Tunneling chemical reactions D + H2 --> DH + H and D + DH --> D2 + H in solid D2-H2 and HD-H2 mixtures: an electron-spin-resonance study

Tunneling chemical reactions D + H2 --> DH + H and D + DH --> D2 + H in solid HD-H2 and D2-H2 mixtures were studied in the temperature range between 4 and 8 K. These reactions were initiated by UV photolysis of DI molecules doped in these solids for 30 s and followed by measuring the time course of electron-spin-resonance (ESR) intensities of D and H atoms. ESR intensity of D atoms produced by the photolysis decreases but that of H atoms increases with time. Time course of the D and H intensities has the fast and slow processes. The fast process, which finishes within approximately 300 s after the photolysis, is assigned to the reaction of D atom with one of its nearest-neighboring H2 molecules, D(H2)n(HD)(12-n) --> H(H2)(n-1)(HD)(13-n) or D(H2)n(D2)(12-n) --> H(HD)(H2)(n-1)(D2)(12-n) for 12 > or = n > or = 1. Rate constant for the D + H2 reaction between neighboring D atom-H2 molecule pair is determined to be (7.5 +/- 0.7) x 10(-3) s(-1) in solid HD-H2 and (1.3+/-0.3) x 10(-2) s(-1) in D2-H2 at 4.1 K, which is very close to that calculated based on the theory of chemical reaction in gas phase by Hancock et al. [J. Chem. Phys. 91, 3492 (1989)] and Takayanagi and Sato [J. Chem. Phys. 92, 2862 (1990)]. This rate constant was found to be independent of temperature up to 7 K within experimental error of +/-30%. The slow process is assigned to the reaction of D atom produced in a cage fully surrounded by HD or D2 molecules, D(HD)12 or D(D2)12. This D atom undergoes the D + DH reaction with one of its nearest-neighboring HD molecules in solid HD-H2 or diffuses to the neighbor of H2 molecules to allow the D + H2 reaction in solid HD-H2 and D2-H2. The former is the main channel in solid HD-H2 below 6 K where D atoms diffuse very slowly, whereas the latter dominates over the former above 6 K. Rate for the reactions in the slow process is independent of temperature below 6 K but increases with the increase in temperature above 6 K. We found that the increase is due to the increase in hopping rate of D atoms to the neighbor of H2 molecules. Rate constant for the D + DH reaction was found to be independent of temperature up to 7 K as well.     

The Reaction X + Cl2→XCl + Cl (X = Mu,H, D). II. Comparison of experimental data with theoretical results derived from a new potential energy surface

We consider experimental implications for the Mu + Cl₂, H + Cl₂, and D + Cl₂ reactions of the extended London—Eyring—Polanyi—Sato (LEPS) potential energy surface derived from experimental data in paper I. In the present calculations, it is necessary to make additional implicit and explicit assumptions concerning the three-dimensional (3D) nature of the potential surface, since the inversion procedure of paper I yields information only on the collinear (1D) part of the surface. We have performed accurate 1D quantum calculations of reaction probabilities, which are then transformed into 3D by an information theoretic 1D → 3D transformation incorporating a constraint to allow for angular momentum transfer effects in light+heavy—heavy atom reactions. This procedure implicitly accounts for the 3D nature of the potential surface. The calculated vibrational and vibrotational product distributions are in good agreement with those determined in thermal chemiluminescence experiments. The Sato parameters for the 1D surface also define a full 3D surface. This is used as an approximation to the true surface, and its properties are explored in 3D quasiclassical trajectory calculations. Comparison is made for the H and D reactions with available chemiluminescence, molecular beam and kinetic experimental data for differential and total reaction cross sections, energy disposal, rate coefficients and Arrhenius parameters. Some kinetic isotope effects in the Mu, H, and D reactions are discussed using vibrationally adiabatic theory. Comparison is also made with results from other calculations in the literature for the H + Cl₂ and D + Cl₂ reactions.     

Thermochemistry and accurate quantum reaction rate calculations for H2/HD/D2 + CH3

Accurate quantum-mechanical results for thermodynamic data, cumulative reaction probabilities (for J = 0), thermal rate constants, and kinetic isotope effects for the three isotopic reactions H2 + CH3 --> CH4 + H, HD + CH3 --> CH4 + D, and D2 + CH3 --> CH(3)D + D are presented. The calculations are performed using flux correlation functions and the multiconfigurational time-dependent Hartree (MCTDH) method to propagate wave packets employing a Shephard interpolated potential energy surface based on high-level ab initio calculations. The calculated exothermicity for the H2 + CH3 --> CH4 + H reaction agrees to within 0.2 kcal/mol with experimentally deduced values. For the H2 + CH3 --> CH4 + H and D2 + CH3 --> CH(3)D + D reactions, experimental rate constants from several groups are available. In comparing to these, we typically find agreement to within a factor of 2 or better. The kinetic isotope effect for the rate of the H2 + CH3 --> CH4 + H reaction compared to those for the HD + CH3 --> CH4 + D and D2 + CH3 --> CH(3)D + D reactions agree with experimental results to within 25% for all data points. Transition state theory is found to predict the kinetic isotope effect accurately when the mass of the transferred atom is unchanged. On the other hand, if the mass of the transferred atom differs between the isotopic reactions, transition state theory fails in the low-temperature regime (T < 400 K), due to the neglect of the tunneling effect.     

Angular distributions of H-induced HD and D2 desorptions from the Si(100) surfaces

We measured angular distributions of HD and D2 molecules desorbed via the reactions H+DSi(100)-->HD [abstraction (ABS)] and H+DSi(100)-->D2 [adsorption-induced-desorption (AID)], respectively. It was found that the angular distribution of HD molecules desorbed along ABS is broader than that of D2 molecules desorbed along AID, i.e., the former could be fit with cos(2.0+/-0.2) theta, while the latter with cos(5.0+/-0.5) theta. This difference of the angular distributions between the two reaction paths suggests that their dynamic mechanisms are different. The observed cos2 theta distribution for the ABS reaction was reproduced by the classical trajectory calculations over the London-Eyring-Polanyi-Sato potential-energy surfaces. The simulation suggests that the HD desorption along the ABS path takes place along the direction of Si-D bonds, but the apparent angular distribution is comprised of multiple components reflecting the different orientations of D-occupied Si dimers in the (2 x 1) and (1 x 2) double domain structures.     

The mechanism of proton exchange: guided ion beam studies of the reactions, H(H2O)n+ (n=1-4)+D2O and D(D2O)n+ (n=1-4)+H2O

Reactions of protonated water clusters, H(H(2)O)(n) (+) (n=1-4) with D(2)O and their "mirror" reactions, D(D(2)O)(n) (+) (n=1-4) with H(2)O, are studied using guided-ion beam mass spectrometry. Absolute reaction cross sections are determined as a function of collision energy from thermal energy to over 10 eV. At low collision energies, we observe reactions in which H(2)O and D(2)O molecules are interchanged and reactions where H-D exchange has occurred. As the collision energy is increased, the H-D exchange products decrease and the water exchange products become dominant. At high collision energies, processes in which one or more water molecules are lost from the reactant ions become important, with simple collision-induced dissociation processes, i.e., those without H-D exchange, being dominant. Threshold energies of endothermic channels are measured and used to determine binding energies of the proton bound complexes, which are consistent with those determined by thermal equilibrium measurements and previous collision-induced dissociation studies. A kinetic scheme that relies only on the ratio of isomerization and dissociation rate constants successfully accounts for the kinetic energy dependence observed in the branching ratios for H-D and water exchange products in all systems. Rice-Ramsperger-Kassel-Marcus theory and ab initio calculations confirm the feasibility and establish the details of this kinetic model.     

H/D isotopic exchange between water molecules at ice surfaces

H/D isotopic exchange between H(2)O and D(2)O molecules was studied at the surface of ice films at 90-140 K by the technique of Cs(+) reactive ion scattering. Ice films were deposited on a Ru(0001) substrate in different compositions of H(2)O and D(2)O and in various structures to study the kinetics of isotopic exchange. H/D exchange was very slow on an ice film at 95-100 K, even when H(2)O and D(2)O were uniformly mixed in the film. At 140 K, H/D exchange occurred in a time scale of several minutes on the uniform mixture film. Kinetic measurement gave the rate coefficient for the exchange reaction, k(140 K)=1.6(+/-0.3) x 10(-19) cm(2) molecule(-1) s(-1) and k(100 K)< or =5.7(+/-0.5) x 10(-21) cm(2) molecule(-1) s(-1) and the Arrhenius activation energy, E(a)> or =9.8 kJ mol(-1). Addition of HCl on the film to provide excess protons greatly accelerated the isotopic exchange reaction such that it went to completion very quickly at the surface. The rapid reaction, however, was confined within the first bilayer (BL) of the surface and did not readily propagate to the underlying sublayer. The isotopic exchange in the vertical direction was almost completely blocked at 95 K, and it slowly occurred only to a depth of 3 BLs from the surface at 140 K. Thus, the proton transfer was highly directional. The lateral proton transfer at the surface was attributed to the increased mobility of protonic defects at the molecularly disordered and activated surface. The slow, vertical proton transfer was probably assisted by self-diffusion of water molecules.     

Effusive molecular beam study of C2H6 dissociation on Pt(111)

The dissociative sticking coefficient for C2H6 on Pt(111) has been measured as a function of both gas temperature (Tg) and surface temperature (Ts) using effusive molecular beam and angle-integrated ambient gas dosing methods. A microcanonical unimolecular rate theory (MURT) model of the reactive system is used to extract transition state properties from the data as well as to compare our data directly with supersonic molecular beam and thermal equilibrium sticking measurements. We report for the first time the threshold energy for dissociation, E0 = 26.5 +/- 3 kJ mol(-1). This value is only weakly dependent on the other two parameters of the model. A strong surface temperature dependence in the initial sticking coefficient is observed; however, the relatively weak dependence on gas temperature indicates some combination of the following (i) not all molecular excitations are contributing equally to the enhancement of sticking, (ii) that strong entropic effects in the dissociative transition state are leading to unusually high vibrational frequencies in the transition state, and (iii) energy transfer from gas-phase rovibrational modes to the surface is surprisingly efficient. In other words, it appears that vibrational mode-specific behavior and/or molecular rotations may play stronger roles in the dissociative adsorption of C2H6 than they do for CH4. The MURT with an optimized parameter set provides for a predictive understanding of the kinetics of this C-H bond activation reaction, that is, it allows us to predict the dissociative sticking coefficient of C2H6 on Pt(111) for any combination of Ts and Tg even if the two are not equal to one another.     

Reaction rates for O + HD → OH + D and O + HD → OD + H

We have calculated reaction rates for the reactions O + HD → OH + D and O + DH → OD + H using improved canonical variational transition state theory and least-action ground-state transmission coefficients with an ab initio potential energy surface. The kinetic isotope effects are in good agreement with experiment. The optimized tunneling paths and properties of the variational transition states and the rate enhancement for vibrationally excited reactants are also presented and compared with those for the isotopically unsubstituted reaction O + H₂ → OH + H. The thermal reactions at low and room temperature are predicted to occur by tunneling at extended configurations, i.e., to initiate early on the reaction path and to avoid the saddle point regions. Tunneling also dominates the low and room temperature reactions for excited vibrational states, but in these cases the results are not as sensitive to the nature of the tunneling path. Overbarrier mechanisms dominate for both thermal and excited-vibrational state reactions for T > 600 K. For the excited-state reaction (with initial vibrational quantum number n > 0) a transition state switch occurs for T > 1000 K for the O + HD(n = 1) → OD + H case and for T > 1500 K for the O + DH(n = 1) → OD + H reaction, and this may be a general phenomenon for excited-state reactions at higher temperature. In the present case the switch occurs from an early variational transition state where the vibrationally adiabatic approximation is expected to be valid to a tighter variational transition state where nonadiabatic effects are probably important and should be included.     

Effective rate constants for the surface reaction between solid methanol and deuterium atoms at 10 K

The surface reactions of CH3OH, CH2DOH, and CHD2OH with cold D atoms at 10 K were investigated using an atomic beam source and FTIR. Methyl-deuterated isotopologues CH2DOH, CHD2OH, and CD3OH were produced by exposure of amorphous solid CH3OH to D atoms at 10 K, and the pseudo-first-order rates for the reactions CH3OH + D --> CH2OH + HD, CH2DOH + D --> CHDOH + HD, and CHD2OH + D --> CD2OH + HD were estimated. The ratios of the reaction rates of the second and third reactions to the first reaction were 0.69 +/- 0.11 and 0.52 +/- 0.14, respectively. The difference in reaction rates is thought to be due to a secondary kinetic isotope effect on the H-abstraction reaction from the methyl side by D atoms.     

About the stability of sulfurous acid (H2SO3) and its dimer

The characterization and isolation of sulfurous acid (H2SO3) have never been accomplished and thus still remain one of the greatest open challenges of inorganic chemistry. It is known that H2SO3 is thermodynamically unstable. In this study, however, we show that a Ci-symmetric dimer of sulfurous acid (H2SO3)2 is 3.5 kcal mol-1 more stable than its dissociation products SO2 and H2O at 77 K. Additionally, we have investigated the kinetic stability of the sulfurous acid monomer with respect to dissociation into SO2 and H2O and the kinetic isotope effect (KIE) on this reaction by transition-state theory. At 77 K, the half-life of H2SO3 is 15 x 10(9) years, but for the deuterated molecule (D2SO3) it increases to 7.9 x 10(26) years. At room temperature, the half-life of sulfurous acid is only 24 hours; however, a KIE of 3.2 x 10(4) increases it to a remarkable 90 years. Water is an efficient catalyst for the dissociation reaction since it reduces the reaction barrier tremendously. With the aid of two water molecules, one can observe a change in the reaction mechanism for sulfurous acid decomposition with increasing temperature. The most likely mechanism below 170 K is via an eight-membered transition-state ring; yet, above 170 K, a mechanism with a six-membered transition state ring becomes the predominant one. For deuterated sulfurous acid, this change in reaction mechanism can be observed at 120 K. Consequently, between 120 and 170 K, different predominant reaction mechanisms occur for the decomposition of normal and deuterated sulfurous acid when assisted by two water molecules. However, the much longer half-life of deuterated sulfurous acid and the stability of the sulfurous acid dimer at 77 K are encouraging for future synthesis and characterization under laboratory conditions.     

Molecular beam investigation of hydrogen dissociation on Si(001) and Si(111) surfaces

The influence of molecular vibrations on the reaction dynamics of H2 on Si(001) as well as isotopic effects have been investigated by means of optical second-harmonic generation and molecular beam techniques. Enhanced dissociation of vibrationally excited H2 on Si(001)2 x 1 has been found corresponding to a reduction of the mean adsorption barrier to 390 meV and 180 meV for nu=1 and nu=2, respectively. The adsorption dynamics of the isotopes H2 and D2 show only small differences in the accessible range of beam energies between 50 meV and 350 meV. They are traced back to different degrees of vibrational excitation and do not point to an important influence of quantum tunneling in crossing the adsorption barrier. The sticking probability of H2 on the 7 x 7-reconstructed Si(111) surface was found to be activated both by H2 kinetic energy and surface temperature in a qualitatively similar fashion as H2/Si(001)2 x 1. Quantitatively, the overall sticking probabilities of H2 on the Si(111) surface are about one order of magnitude lower than on Si(001), the influence of surface temperature is generally stronger.     

Kinetics and mechanistic study of the reaction of cyclic anhydrides with substituted phenols. Structure-reactivity relationships

[reaction: see text] The kinetic of the reactions of phthalic and maleic anhydrides with different substituted phenols (Z-PhOH with Z = H, m-CH(3), p-CH(3), m-Cl, p-Cl, and m-CN) were studied in aqueous solution. Two kinetic processes well separated in time were observed. The fast one is attributed to the formation of the aryl ester in equilibrium with the anhydride and allows the determination of the rate of nucleophilic attack of the phenol on the anhydride (k(-)(A)). From the slow kinetic process, the equilibrium constant for this reaction was determined. The Bronsted-type plots for the nucleophilic attack of substituted phenols on the anhydrides were linear with slopes beta(Nuc) of 0.45 and 0.56 for phthalic and maleic anhydride, respectively. The results are consistent with a mechanism involving rate-determining nucleophilic attack and also with a concerted mechanism. The calculated effective charge on the atoms involved in the reactions and the Bronsted beta values are consistent with a mechanism involving a concerted or enforced concerted mechanism where a tetrahedral intermediate with significant lifetime is not formed along the reaction coordinate. The latter mechanism is preferred over the stepwise process.     

Influence of reagent rotation on (H-, D2) and (D-, H2) collisions: a quantum mechanical study

Time-independent quantum mechanical (TIQM) approach (helicity basis truncated at k = 2) has been used for computing differential and integral cross sections for the exchange reaction H- + D2 (v = 0, j = 0-4) --> HD + D- and D- + H2 (v = 0, j = 0-3) --> HD + H- in three dimensions on an accurate ab initio potential energy surface. It is shown that the j-weighted differential reaction cross section values are in good agreement with the experimental results reported by Zimmer and Linder at four different relative translational energies (Etrans = 0.55, 0.93, 1.16 and 1.48 eV) for (H-, D2) and at one relative translational energy (Etrans = 0.6 eV) by Haufler et al. for both (H-, D2) and (D-, H2) collisions. The j-weighted integral reaction cross section values are in good agreement with the crossed beam measurements by Zimmer and Linder in the Etrans range 0.5-1.5 eV and close to the guided ion beam results by Haufler et al. for (H-, D2) in the range 0.8-1.2 eV. Time-dependent quantum mechanical (TDQM) results obtained using centrifugal sudden approximation are reported in the form of integral reaction cross section values as a function of Etrans in the range 0.3-3.0 eV for both reactions in three dimensions on the same potential energy surface. The TDQM reaction cross section values decline more sharply than the TIQM results with increase in the initial rotational quantum number (j) for the D2 molecules in their ground vibrational state (v = 0) for (H-, D2) collisions. The computed j-weighted reaction cross section values are in good agreement with the experimental results reported by Zimmer and Linder for (H-, D2) collisions and guided ion beam results by Haufler et al. for both (H-, D2) and (D-, H2) collisions for energies below the threshold for electron detachment channel.     

Photoionization dynamics in pure helium droplets

The photoionization and photoelectron spectroscopy of pure He droplets were investigated at photon energies between 24.6 eV (the ionization energy of He) and 28.0 eV. Time-of-flight mass spectra and photoelectron images were obtained at a series of molecular beam source temperatures and pressures to assess the effect of droplet size on the photoionization dynamics. At source temperatures below 16 K, where there is significant production of clusters with more than 10(4) atoms, the photoelectron images are dominated by fast electrons produced via direct ionization, with a small contribution from very slow electrons with kinetic energies below 1 meV arising from an indirect mechanism. The fast photoelectrons from the droplets have as much as 0.5 eV more kinetic energy than those from atomic He at the same photon energy. This result is interpreted and simulated within the context of a "dimer model", in which one assumes vertical ionization from two nearest-neighbor He atoms to the attractive region of the He2+ potential energy curve. Possible mechanisms for the slow electrons, which were also seen at energies below IE(He), are discussed, including vibrational autoionizaton of Rydberg states comprising an electron weakly bound to the surface of a large HeN+ core.     

Guided ion beam and theoretical study of the reactions of Hf+ with H2, D2, and HD

The kinetic energy dependences of reactions of the third-row transition metal cation Hf(+) with H(2), D(2), and HD were determined using a guided ion beam tandem mass spectrometer. A flow tube ion source produces Hf(+) in its (2)D (6s(2)5d(1)) electronic ground state level. Corresponding state-specific reaction cross sections are obtained. The kinetic energy dependences of the cross sections for the endothermic formation of HfH(+) and HfD(+) are analyzed to give a 0 K bond dissociation energy of D(0)(Hf(+)-H)=2.11±0.08 eV. Quantum chemical calculations at several levels of theory performed here generally overestimate the experimental bond energy but results obtained using the Becke-half-and-half-LYP functional show good agreement. Theory also provides the electronic structures of these species and the reactive potential energy surfaces. Results from the reactions with HD provide insight into the reaction mechanisms and indicates that Hf(+) reacts via a statistical mechanism. We also compare this third-row transition metal system with the first-row and second-row congeners, Ti(+) and Zr(+), and find that Hf(+) has a weaker M(+)-H bond. As most third-row transition metal hydride cation bonds exceed their lighter congeners, this trend is unusual but can be understood using promotion energy arguments.     

On the application of an oxygen probe in chemical kinetics: The influence of the probe response on kinetic measurement

Membrance covered oxygen probes are frequently used to monitor the progress of chemical reactions involving dissolved oxygen. For reactions that are comparable in rate to the response of the oxygen probe it is difficult to obtain the correct kinetic constants directly from the probe signal with traditional kinetic data manipulation methods. In this article, we apply the method of impulse response function to describe the probe signal of an oxygen probe for three different types of simple chemical reactions. The impulse response function is obtained experimentally. Using the impulse response function we have obtained the relationships between the probe signal and the kinetic parameters of these reactions. The slow response of the probe has two effects on the kinetic curves of the reaction studied: a time-lag and distortion of the shape of the kinetic curve throughout the reaction. The latter effect becomes significant when the reaction is fast. Procedures to obtain the correct kinetic information from the oxygen probe signal are described. © 1995 John Wiley & Sons, Inc.     

Probes of spin conservation in heavy metal reactions: experimental and theoretical studies of the reactions of Re+ with H2, D2, and HD

A guided ion beam tandem mass spectrometer is used to examine the kinetic energy dependence of reactions of the third-row transition metal cation, Re(+), with molecular hydrogen and its isotopologues. A flow tube ion source produces Re(+) in its (7)S(3) electronic ground state. Reaction with H(2), D(2), and HD forms Re H(+)(Re D(+)) in endothermic processes. Modeling of the endothermic reaction cross sections yields the 0 K bond dissociation energy of D(0)(Re(+)-H)=2.29+/-0.07 eV (221+/-6 kJ/mol). The experimental thermochemistry is consistent with ab initio calculations, performed here and in the literature. Theory also provides the electronic structures of these species and is used to examine the reactive potential energy surfaces. Results from reactions with HD provide insight into the reaction mechanisms and indicate that the late metal ion, Re(+), reacts largely via a statistical mechanism. This is consistent with the potential energy surfaces which locate a stable Re H(2) (+)((5)B(2)) complex. Results for this third-row transition metal system are compared with the first-row congener (Mn(+)) and found to have much higher reactivity towards dihydrogen and stronger M(+)-H bonds. These differences can be attributed to efficient coupling among surfaces of different spin along with lanthanide contraction and relativistic effects.     

Vibrational and translational enhancement of endothermic reactions: K + HCl(υ = 0,1) and K + HF(υ = 0,1)

The relative integral cross section for the two endothermic reactions K + HCl(υ = 0 and 1) → KCl + H and K + HF(υ = 0 and 1) → KF + H has been measured as a function of the collision energy E using the crossed molecular beam technique. The vibrationally excited state (υ = 1) has been populated thermally by heating the beam source to temperatures around 2000 K. The variation of the collision energy from thermal up to around 2.1 eV was achieved by seeding the K-beam with various carrier gases. The molecular reaction product was detected by surface ionization in connection with a time-of-flight method. The total energy threshold of the reactions has been found to be equal to or only slightly above the corresponding endothermicities. This suggests a vanishing or very low barrier crest on the potential energy hypersurfaces which is contradictory to recent theoretical results. The inclusion of tunneling in case of K + HF leads to a negligible rise of the barrier heights. The efficacy of translational and vibrational energy in promoting the reactive process has been directly compared over a wide range of collision energies. For K + HCl the vibrational enhancement of the reactivity descends with increasing E from approximately a factor of 10 at E = 0.08 eV to around unity for E ? 0.5 eV. The good agreement of this experimental result with phase space calculations suggests that the marked enhancements are predominantly caused by the long-range attraction between reagents in connection with an “early” barrier on the potential energy surface. In case of K + HF vibrational energy is by a factor of up to 380 more favourable in promoting the reaction than the same amount of translational energy. Again, with rising collision energy its efficacy decreases but promotes the reaction still by a factor of 70 at E = 1.7 eV. Since phase space theory fails here the reaction is certainly non-statistical and we conclude that the observed large efficacy of vibrational energy is due to a “late” barrier. The proposed barrier positions for the two systems are in accordance with theoretical results.     

Low-temperature NMR studies of Zn tautomerism and hindered rotations in solid zincocene derivatives

Using a combination of NMR methods we have detected and studied fluxional motions in the slip-sandwich structure of solid decamethylzincocene (I, [(eta5-C5Me5)Zn(eta1-C5Me5)]). For comparison, we have also studied the solid iminoacyl derivative [(eta5-C5Me5)Zn(eta1-C(NXyl)C5Me5)] (II). The variable temperature 13C CPMAS NMR spectra of I indicate fast rotations of both Cp* rings in the molecule down to 156 K as well as the presence of an order-disorder phase transition around 210 K. The disorder is shown to be dynamic arising from a fast combined Zn tautomerism and eta1/eta5 reorganization of the Cp* rings between two degenerate states A and B related by a molecular inversion. In the ordered phase, the degeneracy of A and B is lifted; that is, the two rings X and Y are inequivalent, where X exhibits a larger fraction of time in the eta5 state than Y. However, the interconversion is still fast and characterized by a reaction enthalpy of DeltaH = 2.4 kJ mol-1 and a reaction entropy of DeltaS = 4.9 J K-1 mol-1. In order to obtain quantitative kinetic information, variable temperature 2H NMR experiments were performed on static samples of I-d6 and II-d6 between 300 and 100 K, where in each ring one CH3 is replaced by one CD3 group. For II-d6, the 2H NMR line shapes indicate fast CD3 group rotations and a fast "eta5 rotation", corresponding to 72 degrees rotational jumps of the eta5 coordinated Cp* ring. The latter motion becomes slow around 130 K. By line shape analysis, an activation energy of the eta5 rotation of about 21 kJ mol-1 was obtained. 2H NMR line shapes analysis of I-d6 indicates fast CD3 group rotations at all temperatures. Moreover, between 100 and 150 K, a transition from the slow to the fast exchange regime is observed for the 5-fold rotational jumps of both Cp* rings, exhibiting an activation energy of 18 kJ mol-1. This value was corroborated by 2H NMR relaxometry from which additionally the activation energies 6.3 kJ mol-1 and 11.2 kJ mol-1 for the CD3 rotation and the molecular inversion process were determined.     

Mechanism and thermochemistry of oxidation of HCl and HBr at high temperatures. The heat of formation of HO2

Using currently available thermochemical and kinetic data and estimation methods to analyze the thermochemistry and the kinetic parameters of the elementary reactions involved in the oxidation of HCl and HBr, reaction mechanisms are proposed which account for the previously reported reaction products, the rate law, and the kinetic data. For oxidation of HCl, two competitive pathways, the radical initiation by hydrogen abstraction and the fourcenter reaction pathway, were invoked to account for the observations. In the oxidation of HBr one must invoke a fast surface reaction of the type to account for the reaction.