Halogen Molecules Research Articles

Cluster-surface impact dissociation of halogen molecules in large inert gas clusters

Molecular dynamics simulations of the dissociation of I 2 embedded in large Ar n ( n = 319, 553) clusters, which impact at high velocities (ν = 7–15 km s −1 1 ) on Pt surfaces, result in information on heterogeneous and homogeneous dissociation mechanisms. A broad distribution of dissociation lifetimes is exhibited, which can be attributed to prompt and retarded heterogeneous dissociation and to prompt, retarded and outbound homogeneous dissociation events. The propagation of a microshock wave within a large cluster can be interrogated by the homogeneous dissociation of a chemical probe, with the velocity of the propagation of the dissociation front being close to the cluster impact velocity.

Ar–I2 interactions: The models based on the diatomics-in-molecule approach

Semiempirical model is developed for studying the electronic structure of the rare gas atom–halogen molecule systems. It is formulated in the frame of diatomics-in-molecule (DIM) approach and takes explicitly into account strong spin–orbit coupling pertinent to heavy halogen molecules. The consistent DIM scheme is realized for intermolecular interactions, whereas the description of valence electronic states of halogen molecule is more approximate being based on the asymptotic wave functions. The corresponding perturbation theory is also put forward. The model is applied to analysis of several features of the Ar...I2 van der Waals complex. First, the calculations on the spectroscopic constants of the B←X transition in the complex reveal the quantitative performance of the model. Second, mechanisms of nonadiabatic dynamics are examined. The results are qualitatively consistent with the current view on the Ar...I2 electronic predissociation and one-atom cage effect. Third, the prediction is made on the valence electronic spectrum of Ar...I2 complex. These examples demonstrate the reliability of the model.

Fundamental Aspects of the Reactions of Thermal and Hyperthermal F, F2, Cl, and Cl2 with Si Surfaces

Reactive neutral halogen interactions with silicon surfaces are being investigated as possibilities for processing technologies which minimize damage and the exposure of ultrathin dielectric films to charged particles. In order to assess the feasibility of this method, it is necessary to determine reactive adsorption probabilities for halogen atoms and molecules as a function of their kinetic and internal energy. In addition, it would be advantageous to determine atomic level mechanisms for the etching reactions under different surface conditions and for both equilibrium and nonequilibrium reactive interactions. In this article, a summary of this information is given for the interaction of silicon surfaces with fluorine and chlorine.

The Pre‐Reactive Complex of H2S and BrCl; Observation and Characterisation by Rotational Spectroscopy

AbstractBy using a fast‐mixing nozzle in a Fourier transform microwave spectrometer, any chemical reaction accompanying mixing of H2S and BrCl was prevented, thus allowing the observation of the pre‐reactive complex H2S ··· BrCl. The rotational spectra of eight isotopomers of the complex were recorded. The analysis of the determined spectroscopic constants shows that the atoms S ··· BrCl are collinear or nearly so and that the H2S plane is approximately perpendicular to the S ··· BrCl axis with r(S ··· Br) = 3.094 (7) Å. This geometry is in agreement with previously stated rules for B ··· XY complexes, where B is a Lewis base and XY is an (inter)halogen molecule. The intermolecular interaction is shown to be relatively weak, both in terms of the intermolecular stretching force constant kσ and the intramolecular electric change redistribution δ within the BrCl subunit that accompanies formation of H2S ··· BrCl.

An Analysis of Electron Donor−Acceptor Complexes: H2O·F2, H2O·Cl2, and H2O·ClF

An extended geminal model has been applied to study the electron donor-acceptor complexes H2O·F2, H2O·Cl2 and H2O·ClF. By adopting a [9s, 6p, 4d, 2f/8s, 5p, 4d, 2f/4s, 3p, 1d] contracted Gaussian-type basis set, the equilibrium O−X (X nearest halogen atom) distances are predicted to be 2.69, 2.84, and 2.60 Å, and the binding energies to be 5.86, 12.51, and 20.24 kJ/mol for the complexes H2O·F2, H2O·Cl2, and H2O·ClF, respectively. The halogen molecule is situated in the plane of the H2O molecule in the H2O·F2 complex, whereas it points approximately toward a lone pair of the O atom in the H2O·Cl2 and H2O·ClF complexes. The charge centroid of the corresponding lone pair geminal is moved considerably toward the halogen molecule as a result of the complex formation in the two latter complexes. Two slightly different decomposition schemes for the potential are considered. For the equilibrium structures the Coulombic interaction between the distorted monomers has the largest magnitude within the primary decomposition scheme, and the potential is not very different from the intermolecular correlation energy.

Charge-Transfer Complexes: Stringent Tests for Widely Used Density Functionals

Density functional calculations are reported for charge-transfer complexes (CT), also called electron donor−acceptor systems, formed from ethylene or ammonia interacting with a halogen molecule (C2H4···X2, NH3···X2, X = F, Cl, Br, and I). In all cases, the local density approximation provides a strong overestimation of the intermolecular interaction. The generalized gradient approximation moves the results in the right direction but, in general, not nearly far enough; large errors remain. We attribute the problem to the too rapid asymptotic decay of the exchange−correlation potential associated with the imperfect cancellation of the self-interaction. This breakdown of the potential is reflected in a set of incorrect eigenvalues (orbital electronegativities) that play a crucial role in governing the charge transfer and, hence, the interaction energy. The inclusion of some Hartree−Fock exchange using hybrid methods provides a large improvement, and the parameters related to the intermolecular interaction fo...

ChemInform Abstract: Chemistry of Organo Halogenic Molecules. Part 140. Role of the Reagent Structure on the Transformations of Hydroxy Substituted Organic Molecules with the N‐Fluoro Class of Fluorinating Reagents.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Chemistry of Organo Halogenic Molecules. 140. Role of the Reagent Structure on the Transformations of Hydroxy Substituted Organic Molecules with the N-Fluoro Class of Fluorinating Reagents

Abstract Hydroxy-substituted organic molecules were used as target molecules in investigations of the role of the reagent structure on the reactivity of three types of N–F class fluorinating reagents: 1-chloromethyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) F-TEDA (1a), N-fluorobis(phenylsulfonyl)amine NSF (1b), and N-fluoropyridinium heptafluorodiborate–pyridine (1/1) NFP (1c). Methanol is stable, but hydroquinone is very quickly transformed in acetonitrile to quinone with F-TEDA at room temperature; on the other hand, NSF is less reactive, while oxidation with NFP is achieved only at an elevated temperature; a structure variation of the hydroquinone derivatives did not influence oxidation. Fluorination was achieved with monohydroxy-substituted aromatic compounds; a similar trend concerning the reactivity of N–F reagent (1) was also observed in reactions with 1- and 2-naphthol, while 9-phenanthrol gave 10,10-difluoro-9-(10H)-phenanthrenone with F-TEDA in acetonitrile and 9,10-phenanthrenequinone with NSF. Dealkylation was observed in a reaction with 4-methoxyphenol; although the thio analogue gave bis(4-methoxyphenyl) disulfide, the reactivity was changed and NSF was more reactive than F-TEDA and NFP.

Quenching Rate Constants of the Xe(5p56p and 6p') States and the Energy-Pooling Ionization Reaction of Xe(5p56s) Atoms

Two-photon excitation by a pulsed dye laser was used to prepare the Xe(5p{sup 5}6p[1/2]{sub 0}, [3/2]{sub 2}, [5/2]{sub 2}) and Xe(5p{sup 5}6p{prime}[HLF]{sub 0}, [3/2]{sub 2}) states in the presence of several reagent gases to measure total quenching rate constants at 300 K. The new measurements together with rate constants from the literature provide a general overview of Xe(6p) atom reactivity, especially with halogen and oxygen-containing molecules. Reactive quenching and/or excitation transfer to the reagent molecule are the dominant quenching mechanisms, rather than intramultiplet transfer to other states of the Xe(6p) manifold. The XeCl(B D) and XeF(B and D) product state distributions from reactions of the Xe(5p{sup 5}({sup 2}P{sub HLF})6p{prime}) states with HCl, Cl{sub 2}, F{sub 2}, NF{sub 3}, CCl{sub 4}, and CCl{sub 2}F{sub 2} were observed in order to record the degree of conservation of the Xe{sup +}({sup 2}P{sub HLF}) core as indicated by XeCl(D) or XeF(D) formation. Except for the HCl reaction, the Xe{sup +}({sup 2}P{sub HLF}) does not have a high degree of conservation in reactions of Xe(6p{prime}) atoms. For conditions of low reagent pressure, radiative decay of the Xe(6p) states can produce a high local concentration of metastable Xe(6s) atoms, which subsequently undergo bimolecular, energy-pooling, associative-ionization reactions.more » 41 refs., 12 figs., 3 tabs.« less

Evidence for reversible dissociative adsorption in the reaction of molecular chlorine with gallium arsenide

The reaction of molecular chlorine with the (100) face of a gallium arsenide crystal has been studied at pressures of Cl2 between 0.10 and 9.0 Torr and in the temperature range from 90 to 110 °C. In contrast to an earlier report, the etch rate was found to be half order with respect to Cl2. The similarity of these results to those obtained for the reaction of molecular chlorine and bromine with silicon points to a mechanism in which the gaseous halogen molecule is first physisorbed on the semiconductor surface and then dissociates into chemisorbed atoms. The data indicate that both steps occur reversibly at higher pressures, where the composite half-order rate constant can be represented by the Arrhenius equation:[Formula: see text]At low pressures the first-order rate constant is given by the equation:[Formula: see text]Keywords: etching, gallium arsenide, molecular chlorine, kinetics.

Intercalation of Halogen Molecules in Alkali Fluoride Lattices: A Theoretical Study

Intercalation of Halogen Molecules in Alkali Fluoride Lattices: A Theoretical Study

Dissociation dynamics of diatomic molecules embedded in impact heated rare gas clusters

Molecular dynamics simulations demonstrate facile dissociation of halogen molecules embedded in rare gas clusters upon impact at a surface at collision velocities up to 10 km/s. Two pathways are discerned: a heterogeneous dissociation of the molecule on the surface and a homogeneous mechanism where rare gas atoms which have rebounded from the surface cause the translational–vibrational coupling. The total yield of dissociation of the clustered molecule can reach up to 100%, whereas the yield of dissociation of the bare, vibrationally cold molecule saturates below 40%. A systematic study of the role of different conditions is made possible by not accounting for the atomic structure of the surface. The role of dissipation at the surface is found, however, to be quite important and is allowed for. Larger clusters, clusters of the heavier rare gases and a more rigid surface, all favor the homogeneous mechanism. Evidence for a shock front which, upon the initial impact, propagates into the cluster; the binary nature of the homogeneous dissociation process; and the absence of a dominant cage effect are discussed. A quantitative functional form of the velocity dependence of the yield of dissociation, which accounts for the size of the cluster, the rigidity of the surface and other attributes, is used to represent the data. The physics of the processes within the cluster is dominated by the novel dynamical features made possible when the duration of the atom–molecule collisions is short compared to the vibrational period. This ‘‘sudden’’ regime is sudden with respect to all modes of the nuclear motion and provides a hitherto unavailable tool for examination of reaction dynamics under extreme conditions.

Binary nuclear molecules of an ?-particle and a heavier cluster

A simple nuclear molecule is used as a model to interpret the band structure observed in elastic α-scattering and α-transfer reactions in the sd-shell. As in spectra of halogen acid molecules the bands have even and odd spins and parities. Due to the broadening and fine structure of the rotational levels they are characterized as giant resonances. The big moment of inertia and the line broadening is discussed in terms of performed measurements of scattering and the (α,p) reaction. A fine structure of prominent peaks in the inelastic scattering crosssections hints at the existence of rotational states with an α-particle orbiting an excited core.

Computational studies of halogen chemistry on rare-gas surfaces. II. Structure of chlorine and bromine submonolayer films on Ar(111) and Xe(111) prepared by molecular beam dosing

The deposition by molecular beam dosing of halogen molecules on rare-gas surfaces has been studied with molecular dynamics simulation. Specifically we have considered films formed by the sequential adsorption reactions: X2g+X2[Θ]ads−Rg(111)[T]→X2(ads)−X2[Θ]ads −Rg(111)[T], where Θ is the film coverage defined by X2[adsorbed]/Rg[surface], T indicates the substrate temperature, X2 is the halogen adsorbate which is either chlorine or bromine, and Rg indicates the rare-gas substrate which is either argon or xenon. The structure of halogen adlayers was studied as a function of coverage for films grown on rare-gas substrates at different temperatures. Chlorine and bromine films on argon exhibit orientational ordering and islanding with increasing coverage. The tendency of the halogen diatoms to align along the surface normal with increasing coverage is strongly enhanced by higher temperatures in the case of chlorine on xenon, moderately enhanced in the case of chlorine on argon, and unchanged in the case of bromine on argon. Chlorine and bromine films form three-dimensional aggregated structures on argon at 15 and 25 K. Chlorine forms a two-dimensional amorphous layer on xenon at 15 K and a highly ordered layer at 50 K. The dynamics of the molecular adsorption event were studied at three different coverages: Θ=0.05, 0.25, and 0.5. At all these coverages, we observed finite possibility of diffusional motion of adsorbate on the surface immediately after it lands: This was not seen at zero coverage.

Visible and near-infrared chemical lasers

We analyze the basic thermodynamic, kinetic, radiative, and spectroscopic properties of halogen and interhalogen molecules which are potentially suitable for the construction of visible and near-IR chemical lasers. We discuss the problem of chemical buildup of electronically excited states (EES) of donor molecules (or atoms). We propose a number of new kinetic schemes with formation of EES of donor particles, for the development of lasers based on electronic transitions (ET). We obtain analytic relations for the lasing condition and for the laser efficiency of a system of a “donor—acceptor” type as applied to ET chemical lasers. Actual results of research into the gain properties and energy characteristics of potential visible-band chemical NF-IF and N2-IF lasers. The possibility of using atoms in chemical lasers is discussed.

Ballistic and dissociative collisions of a rare-gas atom with a halogen molecule

Efficient vibrational excitation and dissociation of halogen molecules have been observed in molecular dynamics simulations of shock-heated rare-gas clusters. We discuss the propensity for such transfers in a single atom—molecule collision and present classical trajectory computational results. Rather large cross sections are reported. It is expected that in a real experiment some of the flux will be diverted to electronic excitation.

Polyol and Blowing Agent Binary System Described by Means of the Flory Model

Binary systems consisting of small halogenated molecules dissolved in oligomers have been investigated. Pressure/composition isothermal data were used to calculate the activities by the Flory equation which provides a good description of such systems and includes the possible immiscibility region calculated from the Gibbs excess energy. The interaction χ parameter can be calculated as well, which accounts for the essentially enthalpic contribution to the non ideality. Deviations from ideality can also be determined by boiling temperature isobaric measurements. The slope of the composition with respect to boiling temperature profile appears as an important term modelling the free rise of a polyurethane system foamed by physical blowing agents. Adopting such a model and utilizing the Flory equation to describe the χ dependence on the temperature, the profile of the blowing agent mole fraction with respect to boiling temperature and its slope have been determined and cream, gel times along with density and bulk temperature profiles have been simulated. Simulation shows that calculated kinetics and density profiles are significantly dependent on using experimental data relative to blowing agents such as CCl3F, CHCl2-CF3 and CH2F-CF3. The last two compounds in this study have been assumed to be model molecules focusing attention on their opposite deviation from ideality with respect to CCl3F. Work is in progress to evaluate how this simulation is suitable to describe the real foaming processes

Two‐Photon Photoassisted Reactions of Collision Pairs Consisting of Xe with Halogen‐Containing Polyatomic Molecules

AbstractExcitation spectra have been acquired for the two‐photon, laser‐assisted reactions between collision pairs of Xe with CCl4, CCl1Br, CH3I, CH2I2, CH2ICl, and SOCl2. In each case the product of the reaction is XeX(B,C); X = halogen. The excitation spectra obtained at 300 K in 100 Torr of Xe and 1.3 Torr of reagent have thresholds near 255 nm and increase toward shorter wavelength. Unexpected resonance‐like features have been found in the excitation spectra of Xe/CH2ICl and Xe/CCl3Br. A set of relative two‐photon excitation rates was obtained at 238 and 227 nm for all pairs by comparison to CCl4, which was selected as a standard. The excitation spectra for the polyatomic molecular pairs can be compared to previously published excitation spectra of diatomic halogen molecules with Xe. The two‐photon, laser‐assisted reaction of the Xe·F2 van der Waals molecule prepared by free‐jet expansion also is reported.

Dipped adcluster model and SAC-CI method applied to harpooning, chemiluminescence and electron emission in halogen chemisorption on alkali metal surface

The methodology of our laboratory, the dipped adcluster model (DAM) and the symmetry adapted cluster configuration interaction (SAC-CI) method, are briefly reviewed and successfully applied to the electron transfer processes, harpooning, surface chemiluminescence, and surface electron emission, in the course of the chemisorption of halogen molecule on an alkali metal surface. This success encourages us to apply our methodology to surface photochemistry.

Photochemistry of alkyl halide dimers

Dimers and other small clusters of CH3I, C2H5I, i- and n-C3H7I, HI, CF3I, CH3Br, and C2H5Br formed in a supersonic expansion are irradiated at 248 and 193 nm and the halogen molecule product probed via laser induced fluorescence spectroscopy. Both dimers and larger clusters of RI (R=H, alkyl) excited at each wavelength yield I2 in its ground electronic state with very little internal energy. Clusters of CF3I and those containing alkyl bromides do not give halogen molecule products after excitation at either wavelength. A model for the dynamics in the dimer excited state which explains these results is presented.