pdn(+). It was found that hydrogen molecules were released from the clusters into the gas phase with increasing temperature until bare Pdn(+) was formed. The threshold energy for desorption, estimated from the TPD curve, was compared to the desorption energy calculated by using DFT, indicating that smaller Pdn(+) clusters (n ≤ 6) tended to have weakly adsorbed hydrogen molecules, whereas larger Pdn(+) clusters (n ≥ 7) had dissociatively adsorbed hydrogen atoms on the surface. Highly likely, the nonmetallic nature of the small Pd clusters prevents hydrogen molecule from adsorbing dissociatively on the surface.

17.
J Phys Chem A ; 119(22): 5545-52, 2015 Jun 04.
Artigo em Inglês | MEDLINE | ID: mdl-25965076

RESUMO

Oxygen deficient cerium oxide cluster ions, Ce(n)O(m)(+) (n = 2-10, m = 1-2n) were prepared in the gas phase by laser ablation of a cerium oxide rod. The reactivity of the cluster ions was investigated using mass spectrometry, finding that oxygen deficient clusters are able to extract oxygen atoms from CO, CO2, NO, N2O, and O2 in the gas phase. The oxygen transfer reaction is explained in terms of the energy balance between the bond dissociation energy of an oxygen containing molecule and the oxygen affinity of the oxygen-deficient cerium oxide clusters, which is supported by DFT calculations. The reverse reaction, i.e., formation of the oxygen deficient cluster ions from the stoichiometric ones was also examined. It was found that intensive heating of the stoichiometric clusters results in formation of oxygen deficient clusters via Ce(n)O(2n)(+) → Ce(n)O(2n-2)(+) + O2, which was found to occur at different temperatures depending on cluster size, n.

18.
J Phys Chem A ; 119(10): 1813-9, 2015 Mar 12.
Artigo em Inglês | MEDLINE | ID: mdl-25651032

RESUMO

Cerium oxide cluster ions, Ce(n)O(2n+x)(+) (n = 2-9, x = -1 to +2), were prepared in the gas phase by laser ablation of a cerium oxide rod in the presence of oxygen diluted in He as the carrier gas. The stable stoichiometry of the cluster ions was investigated using a mass spectrometer in combination with a newly developed post heating device. The oxygen-rich clusters, Ce(n)O(2n+x)(+) (x = 1, 2), were found to release oxygen molecules, and Ce(n)O(2n+x)(+) (x = -1, 0) were exclusively formed by post heating treatment at 573 K. The Ce(n)O(2n-1)(+) and Ce(n)O(2n)(+) clusters were found to be thermally stable, and the oxygen-rich clusters consisted of robust Ce(n)O(2n-1)(+) and Ce(n)O(2n)(+) and weakly bound oxygen atoms. Evaluation of the reactivity of Ce(n)O(2n+x)(+) with CO molecules demonstrated that Ce(n)O(2n)(+) oxidized CO to form Ce(n)O(2n-1)(+) and CO2, and the rate constants of the reaction were in the range of 10(-12)-10(-16) cm(3) s(-1). The CO oxidation reaction was distinct for n = 5, which occurred in parallel with the CO attachment reaction.

19.
J Phys Chem A ; 118(37): 8298-308, 2014 Sep 18.
Artigo em Inglês | MEDLINE | ID: mdl-24725191

RESUMO

Organoeuropium sandwich clusters, comprising europium (Eu) and 1,3,5,7-cyclooctatetraene (COT) (Eu(n)(COT)(m)), were produced in the gas phase using a laser vaporization synthesis method. Photoionization mass spectra revealed an exclusive Eu(n)(COT)(m) formation with three compositions: m = n + 1, m = n, and m = n - 1, which, we propose, correspond to full-sandwich, half-sandwich, and inverted-sandwich structures, respectively. The charge distributions, metal-ligand bonding characteristics, and electronic structures of the clusters were comprehensively investigated by photoionization measurements of Eu(n)(COT)(m) neutrals and by photoelectron spectroscopy of Eu(n)(COT)(m)(-) and isoelectronic Ba(n)(COT)(m)(-) anions. The results confirmed that (1) highly ionic metal-ligand bonding is formed between Eu(2+) and COT(2-) within the sandwich structure (at the termini, ionic forms are Eu(+) and COT(-)) and (2) size dependence of orbital energy can be explained by the Coulombic interaction of simple point charge models between the detaching electrons and dipoles/quadrupoles. When the terminus of the sandwich clusters is Eu(2+), COT(2-), or Eu(0), the orbital energy of the electron detachment channel at the opposite terminus strongly depends on the cluster size. In this case, the molecular stack behaves as a one-dimensionally aligned dipole; otherwise, it behaves as a quadrupole, and the relationship between cluster size and electron detachment energy is much weaker. The study also reports on the 4f orbital energy in Eu ions and the formation mechanism of organoeuropium sandwich nanowires up to 12 nm in length. The nanowires are formed by successive charge transfer at the terminal part, Eu(+) and COT(-), which reduces the ionization energy and increases the electron affinity, respectively.

20.
J Phys Chem A ; 117(47): 12175-83, 2013 Nov 27.
Artigo em Inglês | MEDLINE | ID: mdl-24156250

RESUMO

The reduction of N2O in the gas phase by isolated, neutral platinum clusters, Pt(n) (n = 4-12), was investigated using mass spectrometry. The associated oxygen transfer reactions had the general formula Pt(n)O(m-1) + N2O → Pt(n)O(m) + N2 (m = 1 or 2). The rate constants k1 and k2 for the reactions in which m = 1 and 2, respectively, were ascertained and were found to be similar to one another. Unexpectedly, Pt6O was discovered to be completely unreactive with N2O under the applied experimental conditions. The reaction mechanism was elucidated on the basis of density functional theory (DFT) calculations, which indicated a reaction barrier between Pt6O + N2O and Pt6O2 + N2. The possibility of catalyzing either the reduction of N2O or the oxidation of CO using neutral Pt(n) species was also examined and the results showed that Pt(n) does not exhibit significant catalytic properties and that O and CO instead coadsorb to Pt(n). Desorption of CO2 from the coadsorbed clusters was not clearly identifiable from mass spectra. The reactivities of the platinum clusters were discussed and compared with the properties of the highly catalytically active rhodium clusters.

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