Photodissociation of hydrogen halide molecules on free ice nanoparticles

Photodissociation of water clusters doped with HX(X=Br,Cl), molecules has been studied in a molecular beam experiment. The HX(H2O)n clusters are dissociated with 193 nm laser pulses, and the H fragments are ionized at 243.07 nm and their time-of-flight distributions are measured. Experiments with deuterated species DBr(H2O)n and HBr(D2O)n suggest that the photodissociation signal originates from the presence of the HX molecule on the water cluster, but does not come directly from a photolysis of the HX molecule. The H fragment is proposed to originate from the hydronium molecule H3O. Possible mechanisms of the H3O production are discussed. Experimental evidence suggests that acidic dissociation takes place in the cluster, but the H3O+ ion remains rather immobile.     

Studies on the absorption spectra of aqueous l-tyrosine and its dissociation equilibria

The UV absorption spectra of aqueous tyrosine at various total ionic strengths were measured at known pH values at 298.2+/-0.2 K. The standard third dissociation constant of tyrosine was determined from the spectra by linear extrapolation and polynomial approximation. The results obtained from both methods are in good agreement within experimental error.     

Assignment of charge transfer absorption band in optical absorption spectra of the adsorbate-silver colloid system

In this paper we reported the UV-visible-NIR optical absorption properties of silver colloid, employed as a high efficient substrate in surface-enhanced Raman spectra (SERS), under various conditions. Experimental results revealed that the new absorption band, usually appearing in the longer wavelength region due to the addition of molecules, was related to the direct adsorption of molecules on colloidal silver surface. When the adsorption occurred, this new band would appear. Once the molecules were desorbed from silver surface, the new band could not be observed. Some evidences inferred that the new absorption band was associated with the effect of charge-transfer transition between adsorbates and colloidal silver particles, while not with the effect of the surface plasma resonance due to the silver particles aggregation which was usually attributed to in previous research work.     

Critical comparison of emission spectroscopic determination of dissociation in hydrogen RF discharges

The degree of dissociation of molecular hydrogen in a RF parallel plate plasma reactor is analyzed by three dif erent emission spectroscopic methods. These schemes are based on the comparison of line intensities of atomic and molecular hydrogen and of argon which is admixed as a tracer gas (actinometry). The different schemes yield similar results in the parameter ranges where they are applicable. The basic assumptions, the available cross sections, and the conditions and limitations of applicability of the different schemes are compared and discussed.     

ESR evidence for halogen oxy-radical formation in the study of the heterogeneous decomposition of gaseous hydrogen peroxide in halides

Above 200°C, the reaction of H₂O₂ on walls coated with halides (e.g. KCl, KBr) leads not only to HO^?₂ from H₂O₂ decomposition but also to the formation of halide dioxides from the coating. The possible influence of halogenated compounds eventually formed during hydrocarbon oxidation carried out in halide-coated vessels is suggested.     

IR absorption and Raman spectra of silicon dioxide nanoparticles in the presence of water: Computer experiment

Interactions of (SiO₂)₅₀ clusters with 10, 20, 30, or 40 water molecules are studied by molecular dynamics method. Flat SiO₂ nanoparticle covered with a water layer is formed after the inclusion of water molecules into the cluster. As a rule, the integral intensity of IR and Raman spectra lowers after the absorption of H₂O molecules by the cluster. The power of IR radiation emitted by the cluster increases nonmonotonically with the addition of water molecules to the cluster. The absorption of water molecules by the cluster leads to a significant increase in the absorption coefficient and only a slight increase in the refractive index. The number of electrons participating in the interaction with electromagnetic radiation increases with the addition of water molecules to the cluster.     

Computer simulation of band shapes in electronic absorption spectra of dimers of organic dyes

The band shapes in the absorption spectra of dimers of cyanine dyes were simulated using a combination of an empirical molecular force field for the ground state with quantum-chemical calculations of the electron excitation energy as a function of normal nuclear coordinates. The shape and the width of an absorption band strongly depend on the mutual arrangement of the monomers. If the monomers are located one directly above the other, the sublevels arising from intramolecular vibrations disappear in the spectrum, and a large hypsochromic shift of the 0-0-transition band is observed, which results mainly from through-space interaction of monomer orbitals. If the monomers are strongly shifted relative to each other, the sublevels mentioned are also absent in the spectrum, but the bathochromic shift of the 0-0-transition band is small and results from interaction of dipole moments of electron transitions. A rather broad region of intermediate structures is found between these dimer forms, where the interaction of dipole moments of electron transitions in monomers is low, and the shapes of absorption bands are similar to those of the monomers.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1705–1710, September, 1995.This work was financially supported by the Russian Foundation for Basic Research (Project No. 94-03-08539) and the International Science Foundation (Grant M1Z 000).     

Photochemistry of hydrogen halides on water clusters: simulations of electronic spectra and photodynamics, and comparison with photodissociation experiments

The photochemistry of small HX·(H(2)O)(n), n = 4 and 5 and X = F, Cl, and Br, clusters has been modeled by means of ab initio-based molecular simulations. The theoretical results were utilized to support our interpretation of photodissociation experiments with hydrogen halides on ice nanoparticles HX·(H(2)O)(n), n ≈ 10(2)-10(3). We have investigated the HX·(H(2)O)(n) photochemistry for three structural types: covalently bound structures (CBS) and acidically dissociated structures in a form of contact ion pair (CIP) and solvent separated pair (SSP). For all structures, we have modeled the electronic absorption spectra using the reflection principle combined with a path integral molecular dynamics (PIMD) estimate of the ground state density. In addition, we have investigated the solvent effect of water on the absorption spectra within the nonequilibrium polarizable continuum model (PCM) scheme. The major conclusion from these calculations is that the spectra for ionic structures CIP and SSP are significantly red-shifted with respect to the spectra of CBS structures. We have also studied the photodynamics of HX·(H(2)O)(n) clusters using the Full Multiple Spawning method. In the CBS structures, the excitation led to almost immediate release of the hydrogen atom with high kinetic energy. The light absorption in ionically dissociated species generates the hydronium radical (H(3)O) and halogen radical (X) within a charge-transfer-to-solvent (CTTS) excitation process. The hydronium radical ultimately decays into a water molecule and hydrogen atom with a characteristic kinetic energy irrespective of the hydrogen halide. We have also investigated the dynamics of an isolated and water-solvated H(3)O radical that we view as a central species in water radiation chemistry. The theoretical findings support the following picture of the HX photochemistry on ice nanoparticles investigated in our molecular beam experiments: HX is acidically dissociated in the ground state on ice nanoparticles, generating the CIP structure, which is then excited by the UV laser light into the CTTS states, followed by the H(3)O radical formation.     

Size control of semimetal bismuth nanoparticles and the UV-visible and IR absorption spectra

We introduced a simple chemical method to synthesize semimetal bismuth nanoparticles in N,N-dimethylformamide (DMF) by reducing Bi(3+) with sodium borohydride (NaBH(4)) in the presence of poly(vinylpyrroldone) (PVP) at room temperature. The size and dispersibility of Bi nanoparticles can be easily controlled by changing the synthetic conditions such as the molar ratio of PVP to BiCl(3) and the concentration of BiCl(3). The UV-visible absorption spectra of Bi nanoparticles of different diameters are systematically studied. The surface plasmon peaks broaden with the increasing molar ratio of PVP to BiCl(3) as the size of bismuth nanoparticles decreases. Infrared (IR) spectra of the complexes with different molar ratios of PVP/BiCl(3) show a strong interaction between the carboxyl oxygen (C=O) of PVP and Bi(3+) ion and a weak interaction between the carboxyl oxygen (C=O) of PVP and the Bi atom in nanoparticles. This indicates that PVP serves as an effective capping ligand, which prevents the nanoparticles from aggregation.     

Probing the active sites for CO dissociation on ruthenium nanoparticles

The active sites for CO dissociation were probed on mass-selected Ru nanoparticles on a HOPG support by temperature programmed desorption spectroscopy using isotopically labelled CO. Combined with transmission electron microscopy we gain insight on how the size and morphology of the nanoparticles affect the CO dissociation activity. The Ru nanoparticles were synthesized in a UHV chamber by gas-aggregation magnetron sputtering in the size range from 3 to 15 nm and the morphology was investigated in situ by scanning tunneling microscopy and ex situ by high resolution transmission electron microscopy. Surprisingly, it was found that larger particles were more active per surface area for CO dissociation. It is suggested that this is due to larger particles exposing a more rough surface than the smaller particles, giving rise to a higher relative amount of under-coordinated adsorption sites on the larger particles. The induced surface roughness is proposed to be a consequence of the growth processes in the gas-aggregation chamber.     

Quantum chemical investigation of the structures of ionic liquids based on 1-ethyl-3-methylimidazolium halides: IR spectra and hydrogen bonds

Hydrogen bonds, structures, and vibrational spectra of 1-ethyl-3-methylimidazolium halides ([emim][Hal]) were analyzed in the framework of the Hartree—Fock method (HF/6-31G*, 6-31+G*) and the density functional theory (B3LYP/6-31G*, 6-31+G*, 6-31G**, 6-31+G**). It is shown that the use of the approximation of an isolated ion pair cation—anion is incorrect for the ionic liquids studied. It is more reasonable to consider “clusters” including at least two-three cations and being more appropriate models for the study of hydrogen bonds and vibrational spectra of ionic liquids of this type.     

Effect of hydrogen gas impurities on the hydrogen dissociation on iron surface

A small addition of oxygen to hydrogen gas is known to mitigate the hydrogen embrittlement (HE) of steels. As atomic hydrogen dissolution in steels is responsible for embrittlement, catalysis of molecular hydrogen dissociation by the steel surface is an essential step in the embrittlement process. The most probable role of oxygen in mitigating HE is to inhibit the reactions between molecular hydrogen and the steel surface. To elucidate the mechanism of such surface reaction of hydrogen with the steel in the presence of oxygen, hydrogen, and oxygen adsorption, dissociation, and coadsorption on the Fe(100) surface were investigated using density functional theory. The results show that traces of O₂ would successfully compete with H₂ for surface adsorption sites due to the grater attractive force acting on the O₂ molecule compared to H₂. The H₂ dissociation would be hindered on iron surfaces with predissociated oxygen. Prompted by the notable results for H₂ + O₂, other practical systems were considered, that is, H₂ + CO and CH₄. Calculations were performed for the CO chemisorption and H₂ dissociation on iron surface with predissociated CO, as well as, CH₄ surface dissociation. The results indicate that CO inhibition of H₂ dissociation proceeds via similar mechanism to O₂ induced inhibition, whereas CH₄ traces in the H₂ gas have no effect on H₂ dissociation. © 2014 Wiley Periodicals, Inc.     

Addition of hydrogen halides to alkylidenecyclopropanes: a highly efficient and stereoselective method for the preparation of homoallylic halides

The reaction of alkylidenecyclopropanes with HCl or with HBr proceeds very smoothly at 120°C to produce the corresponding homoallylic halides stereoselectively in good to excellent yields. For example, the reaction of (1-phenylbenzylidene)cyclopropane, (1-butylpentylidene)cyclopropane and octylidenecyclopropane with hydrochloric acid produced the corresponding homoallylic chlorides, 4-chloro-1,1-diphenyl-1-butene, 4-butyl-1-chloro-3-octene and (E)-1-chloro-3-undecene in 99, 96, and 87% yields, respectively. The reaction of (1-butylpentylidene)cyclopropane with hydrobromic acid yielded 1-bromo-4-butyl-3-octene in 95% yield.     

Effect of solvent on absorption and fluorescence spectra of a typical fluorinated azo dye for its acidic and basic structures

The effect of 15 polar solvents on absorption and fluorescence energies of a typical fluorinated azo dye, 4-(2,3,5,6-tetrafluoro-pyridin-4-yl azo)-phenol, was reported for its acidic, MH, and basic, M, structures.For MH, the absorption energy is described on the basis of multi-linear equation with Taft's π* (solvent polarity) and β (hydrogen bond acceptor) parameters while the fluorescence energy varies rectilinearly with free energy of transferring the proton to the surrounding solvent, ΔG_t°.For M, the hydrogen bonding donor ability of protic solvent, α, is a predominant factor which affects the absorption energy while in aprotic solvents, the absorption energy correlates linearly with Kirkwood function. As the ability of the solvent for hydrogen bonding increases, the absorption band width will increase in parallel with the transition energy.     

Substituent dependence of the d-d transition band in the electronic absorption spectra of arylferrocenes and the corresponding arylferricenium salts

The electronic absorption spectra of 22 arylferrocenes and their arylferricenium salts with pentacyanopropenide (PCNP) were recorded. The attemptedHammett correlation of _max of the d-d transition bands of arylferrocenes revealed that this band is substituent dependent only for strong electron-withdrawing substituents. Excellent correlation of _max of the d-d transition bands with ⁺ constants was found in the case of arylferricenium cations. Good correlations of _max was also found with theE _1/2 oxidation potentials measured by cyclic voltametry.

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Substituentenabhängigkeit des d-d-Überganges in den Elektronenabsorptions-Spektren von Arylferrocenen und entsprechenden Arylferricenium-Salzen

Zusammenfassung Elektronenabsorptions-Spektren von 22 Arylferrocenen und ihren Arylferricenium-Pentacyanopropenid-Salzen wurden gemessen. VersuchteHammett-Korrelationen des längstwelligen d-d-Überganges (_max) zeigten, daß diese Bande nur von stark elektronenanziehende Substituenten abhängig ist. Im Fall der Arylferricenium-Kationen wurde eine ausgezeichnete Korrelation zwischen _max der d-d-Bande und den ⁺-Konstanten festgestellt. Eine gute Korrelation der _max-Werte wurde auch mit denE _1/2 Oxidationspotentialen gefunden, die durch cyclische Voltametrie meßbar sind.

     

Electric-field-induced changes in absorption and emission spectra of CdS nanoparticles doped in a polymer film

Electric-field-induced changes in absorption and emission spectra of colloidal CdS nanoparticles ranging in size from 1.0 to 5.0 nm in diameter have been measured by using electric field modulation spectroscopy. The analysis of the electroabsorption spectra indicates that the dipole moment in the first exciton state becomes larger with increasing particle size. The presence of the large dipole moment following photoexcitation into the first exciton band suggests that the CdS nanoparticles have large CT character in the first exciton state. The quantum yields both of the exciton emission and of the trap emission are markedly reduced by application of an electric field. On the basis of the direct measurements of the field-induced change in emission decay profile, it is suggested that the field-induced de-enhancement of these emission yields results from the field-induced decreases both in lifetime and in initial population of each emitting state. It is also found that the emission intensity of CdS nanoparticles increases under the UV light irradiation in air and decreases in a vacuum condition and that fluorescence lifetime in the former case is longer than that in the latter. This enhancement and de-enhancement process in emission intensity is almost reversible at least in several cycles.     

The characteristic red chemiluminescence from reactions with acidic potassium permanganate: further spectroscopic evidence for a manganese(II) emitter

A direct comparison of the laser-induced photoluminescence of manganese(ii) with the chemiluminescence from the reaction between acidic potassium permanganate and sodium borohydride was used to confirm that the characteristic red emission from this widely used chemiluminescence reagent emanates from an electronically excited manganese(ii) species.