Cl Dissociation Research Articles

Kinetics of electron-induced decomposition of CF2Cl2 coadsorbed with water (ice): A comparison with CCl4

The kinetics of decomposition and subsequent chemistry of adsorbed CF(2)Cl(2), activated by low-energy electron irradiation, have been examined and compared with CCl(4). These molecules have been adsorbed alone and coadsorbed with water ice films of different thicknesses on metal surfaces (Ru; Au) at low temperatures (25 K; 100 K). The studies have been performed with temperature programmed desorption (TPD), reflection absorption infrared spectroscopy (RAIRS), and x-ray photoelectron spectroscopy (XPS). TPD data reveal the efficient decomposition of both halocarbon molecules under electron bombardment, which proceeds via dissociative electron attachment (DEA) of low-energy secondary electrons. The rates of CF(2)Cl(2) and CCl(4) dissociation increase in an H(2)O (D(2)O) environment (2-3x), but the increase is smaller than that reported in recent literature. The highest initial cross sections for halocarbon decomposition coadsorbed with H(2)O, using 180 eV incident electrons, are measured (using TPD) to be 1.0+/-0.2 x 10(-15) cm(2) for CF(2)Cl(2) and 2.5+/-0.2 x 10(-15) cm(2) for CCl(4). RAIRS and XPS studies confirm the decomposition of halocarbon molecules codeposited with water molecules, and provide insights into the irradiation products. Electron-induced generation of Cl(-) and F(-) anions in the halocarbon/water films and production of H(3)O(+), CO(2), and intermediate compounds COF(2) (for CF(2)Cl(2)) and COCl(2), C(2)Cl(4) (for CCl(4)) under electron irradiation have been detected using XPS, TPD, and RAIRS. The products and the decomposition kinetics are similar to those observed in our recent experiments involving x-ray photons as the source of ionizing irradiation.

Electron affinities, molecular structures, and thermochemistry of the fluorine, chlorine and bromine substituted methyl radicals

Four independent density functional theory (DFT) methods have been employed to study the structures and electron affinities of the methyl and F-, CI- and Br-substituted methyl radicals and their anions. The methods used have been carefully calibrated against a comprehensive tabulation of experimental electron affinities (Chemical Reviews, 2002, 102, 231). The first dissociation energies together with the vibrational frequencies of these species are also reported. The basis sets used in this work are of double-ζ plus polarization quality with additional s- and p-type diffuse functions, labelled as DZP++. Previously observed trends in the prediction of bond lengths by the DFT methods are also demonstrated for the F-, Cl- and Br-substituted methyl radicals and their anions. Generally, the Hartree-Fock/DFT hybrid methods predict shorter and more reliable bond lengths than the pure DFT methods. Neutral-anion energy differences reported in this work are the adiabatic electron affinity (EAad), the vertical electron affinity (EAvert), and the vertical detachment energy (VDE). Compared with the available experimental electron affinities, the BHLYP method predicts much lower values, while the other methods predict values (EAad, EAvert, VDE) close to each other and almost within the experimental range. For those systems without reliable experimental measurements, our best adiabatic EAs predicted by BLYP are 0.78 (CHF2), 1.23 (CHFCl), 1.44 (CHFBr), 1.61 (CHClBr), 2.24 (CF2Cl), 2.42 (CF2Br), 2.56 (CFBr2), 2.36 (CCl2Br), 2.46 (CClBr2), and 2.44 eV (CFClBr). The most striking feature of these predictions is that they display an inverse relationship between halogen electronegativity and EA. The DZP++B3LYP method determines the vibrational frequencies in best agreement with available experimental results for this series, with an average relative error of ∼2%. The value of using a variety of DFT methods is observed in that BHLYP does best for geometries, BLYP for electron affinities, and B3LYP for vibrational frequencies. These theoretical results serve to resolve several disagreements between competing experiments. Several other experiments appear to have drawn incorrect conclusions. For example, CHCl2 is significantly pyramidal, unlike the experimental inferences, and clearly the experimental CCl2—Cl dissociation energy is too large.

Relationship between Mode Specific and Thermal Unimolecular Rate Constants for HOCl → OH + Cl Dissociation

Recent quantum dynamical calculations [Hauschildt, J.; et al. Chem. Phys. Lett. 1999, 300, 569] have shown that HOCl → OH + Cl dissociation on the ground-state potential energy surface, and for J = 0, occurs predominantly via isolated compound-state resonances, whose rates are highly mode-specific. In this work, these resonance rates are averaged to calculate the HOCl → OH + Cl unimolecular rate constant as a function of temperature and pressure. The result is compared with the standard pressure and temperature dependent RRKM unimolecular rate constant. It is found that the state-specificity makes the pressure-dependent rates significantly lower than the RRKM rates in the intermediate pressure regime.

Electronic spectroscopy of jet-cooled CFCl: Laser-induced fluorescence, dispersed fluorescence, lifetimes, and C–Cl dissociation barrier

The Ã(1A″)–X̃(1A′) transition of jet-cooled chlorofluorocarbene (CFCl) has been measured by laser-induced fluorescence (LIF) excitation and dispersed fluorescence spectroscopy. Over 170 vibronic transitions were measured in the LIF spectrum, consisting of cold bands and hot bands of carbenes containing both Cl35 and Cl37 isotopes. Dispersed fluorescence spectroscopy was used both to map the ground-state vibrational levels and to provide confirmation of the vibronic identity of the emitting level. A predictor–corrector method was used to progressively assign almost all of the vibronic transitions, resulting in the positive assignment and measurement of almost every bound vibrational state within the Ã-state manifold. The vibrational structure is modeled well by a Morse potential with frequencies ν1′=1229 cm−1, ω2′=399.2 cm−1, and ω3′=748.0 cm−1 for CF 35Cl and 1235 cm−1, 397.0 cm−1, and 744.5 cm−1 for the same three vibrations in CF 37Cl. The standard diagonal and cross-anharmonicity constants for a three-coordinate Morse potential were also measured for each isotopic species. Dispersed fluorescence spectroscopy provided a map of ground-state vibrational levels up to about 4000 cm−1. Franck–Condon factors were modeled well by a simple, one-dimensional harmonic potential, and these were also used to confirm assignment of many transitions. The fluorescence lifetime of the excited vibronic states decreased markedly from a consistent 650 ns for most states, to <20 ns for the highest lying observed state. In addition, the Franck–Condon analysis indicates that higher lying members of progressions were missing in the LIF spectrum. This strongly indicated the presence of a nonradiative pathway that opens for energies above T00+4073 cm−1. Analysis of the rotational structure of many transitions indicated that the molecule was not reaching the Renner–Teller intersection, where the à and X̃ states are degenerate. We attribute the nonradiative channel to cleavage of the C–Cl bond directly on the à state, in exact analogy with the observed process in CFBr. The height of the barrier, and the vibrational frequencies are all in reasonable agreement with recent ab initio values.

Dynamics of vibrationally mediated photodissociation of CH3CFCl2

The ∼235 nm photodissociation of CH3CFCl2 pre-excited to three, four, and five quanta of C–H methyl stretches was studied to investigate the effect of internal parent excitation on the dynamics of two- and three-body photofragmentation. The ∼235 nm photons also tagged spin-orbit ground Cl 2P3/2 [Cl] and excited Cl 2P1/2 [Cl*] state photofragments, via (2+1) resonantly enhanced multiphoton ionization in a time-of-flight mass spectrometer. Monitoring the shapes of Cl35 and Cl*35 time-of-arrival profiles revealed their energies and angular distributions and showed broad and unstructured fragment kinetic energy distributions. Although a significant amount (∼50%) of the available energy is transferred into internal energy of the CH3CFCl fragment, the spatial Cl distribution is characterized by a nonvanishing anisotropy parameter, β, which indicates at a fast dissociation of the parent molecule along the C–Cl dissociation coordinate. Moreover, β for Cl changes from a slightly positive value to a negative value, while that for Cl* increases when the pre-excitation is increased from three to five quanta of C–H methyl stretches. This is attributed to the promotion of one of the nonbonding electrons located on the Cl atoms to the σ* antibonding C–Cl orbital and involvement of several upper states with different symmetry properties.

Nonadiabatic transition-state theory: A Monte Carlo study of competing bond fission processes in bromoacetyl chloride

Nonadiabatic Monte Carlo transition-state theory is used to explore competing C–Cl and C–Br bond fission processes in a simple model of [n,π1*(CO)] photoexcited bromoacetyl chloride. Morse potentials are used to represent bond stretching coordinates, and the positions and magnitudes of nonadiabatic coupling between excited state potentials are modeled using ab initio data. The main effect of nonadiabaticity is to favor C–Cl fission over C–Br, despite a larger barrier to C–Cl dissociation. The absolute values of the rate constants are smaller than observed experimentally, but the calculated branching ratios are close to the experimental value. For C–Cl fission, it is shown that the minimum energy crossing point is not sufficient to describe the rate constant, suggesting that care must be taken when using alternative models which make this assumption.

Optical transitions from the chlorine [formula omitted]( 3P 2) ion-pair state

Bound–bound 0 + u ( 3 P 2)→ X0 + g and bound-free 0 + u ( 3 P 2)→(2,3,4)0 + g luminescence spectra of molecular chlorine have been measured in the 210–330 nm spectral range after OODR excitation. The relative dipole moment functions of the transitions as well as potential energy curves of the repulsive 0 + g states correlated with the Cl( 2P 3/2) + Cl( 2P 3/2), Cl( 2P 1/2) + Cl( 2P 3/2) and Cl( 2P 1/2) + Cl( 2P 1/2) dissociation limits have been determined for the first time. The functions of the 0 + u ( 3 P 2) – X0 + g , 0 + u ( 3 P 2) – (3)0 + g and 0 + u ( 3 P 2) – (4)0 + g transition have unusual forms. Possible electronic configurations of the states combined in the optical transitions have been discussed.

Method for selective plating

Absorption transitions to vibrational levels close to the A state dissociation limit of ICI have been examined using a two-photon sequential absorption technique. The discrete rotational structures of I37 Cl bands to within 0.7 cm−1 of the limit have been selectively excited and analysed. A value of 17557.514 ± 0.030 cm−1 has been obtained for the I(2Po32) + Cl(2Po32) dissociation energy De, relative to the minimum of the ICI ground state potential well. The two-photon technique can be used to excite and display separately the high resolution absorption spectra of different isotopic species of a molecule which are contained in a mixture.

Metal-to-Ligand Charge Transfer Photochemistry: Homolysis of the Mn−Cl Bond in the mer-Mn(Cl)(CO)3(α-diimine) Complex and Its Absence in the fac-Isomer

Homolytic breaking of the axial metal−Cl bond is not observed upon irradiation at 488 nm of either fac-Mn(Cl)(CO)3(α-diimine) or the parent Mn(Cl)(CO)5. Surprisingly, it does occur for the equatorial Mn−Cl bond in several mer-Mn(Cl)(CO)3(α-diimine). Using DFT calculations it is shown that this photochemical homolysis can only be understood if strong relaxation of the metal fragment occurs concurrent with the equatorial Cl departure, releasing sufficient energy to make the photodissociation energetically possible. The unrelaxed metal fragment with an equatorial vacancy would be very unstable (by 116 kJ/mol) with respect to the relaxed fragment with an axial vacancy. The migration of an axial CO to the equatorial site invoked in the proposed photodissociation mechanism does not occur on the potential energy surface of the photoactive excited state, which is bound in the Mn−Cl dissociation coordinate. It is proposed to occur in a continuum state (above the asymptotic energy) of the ground-state potential energy surface that is in resonance with the photoactive excited state. The possible importance of this mechanism for TM complex photochemistry, where rearrangement of ligands may often occur upon photodissociation, is stressed.

Ab InitioCalculations and Three Different Applications of Unimolecular Rate Theory for the Dissociations of CCl4, CFCl3, CF2Cl2, and CF3Cl

Previously measured Cl dissociation rate constants for CCl4 and CFCl3 were analyzed with three different kinetics modeling calculations. The three models differ in detail but primarily are distinguished by the manner in which the high-pressure limiting rate constant is determined: model 1 involves a calibration to transport properties of the dissociated fragments, model 2 uses a Gorin model with a hindrance parameter, and model 3 requires variational transition state theory on an ab initio reaction path where all low-frequency motion off the path is presumed to be a free rotation. All three models have two adjustable parameters: the dissociation energy E0 and the average energy transferred to the buffer gas 〈ΔE〉down. All three models are found to give comparable fits to the experiment and produce quite similar values for the adjustable parameters. For CCl4, the values are E0 = (68.2 ± 1.2) kcal mol-1 and 〈ΔE〉down = (750 ± 125) cm-1. For CFCl3, the values are E0 = (76.5 ± 0.5) kcal mol-1 with 〈ΔE〉down = ...

Infrared Multiphoton Dissociation Mechanism of 1,2-Dichlorotrifluoroethane in a Molecular Beam

Abstract The mechanism of the infrared multiphoton dissociation of the title molecule has been studied using photofragmentation translational spectroscopy. The HCl elimination and the C–Cl bond rupture occurred competitively as primary dissociation channels. The HCl elimination reaction accounts for % of the total primary dissociation yields at 16 ± 3 J cm−2. This is consistent with the branching ratio calculated by RRKM theory. A C=C bond rupture of the CF2CFCl molecules and C–Cl bond rupture of C2HF3Cl radicals were also observed as secondary photodissociation processes. The average excitation energy of the dissociating 1,2-dichlorotrifluoroethane molecules was found to be 14—36 kcal mol−1 (1 kcal mol−1 = 4.184 kJ mol−1) above the C–Cl dissociation threshold of the molecules by comparing the observed center-of-mass translational energy distribution for the C–Cl bond rupture with that calculated by Rice–Ramsperger–Kassel–Marcus (RRKM) theory.

Ab initio MO studies on dissociative electron attachment of vinyl chloride. The simplest chlorine-containing hydrocarbon having a CC π system

The potential energy surface of the vinyl chloride radical anion, which is the simplest chlorine-containing hydrocarbon anion having a CC π system, has been studied by ab initio molecular orbital theory. The calculated results indicate that the initial 2Π state is rapidly changed to the repulsive 2Σ state by a CCl out-of-plane deformation during CCl dissociation. While no metastable 2Π state is found on the potential energy surface, the same kind of planar metastable weak complex is found on the minimum energy path as in the case of the methyl chloride radical anion. The differences and similarities between the CCl dissociation minimum energy paths of the vinyl chloride and methyl chloride radical anions are also discussed.

Infrared multiphoton dissociation of 2-chloro-1,1,1,2-tetrafluoroethane in a molecular beam

Mechanism and dynamics of the infrared multiphoton dissociation of 2-chloro-1, 1,1,2-tetrafluoroethane have been studied using a photofragmentation translational spectroscopy. The molecule dissociates competitively through three-centered elimination of HCl and C–Cl bond rupture. The HCl elimination reaction accounts for 74% of the total primary dissociation yields. The center-of-mass translational energy distribution for the HCl elimination indicates that an exit barrier of several kcal/mol exists along the reaction coordinate on the potential energy surface. The infrared multiphoton dissociation of CF3CF produced by the HCl elimination from CF3CHClF also occurs as a secondary process through its dissociation into two CF2 molecules. The average excitation energy of dissociating CF3CHClF has been determined to be about 20 kcal/mol above the C–Cl dissociation threshold of the molecule by comparing the observed center-of-mass translational energy distribution for the C–Cl bond rupture reaction with that calculated by Rice–Ramsperger–Kassel–Marcus (RRKM) theory.

Ab initio characterization of peroxyhypochlorous acid: implications for atmospheric chemistry

The equilibrium structure, dipole moment, harmonic vibrational frequencies, and infrared intensities of ClOOH are determined using the CCSD(T) (singles and doubles coupled-cluster theory plus a perturbational estimate of the effects of connected triple excitations) electronic structure method in conjunction with a TZ2P (triple xi plus double polarization) basis set. The heat of formation of CIOOH is determined (using two different isodesmic reactions) to be +1.5 +/- 1 kcal/mol at 0 K or +0.2 +/- 1 kcal/mol at 298.15 K. Using the computed heat of formation, we examined the stability of ClOOH with respect to the ClO + OH, ClOO + H, and HOO + Cl dissociation limits. Since ClOOH is found to be quite stable, it is argued that the chemistry of ClOOH should be included in any accurate modeling of the stratosphere.

The Dynamic Behaviour and NMR Solution structures of complexes of the type (Bisphosphine)(cycloocta‐1,5‐diene)iridium(I) and the X‐ray crystal structure of (cycloocta‐1,5‐diene)((−)‐norphos)iridium(I) hexafluorophosphate

AbstractA square‐planar coordination geometry was found for the complex [Ir(cod){(−)‐norphos}][PF6] (1b[PF6]; cod = cylcoocta‐1,5‐diene and (−)‐norphos = [(2R,3R)‐8‐9‐10‐trinorborn‐5‐ene‐2,3‐diyl]bis(diphenylphosphine)) in the solid state by X‐ray diffraction. Crystal data: monoclinic, space group P21, a = 10.751 (6), b = 18.669(14), c = 12.037(8) Å, β = 114.82(5)°, Z = 2. A total structural assignment including the configurational and conformational aspects of this and the related compounds [Ir(bishosphine)(cod)]X (bisphosphine = (−)‐chiraphos = (2S,3S)‐2,3‐bis(diphenylphosphino)butane and (−)‐norphos, X = Cl, CF3SO3, or PF6) was carried out in solution by one‐ and two‐dimensional NMR spectroscopy. The complexes containing the CF3SO3− and PF6− anions are four‐coordinate cations with square‐planar geometry, whereas the chlorides are five‐coordinate neutral compounds showing solvent‐dependent dynamic behaviour. In toluene, two diastereoisomers of [IrCl(cod){(−)‐norphos}] (2b) exist and interconvert slowly at room temperature. This interchange is fast in CDCl3 solution, and it is likely to involve Cl dissociation and the formation of the cation [Ir(cod){(−)‐norphos}]+ as an intermediate.

Cl Subcomponent Complexes and C2 Cleavage in Active Systemic Lupus erythematosus

We studied the activation and C1 inactivator-dependent dissociation of the first component of complement, the C1q(C1r-C1s)2 complex, in relation to recruitment of the classical activation pathway in the circulation of 24 patients with systemic lupus erythematosus (SLE). The patients were divided into three groups on a clinical basis, and were investigated during flares of disease activity. Group I had mild symptoms, group II major extrarenal manifestations, and group III manifest renal disease. High serum concentrations of trimer complexes containing C1 inactivator, activated C1r and zymogen C1s(C1 IA-C1r-C1s) were found in the majority of the patients. Some patients with high C1 IA-C1r-C1s concentrations showed no evidence of classical pathway activation, indicating that C1 activation was controlled by the action of C1 IA at the C1r level. By contrast, formation in serum of tetramer complexes in which C1 IA was firmly bound to both C1r and C1s (C1 IA-C1r-C1s-C1 IA) was associated with C2 and C3 cleavage in EDTA plasma, and with manifest hypocomplementemia. Low C1 IA-C1r-C1s-C1 IA values were observed in conjunction with substantial C2 cleavage in a few patients. Thus, C1 IA-C1r-C1s-C1 IA may not always be a sensitive indicator of classical pathway activation. Efficient recruitment of the classical pathway was related to disease severity, with some overlap between the clinical groups. In conclusion, C1 dissociation with formation of C1 IA-containing complexes was consistently found in patients with active SLE. The results suggested that C1 IA-dependent control of C1 activation was of biological significance in the disease.

Monoclonal Antibodies to Complement Components Without the Need of Their Prior Purification. II. Antibodies to Mouse C3 And C4

Rat monoclonal antibodies (MAbs) to mouse complement components C3 and C4 were produced by immunizing rats with cell-bound C3 and C4. This principle involves: a) using mouse thymocytes coated with syngeneic rat antibody isotypes that show high affinity to C1q, b) the intercalation of C1q from serum and c) the subsequent activation of the classical complement pathway leading to deposition of cell-bound complement components. Screening for anti-complement antibodies was performed on antibody coating microtiter plates with mouse serum as source of complement. The reactivity of the MAbs was determined by variations of the ELISA screening system using EDTA-serum to inhibit complement activation by C1 dissociation, serum rendered deficient of functionally active C3 by treatment with cobra venom factor (CVF) or serum of genetically C5-deficient mice. The specificity of the MAbs was confirmed by affinity chromatography followed by SDS-PAGE and immunoblotting. We were able to establish a panel of anti-C3 and anti-C4 MAbs of various isotypes.

228.8 nm Photolysis of 1,1,1-trichloroethane

Room temperature photolysis of up to 100 Torr of 1,1,1-trichloroethane in the presence of iodine at 228.8 nm, and 30 Torr 1,1,1-trichloroethane, also in the presence of iodine, and with added CF 4 was investigated. The main photodecomposition pathways are molecular elimination of HCl and CCl bond scission, with Φ HCl/Φ CCl approximately equal to 3 at low pressure. The yields of CCl 2CH 2 formed by HCl elimination decrease with increasing pressure, while the products of radical reactions resulting from CCl dissociation are independent of pressure. The results are consistent with Cl atom loss occurring from the σ* state populated by 228.8 nm photoexcitation, while HCl elimination does not occur from either σ* or the ground state, and must proceed through the intermediacy of another excited state, suggested to be a triplet state. Analysis of the pressure dependence of CH 2CCl 2 yields gave the average energy transferred per gas kinetic collision as 17 ± 5 kcal mol −1 for self-deactivation and 16 ± 6 kcal mol −1 for deactivation by CF 4.

Energy levels of iodine monochloride near the first dissociation limit

Absorption transitions to vibrational levels close to the A state dissociation limit of ICI have been examined using a two-photon sequential absorption technique. The discrete rotational structures of I 37 Cl bands to within 0.7 cm −1 of the limit have been selectively excited and analysed. A value of 17557.514 ± 0.030 cm −1 has been obtained for the I( 2P o 3 2 ) + Cl( 2P o 3 2 ) dissociation energy D e, relative to the minimum of the ICI ground state potential well. The two-photon technique can be used to excite and display separately the high resolution absorption spectra of different isotopic species of a molecule which are contained in a mixture.

Photolyse en phase gazeuse de quelques méthanes halogénés a λ ⩾ 185 nm

The photolysis of CCl 4, CF 2Cl 2 and CFCl 3 was studied at 22°C in the prescence of Br 2 in the pressure range 10–150 Torr. The competition between the primary photochemical processes, CCl dissociation, CCl 2 or C2Cl dissociation, is discussed. At long wavelengths (λ ⩾ 185 nm) the elimination of molecular Cl 2 occurs and this increases with the quantum energy. Two processes seem responsible for the formation of Cl 2 and one of these is pressure dependent.