A new method for electrocrystallization of AgTCNQF4 and Ag2TCNQF4 (TCNQF4 = 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) in acetonitrile

Semiconducting AgTCNQF₄ (TCNQF₄ = 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) has been electrocrystallized from an acetonitrile (0.1 M Bu₄NPF₆) solution containing TCNQF₄ and Ag(MeCN) ⁺₄ . Reduction of TCNQF₄ to the TCNQF ¹⁻₄ anion, followed by reaction with Ag(MeCN) ⁺₄ forms crystalline AgTCNQF₄ on the electrode surface. Electrochemical synthesis is simplified by the reduction of TCNQF₄ prior to Ag(MeCN) ⁺₄ compared with the analogous reaction of the parent TCNQ to form AgTCNQ, where these two processes are coincident. Cyclic voltammetry and surface plasmon resonance studies reveal that the electrocrystallization process is slow on the voltammetric time scale (scan rate = 20 mV s⁻¹) for AgTCNQF₄, as it requires its solubility product to be exceeded. The solubility of AgTCNQF₄ is higher in the presence of 0.1 M Bu₄NPF₆ supporting electrolyte than in pure solvent. Cyclic voltammetry illustrates a dependence of the reduction peak potential of Ag(MeCN) ⁺₄ to metallic Ag on the electrode material with the ease of reduction following the order Au < Pt < GC < ITO. Ultraviolet-visible, Fourier transform infrared, and Raman spectra confirmed the formation of reduced TCNQF ¹⁻₄ and optical microscopy showed needle-shaped morphology for the electrocrystallized AgTCNQF₄. AgTCNQF₄ also can be formed by solid–solid transformation at a TCNQF₄-modified electrode in contact with aqueous media containing Ag⁺ ions. Chemically and electrochemically synthesized AgTCNQF₄ are spectroscopically identical. Electrocrystallization of Ag₂TCNQF₄ was also investigated; however, this was found to be thermodynamically unstable and readily decomposed to form AgTCNQF₄ and metallic Ag, as does chemically synthesized Ag₂TCNQF₄.

     

Shell correction study of fission of doubly charged silver clusters

Fission of doubly charged silver clusters is investigated by the method of shell corrections. The following fission events are considered: Ag ₂₂ ²⁺ → Ag _n ⁺ + Ag _{22 ?n} ⁺ , (n=11, 10, 9, 8); Ag ₂₁ ²⁺ → Ag _n ⁺ + Ag _{21 ?n} ⁺ , (n=10, 9, 8, 7); Ag ₁₈ ²⁺ → Ag _n ⁺ + Ag _{18 ?n} ⁺ , (n=9, 8, 7, 6). It is found that the shell correction energy is comparable to or larger than the deformation energy of the liquid drop. Threshold energies for the fission events are calculated and compared with the experimental abundance spectra obtained by Katakuse et al. (1990). Correspondence between the calculated threshold energies with the shell corrections and the experimental abundance is very good, showing products from lower threshold fission channels yield more abundance. The threshold energies without the shell corrections are almost constant irrespective of the fission channels and cannot explain the experimental abundance. Abundance of some products are too small to be accounted for only by the threshold energies. The low abundance of those products may be explained by the presence of competing fission channels that have similar minimal energy paths. It is found in fission of Ag ₁₈ ²⁺ that the shell correction overwhelms the Coulomb energy and the fission channel to Ag₈ + Ag ₁₀ ²⁺ is preferred over the fission channel to Ag ₈ ⁺ + Ag ₁₀ ⁺ .     

Formation mechanism of nanostructured Ag films from Ag2O particles using a sonoprocess

Nanostructured Ag films composed of nanoparticles and nanorods can be formed by the ultrasonication of ethanol solutions containing Ag₂O particles. The present work examined the formation process of these films from ethanol solutions by two different agitation methods, including ultrasonication and mechanical stirring. The mass-transfer process from Ag₂O particles to ethanol solvent is accelerated by the mechanical effects of ultrasound. Ag⁺ ions and intermediately reduced Ag clusters were released into the ethanol. These Ag⁺ ions and Ag clusters provide absorption bands at 210, 275 and 300 nm in UV-vis spectra. These bands were assigned to the absorption of Ag⁺, Ag ₄ ²⁺ and Ag_n (n?≈?3). The Ag_n clusters that readily grow to become Ag nanoparticles were formed due to the surface reaction of Ag₂O particles with ethanol under ultrasonication. The reactions of Ag⁺ ions in ethanol to form Ag nanomaterials (through the formation of Ag ₄ ²⁺ clusters) were also accelerated by ultrasonication.     

Low-energy decay pathways of doubly charged charged silver clusters Ag n 2+ n = 9–24)

The low-energy dissociation channels of mass selected silver cluster ions Ag _n ²⁺ (n = 9–24) are determined by collision induced dissociation (CID) in a Penning trap. While all clusters of the size n ≥ 17 evaporate neutral monomers, most smaller clusters undergo asymmetric fission of the form Ag _n ²⁺ → Ag _n?3 ⁺ + Ag _{3} ⁺ . However, Ag ₁₅ ²⁺ and Ag ₁₁ ²⁺ emit monomers which indicates shell or odd-even effects. The observed fragmentation pathways are different from previous reports of measurements with sputtered Ag _n ²⁺ .     

Reactivity of positively charged cobalt cluster ions with CH4, N2, H2, C2H4, and C2H2

Reactivity of positively charged cobalt cluster ions (Co _n ⁺ ,n=2?22), produce by laser vaporization, with various gas samples (CH₄, N₂, H₂, C₂H₄, and C₂H₂) were systematically investigated by using a fast-flow reactor. The reactivity of Co _n ⁺ with the various gas samples is qualitatively consistent with the adsorption rate of the gas to cobalt metal surfaces. Co _n ⁺ highly reacts with C₂H₂ as characterized by the adsorption rate to metal surfaces, and it indicates no size dependence. In contrast, the reactions of Co _n ⁺ with the other gas samples indicate a similar cluster size dependence; atn=4, 5, and 10?15, Co _n ⁺ highly reacts. The difference can be explained by the amount of the activation energy for chemisorption reaction. Compared with neutral cobalt clusters, the size dependence is almost similar except for Co ₄ ⁺ and Co ₅ ⁺ . The reactivity enhancement of Co ₄ ⁺ and Co ₅ ⁺ indicates that the cobalt cluster ions are presumed to have an active site for chemisorption atn=4 and 5, induced by the influence of positive charge.     

Mechanism of silver nucleation upon the radiation-induced reduction of its ions in polyphosphate-containing aqueous solutions

Long-lived (hours to days) silver clusters, Ag ₄ ²⁺ , Ag ₄ ⁺ , Ag ₈ ²⁺ , etc., are formed upon the radiation-induced reduction of Ag⁺ ions in aqueous solutions containing sodium polyphosphate. The efficiency of the cluster formation decreases and the stability of the clusters increase with a rise in the concentration of the polymeric stabilizer. In the course of the aggregation of clusters, their sizes increase, quasi-metallic particles emerge, and the process terminates with the formation of silver nanoparticles. The mechanism of silver nucleation upon the radiation-induced reduction of silver ions in aqueous solutions is discussed.     

Gemischt quantenmechanisch-statistische Modellrechnungen nach der Einzentrenmethode an Wasserstoffverbindungen tetraedrischer Symmetrie von Elementen der III., IV. und V. Hauptgruppe

Using a one-center-method, treating the inner shells statistically, the valence-shell, however, by quantum mechanics, the equilibrium internuclear distances and total molecular energies have been computed for CH₄, SiH₄, GeH₄, SnH₄, PbH₄, BH ₄ ^? , AlH ₄ ^? , GaH ₄ ^? , InH ₄ ^? , TlH ₄ ^? , NH ₄ ⁺ , PH ₄ ⁺ , AsH ₄ ⁺ , SbH ₄ ⁺ , and BiH ₄ ⁺ . The results are in good agreement with experimental data as well as with theoretical values.     

Stripping voltammetric determination of azide ions

A procedure is proposed for the stripping voltammetric determination of N ₃ ^? ions at a mercury film electrode. It is based on the reduction of the mercury azide formed upon the oxidation of mercury in the presence of N ₃ ^? at ?0.02 to ?0.04V (in reference to an Ag/AgCl electrode) in a 0.1 M Na₂SO₄ supporting electrolyte solution. A linear dependence of the cathodic current peak on the N ₃ ^? concentration is observed in the concentration range from 4 × 10^?9 to 1 × 10^?3 M.     

Absolute cross sections for electron impact ionization of Ag2

The previously measured relative cross section function for electron impact ionization (EII) of neutral Ag₂ has now been calibrated quantitatively by combining the electron impact ionization with in situ non resonant two photon ionization (NR2PI). By comparing the NR2PI saturation intensities measured for Ag ₂ ⁺ and Ag⁺ with the corresponding EII intensities, the ratio between the electron impact ionization cross sections (EIICS) of neutral Ag₂ and Ag was determined to be σ_Ag2/σ_Ag=1.53 for an electron energy of 46 eV. This result agrees well with the geometricn ^2/3-rule \((\sigma X_n \sim n^{2/3} )\) commonly proposed for the dependence of the EIICS of clustersX _n on the cluster sizen.     

Nucleophilic assistance in methane activation by superelectrophiles with halogen-centered cationic sites

Simulation of fragments of potential energy surface for systems CH₄ + CBr ₃ ⁺ , CH₄ + CBr ₃ ⁺ AlBr ₄ ^? , CH₄ + CCl ₃ ⁺ AlCl ₄ ^? , and CH₄ + CCl ₃ ⁺ Al₂Cl ₇ ^? was performed by DFT-B3LYP and DFT-PBE methods. The important role of nucleophilic assistance in methane halogenation by these superelectrophiles was confirmed. These reactions occur with a synchronous hydride transfer from methane to the electrophile within the cyclic transition states in linear C-H-C fragment of the rings and a generation of a C-Hlg bond between the carbon atom of the arising methyl group and the halogen atom of the electrophile. The nucleophilic assistance from the unshared electron pair of this halogen atom provides the lowering of the potential barriers to methane halogenation by complexes CBr ₃ ⁺ AlBr ₄ ^? , CCl ₃ ⁺ AlCl ₄ ^? , and CCl ₃ ⁺ Al₂Cl ₇ ^? to the values of the order of 20 kcal mol^?1. These essential features of the mechanism of methane halogenation are independent of the halogen nature and are retained on going from the model electrophiles to the real ones.     

Ab initio study of small He cluster ions He n + ,n=2, 3, 4, 5, and low-lying Rydberg states of He4

SCF and CEPA calculations are applied to study the structure of small He cluster ions, He _n ⁺ ,n=2, 3, 4, 5 and some low-lying Rydberg states of He₄. The effect of electron correlation upon the equilibrium structures and binding energies is discussed. He ₃ ⁺ has a linear symmetric equilibrium geometry with a bond length of 2.35a ₀ and a binding energyD _e =0.165 eV with respect to He ₂ ⁺ +He (experimentally:D ₀=0.17 eV which corresponds toD _e ≈0.20 eV). He ₄ ⁺ is a very floppy molecular ion with several energetically very similar geometrical configurations. Our CEPA calculations yield a T-shaped form with a He ₃ ⁺ centre (R _e = 2.35a ₀) and one inductively bound He atom (4.39a ₀ from the central He atom of He ₃ ⁺ ) as equilibrium structure. Its binding energy with respect to He ₃ ⁺ +He is 0.031 eV. A linear symmetric configuration consisting of a He ₂ ⁺ centre with a bond length of 2.10a ₀ and two inductively bound He atoms (4.20a ₀ from the centre of He ₂ ⁺ ) is only 0.02–0.03 eV higher in energy. We expect that in larger He cluster ions structures with He ₂ ⁺ and He ₃ ⁺ centres andn?2 orn?3 inductively bound He atoms have nearly the same energies. In He₄ a low-lying metastable Rydberg state (³ Π symmetry for linear He ₄ ^* ,³ B ₁ for the T-shaped form) exists which is slightly stronger bound with respect to He ₃ ^* +He than the corresponding ion.     

The thermochemical characteristics of the solvation of amines and silver ions in methanol-dimethylformamide mixtures

The heat effects of solution of ethylenediamine, sodium perchlorate, and silver perchlorate in the nonaqueous methanol-dimethylformamide binary solvent were determined calorimetrically at 298.15 K. The suggestion of equal enthalpies of solvation of Ph₄P⁺ and PhB^? was used to calculate the enthalpies of transfer of the ClO ₄ ^? and Ag⁺ ions from methanol to dimethylformamide. The influence of the composition of methanol-dimethylformamide solvents on the energy characteristics of solvation of ethylenediamine, Ag⁺, and ClO ₄ ^? was considered.     

Ab initio calculation of the vertical excitation energies of small helium cluster ions

Quantum chemical ab initio calculations have been performed for the vertical excitation energies and oscillator strengths of all low-lying electronically excited states of small helium cluster ions, He _n ⁺ ,n=2, ..., 7. The geometrical structures of the ions were fixed at the equilibrium geometries of the respective ground states, for He ₄ ⁺ and He ₅ ⁺ also one alternative structure was considered. The low-lying excited states can be classified into two categories: the electronic transition can occur either within the central He ₂ ⁺ or He ₃ ⁺ unit or from the peripheral weakly bound He atoms to this unit. The latter transitions are very weak (f≈0.001), closely spaced, with vertical excitation energies of about 5.7 eV. The He ₂ ⁺ and He ₃ ⁺ units have strong transitions at 9.93 and 5.55 eV, respectively; these transitions are only slightly blue-shifted if He ₂ ⁺ or He ₃ ⁺ are placed as “chromophores” into the centre of a larger He _n ⁺ cluster. The large difference in the vertical excitation energy of the strong transition should enable an experimental decision of the question whether the cluster ions have He ₂ ⁺ or He ₃ ⁺ cores.     

Guided ion beam studies of the reactions of O+, O 2 + , N+ and N 2 + with silane

Guided ion beam mass spectrometry is used to measure the cross sections as a function of kinetic energy for reaction of SiH₄ with O⁺(⁴S), O ₂ ⁺ (²Π_g,v=0), N⁺(³P), and N ₂ ⁺ (²Σ _g ⁺ ,v=0). All four ions react with silane by dissociative charge-transfer to form SiH _m ⁺ (m=0?3), and all but N ₂ ⁺ also form SiXH _m ⁺ products where (m=0?3) andX=O, O₂ or N. The overall reactivity of the O⁺, O ₂ ⁺ , and N⁺ systems show little dependence on kinetic energy, but for the case of N ₂ ⁺ , the reaction probability and product distribution relies heavily on the kinetic energy of the system. The present results are compared with those previously reported for reactions of the rare gas ions with silane [13] and are discussed in terms of vertical ionization from the 1t ₂ and 3a ₁ bands of SiH₄. Thermal reaction rates are also provided and dicussed.     

Spontaneous disproportionation of NO2 (N2o4) by aromatic hydrocarbons. Formation of nitrosonium EDA complexes as chemical intermediates

Strong (orange/red) colourations resulting immediately upon the exposure of nitrogen dioxide and its equilibrium dimer (dinitrogen tetroxide) to various aromatic hydrocarbons (ArH) are shown to arise from the nitrosonium EDA or electron donor-acceptor complexes [ArH, NO⁺NO ₃ ^? ]. The latter exhibit diagnostic charge-transfer absorptions and characteristic N-O stretching bands in the UV-vis and IR spectra, respectively, that relate directly to ArH/NO⁺ interactions extant in the EDA complexes previously derived from the authentic nitrosonium salt, NO⁺PF ₆ ^- . Time-resolved picosecond spectroscopy establishes the charge-transfer excited state of [ArH, NO⁺NO ₃ ^? ] to be essentially identical to that from [ArH, NO⁺BF ₄ ^? ]. Furthermore, the same temporal decay of the spectral transients (ArH^+?) from both systems indicates minimal ion-pairing effects of the counterions (NO ₃ ^? and BF ₄ ^? ) on the kinetics of back electron transfer.     

Photoelectron spectroscopy of transition metal-sulfur cluster anions

Metal (M)-sulfur cluster anions (M = Ag, Fe and Mn) have been studied using photoelectron spectroscopy (PES) with a magnetic-bottle type time-of-flight electron spectrometer. The M_nS _m ^? cluster anions were formed in a laser vaporization cluster source. For Ag-S, the largest coordination number of Ag atoms (n _max) is generally expressed as n _max =2m ? 1 in each series of the number of S atoms (m). For Fe?S and Mn?S, it was found that the stable cluster ions are the ones with compositions of n=m and n=m±1. Their electron affinities were measured from the onset of the PES spectrum. For Ag?S, the EAs of Ag₁S_m are small and around 1 eV, whereas those of Ag_nS_m (_n ≥ 2) become large above 2 eV. The features in the mass distribution and PES suggest that Ag₂S unit is preferentially formed with increasing the number of Ag atoms. For Fe?S and Mn?S, the PES spectra of Fe_nS _m ^? /Mn_nS _m ^? show a unique similarity at n ≥ m, indicating that the Fe/Mn atom addition to Fe_nS _n ^? /Mn_nS _n ^? has little effect on the electronic property of Fe_nS_n/Mn_nS_n. The PES spectra imply that the Fe_nS_n cluster is the structural framework of these clusters, as similarly as the determined structure of the Fe_nS_n cluster in nitrogenase enzyme.     

Novel metal cluster complexes synthesized by matrix deposition of mass-selected clusters

Metal complexes produced by depositing size selected Fe and Ag cluster cations in N₂ and O₂ matrices respectively are studied by infrared spectroscopy. Unknown species such as Fe(N₂)x, Fe₃ (N₂)x and Ag₃(O₂)x are observed. The IR spectra of Ag⁺, Ag ₂ ⁺ and Ag ₉ ⁺ in excess O₂ indicate that no complexes involving molecular oxygen are formed. However, the strong silver cluster UV-visible absorptions detected in Ar matrices disappear in the oxygen matrices, suggesting that silver-oxygen complexes are formed with dissociated oxygen.     

Octacoordination of the nitrogen atom in M4NO 4 + systems (M = Li, Na, K)

The electronic and geometric structures of M₄NO ₄ ⁺ compounds (M = Li, Na, K) in classical and nonclassical isomeric forms were studied by the ab initio (MP2(full)/6-311+G**) and density functional theory (B3LYP/6-311+G**) methods. For all M atoms, structurally stable nonclassical isomers were found with octacoordinated nitrogen atoms and tetracoordinated oxygen atoms. For Li₄NO ₄ ⁺ , the classical structure with the tetracoordinated nitrogen atom is energetically more stable, whereas nonclassical structures with the octacoordinated nitrogen center are more stable for Na₄NO ₄ ⁺ and K₄NO ₄ ⁺ .     

Analysis of optical absorption spectra of transition metal cluster ions by the spin-polarized DV-Xα method

Optical absorption spectra of cobalt cluster ions, Co _n ⁺ , and vanadium cluster ions, V _n ⁺ , were analyzed by a theoretical calculation based on the spin-polarized DV-Xα method, and their electronic and geometric structures were obtained. Relative absorption cross section associated with each electronic transition was calculated; the calculation enables a qualitative comparison of calculated spectrum with a measured one not only in its transition energy but also in its intensity profile. This analysis shows that Co ₄ ⁺ , Co ₃ ⁺ , and V ₄ ⁺ have, respectively, a tetrahedral structure with a bond distance of 2.00Å, an equilateral triangle with a bond distance of 2.30Å, and a distorted tetrahedral structure with five bonds having a distance of 2.34 Å and one of 2.89Å. The differences in the population between majority and minority spins (spin-difference) evaluated from the electronic structure thus obtained were 2.0, 1.7, and zero per atom in Co ₃ ⁺ , Co ₄ ⁺ , and V ₄ ⁺ , respectively. These spin differences indicate a ferromagnetic and an antiferromagnetic spin-coupling in the cobalt and vanadium cluster ions, respectively.