Halogen Molecules Research Articles

Oxidation of CH3CCI3 and CH3CFCI2 in the atmosphere : kinetic study of OH reactions

The kinetics of the reactions OH + CH3CCl3 (1) and OH + CH3CFCl2 (2) have been studied using the EPR discharge-flow method. Highly purified reagents were used in order to avoid any overestimation of the measured rate constants due to the presence of reactive impurities. The results are : k1 = (1.2 ± 0.2) x 10-14 at 298 K and k2 = (2.4 ± 0.6) x 10-12 exp [-(1790 ± 100)/T] between 298 K and 479 K, in cm3 molecule-1S-1. These data are compared with earlier ones, and their atmospheric significance is discussed in relation with the calculation of the tropospheric lifetimes or ozone depletion potentials of man-made halogenated molecules.

The modified all‐valence INDO method with the inclusion of spin–orbit coupling

AbstractThe all‐valence INDO method has been modified for the inclusion of spin–orbit coupling effects. In the method presented, the Hamiltonian includes spin–orbit coupling and the basis set constitutes the singlet and triplet determinental wave functions constructed from molecular orbitals resulting from nonrelativistic calculations. Eigenvectors obtained are later used for the evaluation of transition probabilities among different states. The results presented include lifetimes of different states of organic molecules and transition energies for halogen molecules and they are in a good agreement with experimental results. © 1992 John Wiley & Sons, Inc.

Femtosecond real-time probing of reactions. VIII. The bimolecular reaction Br+I2

In this paper, we discuss the experimental technique for real-time measurement of the lifetimes of the collision complex of bimolecular reactions. An application to the atom–molecule Br+I2 reaction at two collision energies is made. Building on our earlier Communication [J. Chem. Phys. 95, 7763 (1991)], we report on the observed transients and lifetimes for the collision complex, the nature of the transition state, and the dynamics near threshold. Classical trajectory calculations provide a framework for deriving the global nature of the reactive potential energy surface, and for discussing the real-time, scattering, and asymptotic (product-state distribution) aspects of the dynamics. These experimental and theoretical results are compared with the extensive array of kinetic, crossed beam, and theoretical studies found in the literature for halogen radical–halogen molecule exchange reactions.

Ab initio MO calculations and experimental observations on the reactivities of elemental fluorine and chlorine with graphite in the presence of HF

Ab initio MO calculations have been carried out on F 2  HF and Cl 2  HF complexes in order to shed light on the reactivities of elemental fluorine and chlorine with graphite. The results show that the F 2  HF and Cl 2  HF complexes are stabilized by 10.50 and 5.2kJ/mol respectively. A partial charge transfer from the halogen molecules (X 2) to HF occurs. The X 2 molecules are polarized and the X atoms which are not hydrogen-bonded to HF carry a partial positive charge, making the complexes suitable for the dipole-induced dipole interaction with graphite. Furthermore, the X 2 molecules interacting with HF can be regarded as easily polarizable, large “soft oxidants“, facilitating the subsequent charge transfer from graphite. HF plays a catalytic role both in intercalation and deintercalation processes. In contrast with fluorine, the reaction of chlorine with graphite in the presence of liquid HF barely yielded a stage-two compound after a prolonged reaction time, which was in agreement with the results of calculation. The lower reactivity of chlorine is attributed to the weaker interaction of Cl 2 with HF and its smaller oxidizing ability.

Rate constants for the etching of gallium arsenide by molecular iodine

The etching of the (100) face of gallium arsenide with molecular iodine has been studied at I2 pressures between 0.10 and 1.25 Torr and in the temperature range from 270 to 330 °C. GaAs was found to be etched continuously at rates between 0.05 and 1.70 μm min−1 under these conditions. Although the etch rate appears to be first order with respect to I2 at pressures below 0.3 Torr, the order decreased at higher pressures. The data was analyzed in terms of two mechanisms, which have been proposed for the etching of semiconductors by halogen molecules. These are the ‘‘reversible dissociative adsorption’’ (RDA) mechanism, and the ‘‘surface site saturation’’ (SSS) mechanism. In the RDA mechanism the dissociative adsorption, which explains the nonlinear pressure dependence, occurs irreversibly at low pressure giving rise to first order kinetics with a first order rate constant (k1) which can be expressed in the following Arrhenius form: k1=104.7±0.2 μm min−1 Torr−1 exp[−(55±2) kJ mol−1/RT]. At higher pressures, the dependence on I2 changes to half order and the composite half order rate constant (k1/2) can be represented by the equation: k1/2=106.5±0.9 μm min−1 Torr−1/2 exp[−(69±10) kJ mol−1/RT]. On the other hand, the SSS mechanism involves the reversible physisorption of I2 on the GaAs surface (governed by an equilibrium constant K) followed by a rate controlling reaction of this physisorbed species leads to products (governed by the rate constant k4). The values obtained for these two constants are: K=10−4.8±0.7 Torr−1 exp[+(49±7) kJ mol−1/RT], and k4=109.2±0.4 μm min−1 exp[−(96±5) kJ mol−1/RT]. The etching is isotropic, and the major products are GaI3 and AsI3.

Thermodynamic estimation of dissociation pressure parameters for solid molecular substances

Thermochemical and structural chemistry methods of estimating the dissociation pressure of molecular substances are proposed. For halogen, chalcogen, hydrogen and oxygen molecules, satisfactory agreement with experimental data is demonstrated.

The VUV spectroscopy of deuterated hydrazine, N 2D 4

The VUV absorption spectrum of N 2D 4 has been measured in the spectral region 1050–2100 Å using synchrotron radiation. The spectrum, which is largely unstructured, gradually rises from a threshold of 2630 Å and is dominated by three broad peaks of increasing magnitude at ≈ 1940, 1685 and 1400 Å followed by a drop in the absorption at ≈ 1340 Å and then a monotonic rise to the low-wavelength experimental cutoff at 1050 Å. The spectrum can be interpreted either in terms of Rydberg excitations of the nonbonding nitrogen electrons with the broad features resulting from spectral congestion or alternatively, the broad nature of the spectrum may be due to absorption of molecular valence states involvingσ ∗←n and σ ∗←σ transitions. Mulliken proposed the existence of such valence transitions by analogy with corresponding feature in the isoelectric diatomic halogen molecules.

OPTICAL PUMPING OF COPPER METASTABLE ATOMS USED FOR LIFETIME MEASUREMENTS OF HIGH VIBRATIONAL LEVELS OF ELECTRONICALLY EXCITED COPPER HALIDES FORMED BY REACTIVE COLLISIONS

We present a new method for lifetime measurements of excited cogper halides formed by reactive collosion of a metastable D copper atom with a halogen compound. The method is applied for excited states in CuF, CuCl and CuBr, including high vibrational leveJs. Additionally, the molecular constants for the Aniand a new A' 112 states in CuBr are reported. Spectra of copper halides CuX are fairly complex and not fully understood. The large spin-orbit interaction in Cu mixes triplet states of the same symmetry, allowing electronically excited states of both multiplicities to radiate to the ground state xil+. Consequently, assignment of all of the experimentally observed states is not straightforward. This is particularly the case when CuX is formed following the reactive collision of a copper atom with a halogen compound, which popuJates very high vibrational levels of electronically excited CuX molecules and leads to a very dense emission spectrum [ 1 ,21 . Radiative lifetime measurement, following excitation from the ground state with a short laser pulse, is a powerful method to discriminate different excited states. In particular, triplet states are expected to have longer lifetimes than the singlets. However, due to the diagonal nature of the Franck-Condon factors in these molecules, the measurements [3,4] were restricted to the lowest vibrational levels in each excited state, which can be optically excited from the thermally populated low v levels of the ground state. To overcome this difficulty, we have developed an alternative method of lifetime measurements in copper halides, free from the restriction mentioned above. The method is based on observation of the temporal decay of population of the excited levels, following a fast decrease in the source term populating these levels. A fast flow reactor, combined with a hollow-cathode spgttering source, was used to carry copper metastable atoms Cu ('D) to the reaction zone [ 2 I . Here the reactant gas (5% of F2, C12 or BrZ diluted in a noble gas) was added, producing a bright cherniluminescence flame. It was due to the reaction of the metastable copper atoms with halogen molecules: Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:19917130 JOURNAL DE PHYSIQUE IV * - CUX -4 CUX + FLV (2 where n and v denote particular electronic state and vibrational level. Under steady-state conditions we observed chemiluminescence from a given molecular state with a constant intensity, corresponding to equilibrium between formation of CUX* molecules in reaction (1) and radiative (2) and collisional decay. Then a short pulse of a copper vapour laser (pulse duration %&Op, repetition rate 6.5kH2, energy Im~/pulse) depopulated Cu ( D) metastable atoms in the reaction zone [21. Since the chemiluminescence of copper halides is almost uniquely due to the chemical reaction (1) involving metastable copper atoms and halogen molecules 121, we were able to observe an exponential decay of population of the excited molecular states. The decay of the (spectrally resolved) chemiluminescence signal was averaged with a transient 64 channel analyzer and sent to the microcomputer for further data treatment. The typical shape of the time resolved chemiluminescence is shown in Fig.1. The first part of the curve, corresponding to the exponential decay, allows to determine the lifetime of the observed molecular state. Using the experimental method presented above we measured lifetimes of several excited states in CuF, CuCl and CuBr molecules (Fig-2), including high vibrational levels. Results are presented in Table I. A discrepancy of some of our results with the previous measurements [41 will be discussed in the forthcoming paper. Additionally, we have examined in detail chemiluminescence of CuBr moiecule in the spectral range of 465-535nm. Beside of the known A niX'X+ system a new band system was observed in this range, corresponding to the transition from a formerly unknown excited A' state to the groznd X state. The most plausible assignment of the A' state is n-, i.e. as the n=2 fine structure L component of the an manifold, to which belongs also the ~~n~ state. The molecular constants of the $' state were investigated and the vibrational constants of the Ani state were determined with precision superior to previous measurements (Table 11). REFERENCES /I/ BALTAYAN,P, HARTMANN,F, PEBAY-PEYROULA,J, SADEGHI,N, Chem.Phys. 120 (1988) 123. / 2 / BALTAYANIPl HARTMANN,F, HIKMET,I, PEBAY-PEYROULA,J, SADEGHI,N, Chem.Phys.Lett. 160 (1989) 549. /3/ DELAVAL,J, LEFEBVRE,Y, BOCQUET,H, BERNAGE,P, NIAY,P, Chem.Phys. 111 (1987) 129. / 4 / LEFEBVRE,Y, DELAVAL,J, BERNAGE,P, NIAY,P, Chem.Phys.Lett. 139 (1987) 212.

Reactive collision between alkali-metal atoms and halogen molecules.

The Feynman diagram method for atomic collisions developed by us has been applied to reactive collisions between alkali-metal atoms and halogen molecules. The velocity-averaged differential and total cross sections for the production of alkali halide molecules have been calculated and compared with available experimental results.

Kinetics of the reaction of chlorine with formic acid in aqueous sulfuric acid

In sulfuric acid solution, the rate of reaction of chlorine with formic acid is proportional to formic acid and chlorine concentrations and is inversely proportional to the acidity function h. The kinetic parameters are nearly identical with those for the reduction of bromine to bromide ions by formic acid. The two reactions appear to follow similar mechanisms - equilibrium ionization of formic acid followed by direct reaction of a formate ion with a halogen molecule.

Continuum resonance Raman scattering in diatomic molecules: Experiment and theory

Experimental and theoretical investigations of continuum resonance Raman scattering and related photoinduced, unimolecular dissociation in halogen and interhalogen molecules are discussed. Experimentally observed spectra are simulated using both time-independent and time-dependent theory. The sensitivity of the simulated spectra to small changes in the scattering potentials is demonstrated. The time-dependent approach further allows an estimation of the scattering time for this type of resonance Raman scattering. With iodine chloride, dissociation is monitored by electronic Raman scattering on an iodine atomic transition.

Rotational energy transfer in excited states of halogen molecules. I. Transfer from v′=6, J′=72 in IF B 3Π(0+)

A cw laser-induced fluorescence experiment is described in which state-to-state rate coefficients for rotation-to-translation energy transfer have been determined from J′=72, v′=6 in IF B 3Π(0+). Using the collision partners He, Ne, Ar, Kr, Xe, N2, and CF4, nearly 500 individual rate coefficients have been measured. The rare-gas atoms display a characteristic behavior with the heavier collision partners more efficiently exchanging large amounts of angular momentum in a single collision.

ChemInform Abstract: Chemistry of Organo Halogen Molecules. Part 100. Comparative Behavior of Xenon Difluoride and Cesium Fluoroxysulfate in the Fluorination of Enol Acetates and Ketones.

ChemInform Abstract: Chemistry of Organo Halogen Molecules. Part 100. Comparative Behavior of Xenon Difluoride and Cesium Fluoroxysulfate in the Fluorination of Enol Acetates and Ketones.

Continuum resonance Raman scattering in diatomic molecules: experiment and theory

Experimental and theoretical investigations of continuum resonance Raman scattering and related photoinduced, unimolecular dissociation in halogen and interhalogen molecules are discussed. Experimentally observed spectra are simulated using both time-independent and time-dependent theory. The sensitivity of the simulated spectra to small changes in the scattering potentials is demonstrated. The time-dependent approach further allows an estimation of the scattering time for this type of resonance Raman scattering. With iodine chloride, dissociation is monitored by electronic Raman scattering on an iodine atomic transition.

Structure determination in intercalated graphite by polarized XAFS analysis

Highly oriented pyrolytic graphite (HOPG), intercalated with the halogen molecules ICl and IBr, was investigated by x-ray absorption spectroscopy. Information on the structural arrangement of the intercalated halogen molecules was derived from the polarization and temperature dependent near-edge (NEXAFS) and extended (EXAFS) x-ray absorption fine structure of the respective Cl-K, I-LI, III′ and Br-K thresholds. The observed change in the intramolecular bondlength of the halogen molecules in different surroundings is correlated with the energy separation of the σ∗- and π-resonances observed in the NEXAFS spectra.

Intercalate structure in halogen-intercalated graphite

Highly oriented pyrolytic graphite (HOPG), intercalated with the interhalogen molecules ICl and IBr, was investigated by x-ray absorption spectroscopy. From the polarization dependent near-edge (NEXAFS) and extended (EXAFS) x-ray absorption fine structure observed at the ClK, IL I,III, and BrK thresholds, we derive information on the structural arrangement of the intercalated halogen molecules. Of particular interest is the observation of an intercalation induced increase in the intramolecular halogen bondlength, which is found to be stage dependent for ICl-graphite and associated with the charge transfer from graphite to the halogen molecules.

On the effect of low-energy ion bombardment on polystyrene and polymethylmethacrylate etch rates in rf plasmas

The effect of low-energy ion bombardment on CD4/O2 and CF3X (X=F, Cl, Br) plasma etching has been assessed by applying controlled rf bias voltages on polystyrene (PS) and polymethylmethacrylate (PMMA) samples. In both cases ion bombardment has been found to have a “chemical” effect. In the case of CF4/O2 discharges, ion bombardment has been found to change the relative etching efficiency of different mixtures. In the case of CF3X plasmas, ion bombardment has been found to alter PMMA and PS etch rates in a different way. In particular, the ratios between CF4 and CF3X (X=Cl, Br) etch rates of PS have been found to decrease with increasing bias voltage. This effect has been tentatively attributed to an ion bombardment-induced enhancement of the reaction between the aromatic ring and halogen molecules formed in CF3Cl and CF3Br discharges.

Pseudopotential and multiple scattering xα calculations of nuclear quadrupole coupling constants and other properties of diatomic halogen molecules and their monoanions and monocations

An SCF molecular orbital Pseudopotential method has been used to calculate the equilibrium bond lengths, dissociation energies and harmonic force constants for the homonuclear diatomic halogen molecules and their monoanions and monocations (X 2, X 2 −, and X 2 +, X = Br, I). The results were used to calculate the vertical and adiabatic ionization energies and electron affinities of Br 2 and I 2. These calculations were carried out using a modified version of Pople's GAUSSIAN 82 program in which the core electrons on Br and I are described by quasirelativistic core pseudopotentials. The SCF MS-Xα method has been used to calculate the nuclear quadrupole coupling constants for X 2, X 2 − and Xa 2 + for X = Cl, Br, I using the equilibrium intemuclear distances obtained from the Gaussian SCF calculations. For X 2 and X 2 −, agreement between the calculated and observed coupling constants is reasonable (within 10% for X 2 and 20% for X 2 −). In agreement with experiment, the ratio e 2 qQ(X 2 −)/e 2qQ(X 2) is less than 0.5, the value predicted by a simple Townes and Dailey analysis, and possible reasons for this difference are discussed.

Semiconductor surface etching by halogens: Fundamental steps

Abstract The mechanisms underlying the reaction of halogen molecules with Si(100) have been investigated using the surface probes of thermal desorption spectroscopy (TDS) and Auger electron spectroscopy. The formation of a strongly bound monolayer phase followed by a more weakly held silicon halide corrosion phase has been found to occur even at temperatures as low as 100 K. At these low temperatures further adsorption leads to the formation of a molecular physisorbed halogen overlayer. The reaction products from these adsorbed phases have been characterised and are considered in terms of silicon etching processes. The relevance of the data to beam enhanced etching is discussed.

Chemistry of organo halogenic molecules. Part 100. Comparative behaviour of xenon diflouride and caesium fluoroxysulphate in the fluorination of enol acetates and ketones

Xenon difluoride and caesium fluoroxysulphate reacted in methylene chloride or acetonitrile with various enol acetates, diketones, and ketones, yielding mainly α-fluoro ketones, the course of the reaction depending on the reagent and the structure of the organic molecule. Enol acetates from cycloakanones were converted with caesium fluoroxysulphate to α-fluorocycloalkanones in high yield. Xenon difluoride and caesium fluoroxysulphate converted enol acetates of benzocycloalkanones-1 to α-fluorobenzocycloalkanones, while the reactivity of enol acetates of benzocycloalkanones-2 depended on the reagent used. 1,3-Diphenyl- propane-1,3-dione and its enol acetate were converted with XeF 2 and CsSO 4F to mono and difluoro substituted products, the course of the reaction being dependent on the reagent. Xenon difluoride converted 1-indanone to rearranged 2,2-difluorochromane, while caesium fluoroxysulphate reacted to 1-fluoro-2-indanone with 2-indanone.