Dissociation Of HCl Research Articles

Selective dissociation of HCl in Kr from vibrational overtones

Vibrational levels v=1, 2, and 3 of HCl in Kr matrices are populated with tunable IR radiation and the excited molecules are dissociated by UV excitation to the repulsive A 1∏ state. Cl fragments are recorded by laser induced fluorescence of Kr2Cl and dissociation rates are determined from the increase in LIF with UV dose. The enlarged UV Franck–Condon range for overtones allows the study of cage exit of H fragments with small kinetic energy Ekin. A threshold at Ekin=1.4 eV and a steep rise indicate a predominant sudden exit. Monomers, different initial rotational states and transients in the relaxation cascade are preselected with overtone excitation and the feasibility of a discrimination between isotopes, aggregates, and local structures is illustrated.

Redox-linked protonation of cytochrome c oxidase: the effect of chloride bound to CuB.

The effect of bound Cl- on the redox-linked protonation of soluble beef heart cytochrome c oxidase (CcOX) has been investigated at pH 7.3-7.5 by multiwavelength stopped-flow spectroscopy, using phenol red as the pH indicator in an unbuffered medium. Reduction by Ru-II hexamine of the Cl-bound enzyme is associated with an overall apparent uptake of 1.40 +/- 0.21 H+/aa3, whereas 2.28 +/- 0.36 H+/aa3 is taken upon reduction of the Cl-free enzyme. Bound Cl- has no effect on the extent of H+ uptake coupled to heme a reduction (0.59 +/- 0.06 H+/aa3), but significantly decreases (by approximately 0.9 H+/aa3) the apparent stoichiometry of H+ uptake coupled to heme a3-Cu(B) reduction, by eliminating the net H+ uptake linked to Cu(B) reduction. To account for these results, we propose that, after the transfer of the first electron to the active site, reduction of Cu(B) is associated with Cl- dissociation, addition of a H+, and diffusion into the bulk (with subsequent dissociation) of HCl. In the physiologically competent Cl--free enzyme, an OH- likely bound to oxidized Cu(B) is protonated upon arrival of the first electron, and dissociates as H2O. The relevance of this finding to the understanding of the enzyme mechanism is discussed.

Large enhancement in dissociative electron attachment to HCl adsorbed on H2O ice via transfer of presolvated electrons

We report that dissociative electron attachment (DEA) to HCl is strongly enhanced by adsorption on the surface of H2O ice. The absolute DEA cross section at ∼0 eV for HCl adsorbed on ice is measured to be ∼4.0×10−15 cm2, which is two orders of magnitude higher than in the gas phase. This enhancement is essentially due to electron transfer from precursor states of the solvated electron in ice to an unfilled molecular orbital of HCl followed by its dissociation. This study indicates that electron-induced dissociation may be a significant process leading to HCl dissociation on ice surfaces in polar stratospheric clouds due to ionization by cosmic rays.

Electrospray mass spectrometry of trans-[Ru(NO)Cl(dpaH) 2] 2+ (dpaH=2,2′-dipyridylamine) and ‘caged NO’, [RuCl 3(NO)(H 2O) 2]: loss of HCl and NO from positive ions versus NO and Cl from negative ions

The positive ion electrospray mass spectrometry (ESI-MS) of trans-[Ru(NO)Cl)(dpaH) 2]Cl 2 (dpaH=2,2′-dipyridylamine), obtained from the carrier solvent of H 2O–CH 3OH (50:50), revealed 1+ ions of the formulas [Ru II(NO +)Cl(dpaH)(dpa)] + ( m/ z=508), [Ru IIICl(dpaH)(dpa −)] + ( m/ z=478), [Ru II(NO +)(dpa) 2] + ( m/ z=472), [Ru III(dpa) 2] + ( m/ z=442), originating from proton dissociation from the parent [Ru II(NO +)Cl(dpaH) 2] 2+ ion with subsequent loss of NO (17.4% of dissociative events) or loss of HCl (82.6% of dissociative events). Further loss of NO from the m/ z=472 fragment yields the m/ z=442 fragment. Thus, ionization of the NH moiety of dpaH is a significant factor in controlling the net ionic charge in the gas phase, and allowing preferential dissociation of HCl in the fragmentation processes. With NaCl added, an ion pair, {Na[Ru II(NO)Cl(dpa) 2]} + ( m/ z=530; 532), is detectable. All these positive mass peaks that contain Ru carry a signature ‘handprint’ of adjacent m/ z peaks due to the isotopic distribution of 104Ru, 102Ru, 101Ru, 99Ru, 98Ru and 96Ru mass centered around 101Ru for each fragment, and have been matched to the theoretical isotopic distribution for each set of peaks centered on the main isotope peak. When the starting complex is allowed to undergo aquation for two weeks in H 2O, loss of the axial Cl − is shown by the approximately 77% attenuation of the [Ru II(NO +)Cl(dpaH)(dpa)] + ion, being replaced by the [Ru II(NO +)(H 2O)(dpa) 2] + ( m/ z=490) as the most abundant high-mass species. Loss of H 2O is observed to form [Ru II(NO +)(dpa) 2] + ( m/ z=472). No positive ion mass spectral peaks were observed for RuCl 3(NO)(H 2O) 2, ‘caged NO’. Negative ions were observed by proton dissociation forming [Ru II(NO)Cl 3(H 2O)(OH)] − in the ionization chamber, detecting the parent 1− ion at m/ z=274, followed by the loss of NO as the main dissociative pathway that produces [Ru IIICl 3(H 2O)(OH)] − ( m/ z=244). This species undergoes reductive elimination of a chlorine atom, forming [Ru IICl 2(H 2O)(OH)] − ( m/ z=208). The ease of the NO dissociation is increased for the negative ions, which should be more able to stabilize a Ru III product upon NO loss.

Theoretical study of the protolytic dissociation of HCl in water clusters

Reaction mechanisms for the acidic dissociation of HCl in water clusters are considered. Intermediates in the reaction are obtained from stationary points on the potential energy surface of the systems HCl–(H2O)n with n=4 and 5. These points have been determined by the B3LYP density functional method in an aug-cc-pVDZ atomic orbital (AO) basis. The total energies of the stationary points are checked by the coupled cluster single-double-triple [CCSD(T)] method in the same AO basis. For the case of n=4 a multibody analysis of the interaction energies is performed by the CCSD(T) method as well as by symmetry adapted perturbation theory. The clusters have a completely dissociated form as their energetically lowest minimum.

Two possible reaction pathways for the formation of a ruthenium carbene complex by addition of acetylene to [RuH2Cl2(PH3)2]: a quantum chemical study

Abstract [RuH2Cl2(PiPr3)2] reacts with terminal alkynes to give the vinylidene complex [RuCl2(CCHR)(PiPr3)2]. As a side product the carbene complex [RuCl2(CHR)(PiPr3)2] is formed. The formation of the vinylidene compound has been studied widely and is well understood whereas the reaction mechanism that leads to the carbene complex is still unclear. We have studied two possible reaction paths at the B3LYP level of theory: on the one hand the addition of acetylene and two subsequent 1,3-H shifts from the metal center to the C2 carbon of the acetylene ligand; on the other hand the dissociation of HCl from the starting compound, rearrangement of acetylene to vinyl and the formation of the carbene by addition of HCl. Both reaction paths have been found to be possible. The former can be understood as a 1,3-H shift followed by a 1,2-H shift due to the unusual η2 coordination mode of the vinyl intermediate. The latter proceeds via protonation of the vinyl ligand and addition of Cl− to the metal center.

Observation of a continuum-continuum interference hole in ultrafast dissociating core-excited molecules.

The femtosecond dissociation of HCl after core excitation has been studied through the resonant Auger decay. The spectra contain contributions from decay occurring at both "molecular" and "atomic" internuclear distances. We have observed a new interference mechanism in these spectra: An atomic spectral line develops into a negative spectral contribution, a "hole," when detuning the excitation energy from the maximum of the Cl2p(-1)sigma(*) resonance. Resonant x-ray scattering theory quantitatively explains this behavior as due to a novel destructive continuum-continuum interference between molecular and atomic contributions to the Auger decay.

A systematic study of the ancillary effect of different molecules on the acetonitrile hydrolysis

Transition states for the acetonitrile hydrolysis reaction (including hydration and isomerization steps) catalyzed by different proton donors have been studied by different quantum chemical methods (RHF, DFT and MP2), using different cluster models. HCl(H2O), HCl(H2O)2, (H2O)3, HCl(H2O)3 and H3O+(H2O)2 clusters were used for the hydration transition state and H2O, HCl, HCl(H2O) and H3O+ for the tautomerism (intramolecular proton migration) transition state. All ab initio methods showed complete qualitative agreement in the hydration transition state geometry. In the tautomerization case, inclusion of electron correlation was required to obtain the correct transition state geometry. The calculated activation barrier energies changed considerably for the different clusters. An ancillary effect of water and HCl was observed. In complementary studies, the effect of polar environment on HCl dissociation was analyzed via embedding calculations and by using HCl(H2O)5 cluster. Although HCl dissociates in H2O, it was found not to protonate acetonitrile.

Catalysis of the reaction HCl+HOCl→H2O+Cl2 on an ice surface

Abstract Ab initio molecular dynamics simulations are performed for the reaction between HCl and HOCl on an ice surface. The interaction between the two chlorine atoms, and the role played by the acidic dissociation of HCl, are elucidated. The catalytic effect of the ice surface is reproduced and explained.

Proton Transfer Processes in Water Clusters

AbstractElucidation of some of the basic mechanisms and principles of proton transfer in clusters is achieved by systematic evaluation of experimental and theoretical results that were obtained throughout the last decade from the authors' studies on ionic water clusters and, more recently, on protonated rare gas clusters. Among the processes studied are: Blackbody radiation‐induced, stepwise desolvation of clusters when stored in an ion trap; ionic dissociation and recombination of HCl in water clusters; metal oxidation in aqueous clusters; formation of nascent hydrogen in reactions of solvated metal cations with acids; and the behavior of ionic water clusters as acidic or basic micro‐solutions. Infrared spectroscopic studies of size‐selected water clusters are expected to advance our knowledge of structure and dynamics of water clusters and of excess protons therein.

Heterogeneous uptake of HCl by sulfuric acid solutions

The uptake of HCl molecules by aqueous sulfuric acid droplets was measured in the temperature range 230–264 K at 39, 49, 54, 59, and 69 wt % acid and as a function of time (2–15 ms). These experiments utilized a droplet train apparatus in which a stream of monodisperse droplets (120–250 μm in diameter) is passed through a low‐pressure flow containing HCl(g). The droplet area is changed in a step‐wise fashion, while the HCl(g) density is continuously monitored by infrared absorption. The uptake coefficient is obtained from the measured change in the HCl density. The product of H*Dl1/2 (H*, solubility; Dl, liquid phase diffusion coefficient) and the mass accommodation coefficient a of the species as a function of temperature and sulfuric acid concentration were obtained from the uptake coefficient. The good agreement of measured and modeled H*Dl1/2 values validates current formulations of HCl reactivity in stratospheric aerosols. While the solubility of HCl decreases steeply with sulfuric acid concentration because increasing acidity reduces the dissociation of HCl into H+ and Cl− in solution, the mass accommodation coefficient is independent of acid concentration in the region studied. As with previously studied species, α is inversely proportional to temperature increasing from ∼0.06 at 294 K to near unity at ∼230 K. The mass accommodation coefficient is well expressed in terms of an observed Gibbs free energy as α/(1 − α) = exp (−ΔGobs/RT), suggesting that the clustering model for the accommodation process is applicable in this case as well. The mass accommodation measurements are well fitted by the parameters ΔHobs = −13.8±0.9 kcal mol−1 and ΔSobs = −52.2±0.3 cal mol−1 K−1. Under stratospheric conditions α for HCl is unity. Implications of the HCl uptake studies for atmospheric chemistry are examined.

Structure of Concentrated HCl Solutions

A pentagonal ring is suggested as the basic structural unit of HCl(H2O)6 and (HCl)2(H2O)6 in solution. Modeled after the X-ray structure of a caged H13O6+ compound, it contains bridged H5O2+ and Cl- ions, each with a coordination number around four. In the most concentrated HCl solutions, one of the ligands solvating Cl- is HCl, giving rise to a bichloride moiety. The proposed structure explains qualitatively the stoichiometry, ion mobility, (IR, Raman) spectroscopy and (X-ray, neutron) diffraction data of concentrated HCl solutions and provides a semiquantitative fit to the X-ray radial distribution function. The ring structure is a likely candidate for the structure of protonated HCl clusters in the gas phase and for the contact ion-pair formed following HCl dissociation in solution.

Evidence of enhanced atomic hydrogen production with halogens in diamond MWPACVD

Abstract Optical emission spectroscopy measurements and exhaust gas mass spectrometry were used for diagnostics in a microwave plasma assisted chemical vapor deposition reactor for diamond growth. An actinometry technique was used to study the atomic hydrogen generation with additions of CF4 and CCl2F2 in the feed gas. The relative emission intensity of the atomic hydrogen line Hα (656.3 nm) was measured with CF4 and CCl2F2 concentrations in the range of 0–5%. The results show an increase in atomic hydrogen concentration up to 80 and 200% when CF4 and CCl2F2 are added in the mixture, respectively. The -ideal actinometry conditions and the validity of the observed increase of atomic hydrogen with an change in electron temperature and density in the plasma was investigated. The dissociation process of CF4 and CCl2F2 associated with HF and HCl formation were discussed by measurements of stable species by mass spectrometry of the exhaust gas. The increase of the atomic hydrogen concentration is correlated with the easy HCl and/or HF dissociation by electron-impact.

Hybrid quantum and classical mechanical Monte Carlo simulations of the interaction of hydrogen chloride with solid water clusters

Monte Carlo simulations using a hybrid quantum and classical mechanical potential were performed for crystal and amorphous-like HCl(H 2O) n clusters ( n≤24). The subsystem composed by HCl and one water molecule was treated within density functional theory and a classical force field was used for the rest of the system. Simulations performed at 200 K suggest that the energetic feasibility of HCl dissociation strongly depends on its initial placement within the cluster. An important degree of ionization occurs only if HCl is incorporated into the surface. We observe that local melting does not play a crucial role in the ionization process.

Femtosecond Dissociation of Core-Excited HCl Monitored by Frequency Detuning

Core excitation of gaseous HCl to the dissociative ${\ensuremath{\sigma}}^{*}$ state leads to a deexcitation spectrum with two qualitatively different contributions---a broad molecular background and narrow atomic lines. The experiment demonstrates that the ratio of integrated intensities from these two contributions depends crucially on the excitation frequency; the molecular background is enhanced with increased detuning of the photon frequency. This is discussed in a time-dependent model, using the concept of an effective duration time for the resonant x-ray scattering process.

Experimental study of dissociation of HCl from 350 to 500°C and from 500 to 2500 bars: Thermodynamic properties of HCl°(aq)

New values of the dissociation constants of HCl° referring to low density supercritical solutions and near-critical temperatures of water (350–500°C and 500–2500 bars) have been obtained based on comparison of AgCl(s) solubility in NaCl or KCl solutions with Ag(s) solubility in HCl + NaCl or KCl solutions at controlled hydrogen fugacities. During the course of this study the thermodynamic properties of AgCls− were refined with the aid of the revised HKF equation of state (Tanger and Helgeson, 1988). The dissociation constants of HCl° obtained in the present work are higher than that found from electrical conductance measurements (Frantz and Marshall, 1984) by more than an order of magnitude at pressures of about 500 bars, but the difference becomes smaller as the pressure increases. Both sets of dissociation constants agree well at high pressures where the isothermal compressibility of water is less than about 1.42·10−4 bar−1.To reliably compare experimental data obtained by different methods, the Redlikh-Kwong equation of state was applied to available literature data as well as to the results of this study. Finally, the standard state thermodynamic properties and HKF parameters for HCl°(aq) were established. These results allow extrapolation of the thermodynamic properties of HCl°(aq) and consequently the HCl dissociation constant up to 700°C and 5000 bars.

A theoretical study of the ionic dissociation of HF, HCl, and H2S in water clusters

The ionic dissociation of HF, HCl, and H2S in water is examined using density functional theory (DFT), Hartree–Fock (HF), and Mo/ller–Plesset theory to second order (MP2). The calculations show that HF, HCl, and H2S form fully dissociated stable clusters with four water molecules. Each cluster appears to be stabilized by the formation of six hydrogen bonds. These calculations also indicate that a minimum of four water molecules are required to form stable structures in which positive and negative ions coexist in the cluster. The hydrogen transfer between the acid and water molecules is very similar to the mechanism proposed for hydrogen transfer in water solutions. The binding energies of the hydrated hydrofluoric acid, hydrated hydrochloric acid, and hydrated hydrogen disulfide estimated with B-LYP are 37.51, 41.17, and 20.68 kcal/mol, respectively.

Electron-impact dissociation of HCl: Translational energy and angular distributions of excited hydrogen atoms

Electron-impact dissociation of HCl for the formation of excited hydrogen atoms (n=4) has been investigated by measuring Doppler profiles of the Balmer lines and their angular dependence at a high optical resolution using a Fabry–Perot interferometer. The translational energy distribution (TED) and the angular difference Doppler profile were obtained. There are five major dissociation processes for the formation of H* (n=4). The threshold energy and the TED peak of the five components are (1) 19 and 2.5; (2) 25 and 7.2; (3) 29 and 1.7; (4) 36 and 5.1; and (5) ≳40 and 8–12 eV, respectively. Formation of components 1 and 4 is anisotropic and parallel with respect to the electron beam. Component 1 should be produced by predissociation through the Rydberg states with the Σ symmetry converging to either the 4Σ or 2Σ− state and then those converging to the A 2Σ+ state. The asymmetry parameter (β) of component 1 was determined to be about 0.62, and the intermediate excited state for the formation of component 1 has a lifetime equal to the rotational period. Component 2 would be produced through the Rydberg states converging to the 2Π state. Component 3 would be produced through high-lying doubly excited Rydberg states converging to either the (4)2Π or (4)2Σ+ state. Component 4 should be produced through doubly excited repulsive states with the Σ symmetry.

Resonances in the photodissociation of HCl in the Ar–HCl van der Waals complex: How prominent are they?

We investigate the cage effect in the ultraviolet (UV) photodissociation of the Ar...HCl van der Waals complex, especially the possibility of resonance structures caused by trapping of the hydrogen atom between its heavy partners as recently highlighted by Garcia-Vela and Gerber [J. Chem. Phys. 98, 427 (1993)]. The dynamics is described by solving the time-dependent Schrödinger equation employing the standard Jacobi coordinates which are routinely used for triatomic systems. Due to the large size of the required grid, exact three-dimensional (3D) wave packet calculations are extremely time consuming and could be followed up to 20 fs only. This time is sufficient for calculating the absorption spectrum, but too short for determining the final kinetic energy distributions of the fragment atoms. Therefore, the photodissociation dynamics is mainly treated in a vibrationally sudden approximation, in which the dynamical calculations are performed for a range of fixed ArCl bond distances, and the results averaged over this bond length. 3D classical trajectory calculations show that the energy transfer out of the dissociative HCl mode is very weak (∼5% of the total energy), supporting the application of the sudden approximation. In this approximation, both the absorption spectrum and the kinetic energy distribution associated with the dissociating HCl motion exhibit very weak diffuse structures (resonances) which, following the work of Garcia-Vela and Gerber, can be assigned to the transient vibrational motion of hydrogen between Ar and Cl. However, in our calculations these structures are much less pronounced than in the work of Garcia-Vela and Gerber. The very small amplitudes of the resonance features indicate that trapping in the dissociation of HCl in Ar...HCl is marginal, and much less important than suggested by the previous studies of Garcia-Vela et al. Furthermore, in contrast to the work reported by Garcia-Vela et al., we do not find any evidence for the narrow, irregular features superimposed on the resonance structures.

Self-consistent modelling of X-ray preionized XeCl-laser discharges

In this theoretical work a 0-D model for a self-sustained X-ray preionized XeCl-laser discharge is presented. The model is self-consistent in the sense that it simultaneously solves, contrarily to the usual decoupling procedure, the Boltzmann equation for electrons, the kinetic equations for excited and ionic species, the equations for the electrical circuit and the laser photon density. It includes a rather complete kinetics of HCl(v) vibrational excitation, dissociation and dissociative attachment. The influence of electron collisions with excited species and of e-e Coulomb collisions on the plasma parameters and transport coefficients is discussed. Some evidence of the non-stationary equilibrium between the electron distribution and the reduced electric field E/N is given. Results of the model are compared with experimental ones corresponding to a XeCl-laser discharge driven by a L-C inversion circuit. The model predicts well the main trends for the variation of the laser energy in a large range of experimental conditions. The discrepancy between experiment and model for absolute values of the laser energy is discussed.