Rates of processes initiated by pulsed laser production of F atoms in the presence of HCl,CH4, and CF3H

Time-resolved vibrational chemiluminescence from HF has been recorded following the production of F atoms by the pulsed laser photolysis (λ = 266 nm) of F₂ in the presence of HCl, CH₄, and CF₃H. In the first two cases, experiments have been conducted by observing emission from HF(ν = 3) at four temperatures from 295 to 139 K. Rate constants have been determined over this range of temperature for the reactions of F atoms with HCl and CH₄ and of CH₃ radicals with F₂, and for the relaxation of HF(ν = 3) by HCl and CH₄. The reaction of F atoms with CF₃H is slower than those with HCl and CH₄ and measurements on the emission from HF(ν = 2) have been used to infer rate constants for reaction and relaxation only at 295 K. © 1994 John Wiley & Sons, Inc.     

Vibrational spectra and normal coordinate analysis of germylazide-do and -d3

The infrared spectra (4000–20 cm^?1) of gaseous and solid GeH₃NNN and GeD₃NNN and the Raman spectra (3000–0 cm^?1) of liquid and solid GeH₃NNN and GeD₃NNN have been recorded. The vibrational spectrum has been assigned on the basis of C_s symmetry. The GeNN bend was observed in the Raman spectrum of the gas at ~150 cm^?1; however, the frequency of the GeH₃ torsion could not be determined. A normal coordinate calculation has been carried out by utilizing a modified valence force field. The GeN stretching force constant was found to have a significantly smaller value than the corresponding force constant in GeH₃NCO and GeH₃NCS. A considerable amount of mixing was found between the GeN stretch and the NNN in-plane bend. The frequencies of the normal modes and the normal coordinate analysis of GeH₃NNN are discussed in view of the recent results reported for GeH₃NCO and GeH₃NCS.     

The reaction of OH with acetaldehyde and deuterated acetaldehyde: Further insight into the reaction mechanism at both low and elevated temperatures

The pulsed laser photolysis/laser‐induced fluorescence technique has been used to conduct additional measurements of the gas‐phase CH₃CHO + OH reaction. These measurements were conducted to verify the complex temperature dependence previously observed by the authors and to acquire mechanistic information about the reaction mechanism in the form of primary kinetic isotope effects. Primary kinetic isotope effect measurements at temperatures of 297, 383, 600, and 860 K indicate that H abstraction from the acetyl group dominates that of the methyl group at low to modest temperatures (?600 K) and H abstraction from the methyl group dominates that from the acetyl group at higher temperatures (860 K). A bi‐exponential © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 489–495, 2006     

Kinetics and mechanism of the germane decomposition

The homogeneous gas-phase thermal decomposition kinetics of germane have been measured in a single-pulse shock tube between 950 and 1060 K at pressures around 4000 torr. The initial decomposition is GeH₄ → GeH₂ + H₂ in its pressure-dependent regime, with log k = 13.83 ± 0.78 – 50,750 ± 3570 cal/2.303RT. RRKM calculations suggest that the high-pressure Arrhenius parameters are log k GeH₄(M → ∞) = 15.5 – 54,300 cal/2.303RT. Extrapolations to static system pyrolysis conditions (T ～ 600 K, P ～ 200 torr) give homogeneous reaction rates which are much slower than those observed, hence the static system pyrolysis of germane must be predominantly heterogeneous. Shock-initiated pyrolysis reaction stoichiometry is 2 mol H₂ per mole GeH₄, suggesting that the subsequent decomposition of germylene is essentially quantitative. Investigations of the hydrogen product yields for pyrolysis of GeD₄ in øCH₃ further indicate that the germylene decomposition reaction is mainly GeH₂ → H₂ + Ge, but that a small amount of reaction to H atoms may also occur.     

First Experimental and Theoretical Kinetic Study of the Reaction of 4‐Hydroxy‐4‐methyl 2‐pentanone as a Function of Temperature

In this work, experimental and theoretical rate coefficients were determined for the first time for the gas‐phase reaction of 4‐hydroxy‐4‐methyl‐2‐pentanone (4H4M2P) with OH radicals as a function of temperature. Experimental studies were carried out over the pressure range of 5–80 Torr and the temperature range of 280–365 K, by using a cryogenically cooled cell coupled to the pulsed laser photolysis‐laser induced fluorescence (PLP–LIF) technique. A detailed oxidation mechanism of 4H4M2P with OH radicals was discussed theoretically under three hydrogen abstraction pathways by using density functional theory calculations and wave function based MP2 method. Single‐point energy calculations were performed at CCSD(T) level of theory with 6–311++G(d,p) basis set. The H‐atom abstraction from the ‐CH₂ group was found to be the dominant channel. The reaction force analysis predicts that the abstraction process is mainly dominated by structural rearrangement. Linear kinetic behavior for all the pathways was found in the range of 278–365 K. An atmospheric lifetime less than 3 days was evaluated for 4H4M2P with respect to its reaction with OH, indicating that the reaction with OH of 4H4M2P may be competitive with losses via photolysis.     

Reduction kinetics of gold(III) complexes with calix[4]arenes derivatized with thioether groups on the upper rim

Gold(III) reduction with polydentate calix[4]arenes having thioether groups on their upper rim yields gold(I) complexes and sulfoxides. The reaction can proceed by both intramolecular and intermolecular mechanisms, whose rates depend considerably on the steric and donor properties of the radicals at the sulfur atoms. Rate constants and activation energies are determined for both reaction pathways in the temperature range 293–323 K. The higher the polarity of the solvent, the higher the reduction rate; in passing from toluene or CCl₄ to 1,2-dichloroethane, the reduction rate increases by one order of magnitude, following the increasing order of dielectric constants.     

Absolute rate constants for the reactions of O(3P) atoms and OH radicals with SiH4 over the temperature range of 297–438°K

Absolute rate constants for the reaction of SiH₄ with O(³P) atoms and OH radicals have been determined over the temperature range 297°–438°K using flash photolysis–NO₂ chemiluminescence and flash photolysis–resonance fluorescence techniques, respectively. The Arrhenius expressions obtained are where the error limits in the Arrhenius activation energies are the estimated overall error limits. Rate data for the reactions of SiH₄, CH₄, and H₂S with O(³P), H, and F atoms and with OH, CH₃, and CF₃ radicals are compared, showing that H₂S and SiH₄, which have similar bond energies, have reasonably similar reactivities toward these atoms and radicals.     

Time-resolved,Gas-phase Kinetic and Quantum Chemical Studies of the Reaction of Germylene with Hydrogen Chloride

Time-resolved studies of germylene, GeH₂, generated by laser flash photolysis of 3,4-dimethyl-1-germacyclopent-3-ene at 193 nm and monitored by laser absorption, have been carried out to obtain rate constants for its bimolecular reaction with HCl. The reaction was studied in the gas phase, mainly at a total pressure of 10 Torr (in SF₆ bath gas) at five temperatures in the range 295–558 K. Experiments at other pressures showed that these rate constants were unaffected by pressure. The second-order rate constants at 10 Torr (SF₆ bath gas) fitted the Arrhenius equation: log(k/cm³ molecule⁻¹ s⁻¹)=(−12.06±0.14)+(2.58±1.03 kJ mol⁻¹)/RTln10 where the uncertainties are single standard deviations. Quantum chemical calculations at G4 level support a mechanism in which an initial weakly bound donor-acceptor complex is formed. This can then rearrange and decompose to give H₂ and HGeCl (chlorogermylene). The enthalpy barrier (36 kJ mol⁻¹) is too high to allow rearrangement of the complex to GeH₃Cl (chlorogermane).     

Laser photolysis studies of hydrazine vapor: 193 and 222-nm H-atom primary quantum yields at 296 K,and the kinetics of H + N2H4 reaction over the temperature range 222–657 K

The primary quantum yield of H-atom production in the pulsed-laser photolysis of hydrazine vapor, N₂H₄ + hν → H + N₂H₃, was measured to be (1.01 ± 0.12) at 193 nm relative to HBr photolysis, and (1.06 ± 0.16) at 222 nm relative to 248-nm N₂H₄ photolysis, in excess He buffer gas at 296 K. The H-atoms were directly monitored in the photolysis by cw-resonance fluorescence detection of H(²S) at 121.6 nm. The high H-atom yield observed in the photolysis is consistent with the continuous ultraviolet absorption spectrum of N₂H₄ involving unit dissociation of the diamine from repulsive excited singlet state(s). The laser photodissociation of N₂H₄ was thus used as a ‘clean’ source of H-atoms in excess N₂H₄ and He buffer gas to study the gas-phase reaction, H + N₂H₄ → products; (k₁), in a thermostated photolysis reactor made of quartz or Pyrex. The pseudo-first-order temporal profiles of [H] decay immediately after photolysis were determined for a range of different hydrazine concentrations employed in the experiments to calculate the absolute second-order reaction rate coefficient, k₁. The Arrhenius expression was determined to be k₁ = (11.7 ± 0.7) × 10^?12 exp[?(1260 ± 20)/T] cm³ molec^?1 s^?1 in the temperature range 222–657 K. The rate coefficient at room temperature was, within experimental errors, independent of the He buffer gas pressure in the range 24.5–603 torr. The above temperature dependence of k₁ is in excellent agreement to that we determine in our discharge flow-tube apparatus in the temperature range 372–252 K and in 9.5 torr of He pressure. The Arrhenius parameters we report are consistent with a metathesis reaction mechanism involving the abstraction of hydrogen from N₂H₄ by the H-atom. © 1995 John Wiley & Sons, Inc.     

Depletion kinetics of low-lying states of tungsten in the presence of NO,N2O,and SO2

The gas-phase reactivities of W(a⁵D_J, a⁷S₃) with N₂O, SO₂, and NO in the temperature range of 295–573 K are reported. Tungsten atoms produced by the photodissociation of W(CO)₆. The tungsten atoms were detected by a laser-induced fluorescence technique. The removal rate constants for the 6s²5d⁴ a⁵D_l states were found to be pressure dependent for all of the reactants. Removal rate constants for the 6s¹5d⁵ a⁷S₃ state were found to be fast compared to the a⁵D_J states and often approached the gas kinetic rate constant. The reaction rates for all the states were found to be pressure independent with respect to the total pressure. Results are discussed in terms of the different electronic configurations of the states of tungsten © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 367–375 1997     

Deuterium and Hydrogen Tunneling in the Hydrogenation of 4‐Oxocyclohexa‐2,5‐dienylidene

4‐Oxocyclohexa‐2,5‐dienylidene is a highly reactive triplet ground state carbene that is hydrogenated in solid H₂, HD, and D₂ at temperatures as low as 3 K. The mechanism of the insertion of the carbene into dihydrogen was investigated by IR and EPR spectroscopy and by kinetic studies. H or D atoms were observed as products of the reaction with H₂ and D₂, respectively, whereas HD produces exclusively D atoms. The hydrogenation shows a very large kinetic isotope effect and remarkable isotope selectivity, as was expected for a tunneling reaction. The experiments, therefore, provide clear evidence for both hydrogen tunneling and the rare deuterium tunneling in an intermolecular reaction.     

Kinetic isotope effects in the thermal gas phase reactions of 1, 2-epoxycyclohexane-3, 3, 6, 6-d4

First-order rate constants for formation of cyclohexanone and 2-cyclohexen-1-ol from 1,2-epoxycyclohexane and 1,2-epoxycyclohexane-3,3,6,6-d₄ have been determined over the temperature range of 677–746°K. The observed kinetic isotope effects are used in an attempt to determine the mechanism for formation of products. A distinction between a biradical and a concerted mechanism for the alcohol formation could not be made. However, if a common biradical is the precursor of both cyclohexanone and 2-cyclohexenl-ol then the rate of ring closure of this biradical must be much faster than the rates of hydrogen transfer to give the ketone and the alcohol.     

Kinetics of the gas-phase reaction between iodine and monogermane and the bond dissociation energy D(H3Ge?H)

The title reaction has been investigated in the temperature range of 403–446 K. Monoiodogermane and di-iodogermane together with hydrogen iodide were the main products, although at high conversions at least one other product was formed. GeH₃I is clearly the primary product. Initial rates were found to obey the rate law over a wide range of initial iodine and monogermane pressures. Secondary reactions (of GeH₃I with I₂) affect the subsequent kinetics, although at sufficiently high initial reactant ratios ([GeH₄]₀/[I₂]₀ ≥ 100) an integrated rate equation fits the data with the same rate constants as the initial rate expression. The observed kinetics are consistent with an iodine atom abstraction chain mechanism, and for the step log k₁ (dm³/mol·s) = (11.03 ± 0.13) – (52.3 ± 1.0 kJ/mol)/RT ln 10 has been deduced. From this the bond dissociation energy D(GeH₃? H) = 346 ± 10 kJ/mol (82.5 kcal/mol) is obtained. The significance of this value, together with derived values for Ge–Ge and Ge–C bond strengths, is discussed.     

Kinetics of the initiation step of the thermal decomposition of SiCl4

The initiation reaction of the thermal decomposition of silicon tetrachloride was studied behind reflected shock waves at temperatures between 1550 K and 2370 K and pressures between 1 and 1.5 bar. Atomic resonance absorption spectrometry (ARAS) was applied for time-resolved measurements of H atoms at the L_α-line in SiCl₄/H₂/Ar systems. Additional experiments were performed in the SiCl₄/Ar system following the absorption of SiCl₄ at the L_α-line. Rate coefficients for the reaction (RI) were determined to be: The choice between two possible alternatives of the first decomposition step, namely elimination of either Cl₂ or Cl, has been made in favor of the second reaction on the basis of kinetic and energetic considerations. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 415–420, 1997.     

The bonding,structure, and photochemistry of some stable and unstable germanium species

The ultraviolet and vacuum-ultraviolet photolyses of GeH₄, GeH₃Cl, GeH₂Cl₂, GeHCl₃, GeCl₄, GeH₃Br, and GeH₂Br₂ in argon and carbon monoxide matrices have been performed between 4° and 24° K. The results of these experiments support the successful isolation and characterization of a variety of germanium free-radical species produced as a result of primary and secondary photolytic processes. In order to control, characterize and understand the various elementary processes (photochemical and fragment diffusion) occurring in the matrix, both in situ photolysis as well as simultaneous deposition and photolysis were performed as a function of photolyzing radiation, temperature (of the cold window), and concentration (M/ R = matrix/reactive material).The identification and structure of the photochemically produced free radicals were obtained from the vibrational (infrared) spectrum before and after photolysis. In some cases, complete vibrational assignments were possible. Force field (normal coordinate) calculations were also performed in order to corroborate these assignments, within the limitation of their application to matrix isolated spectra.Finally, the structural and bonding properties of these free radicals, as reflected by their vibrational frequencies, are compared with similar stable and unstable Group IVA species in an effort to understand the major effect(s) controlling their geometry. The geometries obtained on the basis of the experimental observations are compared with those predicted by Walsh's semiempirical MO treatment and Self-Consistent Extended Huckel calculations. In addition, a simple thermodynamic argument in conjunction with elementary quantum mechanics is used as a tool for predicting the structure of some of these simple free-radical species.     

Determination of the rate constant for the OH(X2Pi) + OH(X()Pi) --> O(3P) + H2O reaction over the temperature range 293-373 K

The rate constant for the reaction OH(X2Pi) + OH(X2Pi) --> O(3P) + H2O has been measured over the temperature range 293-373 K and pressure range 2.6-7.8 Torr in both Ne and Ar bath gases. The OH radical was created by 193 nm laser photolysis of N2O to produce O(1D) atoms that reacted rapidly with H2O to produce the OH radical. The OH radical was detected by quantitative time-resolved near-infrared absorption spectroscopy using Lambda-doublet resolved rotational transitions of the first overtone of OH(2,0) near 1.47 microm. The temporal concentration profiles of OH were simulated using a kinetic model, and rate constants were determined by minimizing the sum of the squares of residuals between the experimental profiles and the model calculations. At 293 K the rate constant for the title reaction was found to be (2.7 +/- 0.9) x 10(-12) cm(3) molecule(-1) s(-1), where the uncertainty includes an estimate of both random and systematic errors at the 95% confidence level. The rate constant was measured at 347 and 373 K and found to decrease with increasing temperature.     

Synthetic, structural, and theoretical investigations of alkali metal germanium hydrides--contact molecules and separated ions

The preparation of a series of crown ether ligated alkali metal (M=K, Rb, Cs) germyl derivatives M(crown ether)_nGeH₃ through the hydrolysis of the respective tris(trimethylsilyl)germanides is reported. Depending on the alkali metal and the crown ether diameter, the hydrides display either contact molecules or separated ions in the solid state, providing a unique structural insight into the geometry of the obscure GeH₃^? ion. Germyl derivatives displaying M? Ge bonds in the solid state are of the general formula [M([18]crown‐6)(thf)GeH₃] with M=K ( 1 ) and M=Rb ( 4 ). The compounds display an unexpected geometry with two of the GeH₃ hydrogen atoms closely approaching the metal center, resulting in a partially inverted structure. Interestingly, the lone pair at germanium is not pointed towards the alkali metal, rather two of the three hydrides are approaching the alkali metal center to display M? H interactions. Separated ions display alkali metal cations bound to two crown ethers in a sandwich‐type arrangement and non‐coordinated GeH₃^? ions to afford complexes of the type [M(crown ether)₂][GeH₃] with M=K, crown ether=[15]crown‐5 ( 2 ); M=K, crown ether=[12]crown‐4 ( 3 ); and M=Cs, crown ether=[18]crown‐6 ( 5 ). The highly reactive germyl derivatives were characterized by using X‐ray crystallography, ¹H and ¹³C NMR, and IR spectroscopy. Density functional theory (DFT) and second‐order Møller–Plesset perturbation theory (MP2) calculations were performed to analyze the geometry of the GeH₃^? ion in the contact molecules 1 and 4 .     

Large kinetic isotope effects with unsymmetrical transition states

The results of a series of calculations for a wide, unrestricted variation in the force constants for the making and breaking bonds and their interaction constant are presented for the abstraction reactions of CH₂D₂ with Cl atoms. A wide range of asymmetrical force constants leads to a high kinetic isotope effect as has been pointed out by others for a more restricted range of consideration. These results pointedly contradict the assumed connection between a high kinetic isotope effect and a symmetrical transition state. It is found by examining the atomic displacements of the normal mode that the motion of the H or D atom in the real stretch of the transition state will often have little influence on the isotope effect because the mode can be dominated by end group motions. It is further found that a 3-center model multiplied by a constant factor to account for the contributions of the other vibrations is capable of very satisfactorily reproducing the more rigorous 6-center calculations.     

Flash photolysis resonance fluorescence study of the reaction Cl + O3 → ClO + O2 over the temperature range 213–298 K

Rate constants for the removal of Cl atoms in the reaction Cl + O₃ → ClO + O₂ were measured by the flash photolysis resonance fluorescence technique over the temperature range 213–298 K. The rate constant is given by the Arrhenius expression (2.94 ± 0.49) × 10^?11 exp[?(298 ± 39)/T] in units of cm³ molecule^?1 s^?1. Comparison with recent results from other laboratories are presented.     

Kinetics of the reactions of hydroxyl radicals (OH) and of chlorine atoms (Cl) with methylethylether over the temperature range 274–345 K

The rate constants for the gas-phase reactions between methylethylether and hydroxyl radicals (OH) and methylethylether and chlorine atoms (Cl) have been determined over the temperature range 274–345 K using a relative rate technique. In this range the rate constants vary little with temperature and average values of k_MEE+OH = (6.60_−2.62^+3.88) × 10⁻¹² cm³ molecule⁻¹ s⁻¹ and k_MEE+Cl= (34.9 ± 6.7) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ were obtained. The atmospheric lifetimes of methylethylether have been estimated with respect to removal by OH radicals and Cl atoms to be ca. 2 days and ca. 30–40 days, respectively. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 231–236, 1997.