The Journal of Physical Chemistry 最新文献

Rate coefficients for the reactions C₂H with C₂H₄, C₂H₆, and H₂ are measured over the temperature range 150?359 K using transient infrared laser absorption spectroscopy. The ethynyl radical is formed by photolysis of C₂H₂ with a pulsed excimer laser at 193 nm, and its transient absorption is monitored with a color center laser on the Q₁₁(9) line of the A²Π?X²Σ transition at 3593.68 cm^-1. Over the experimental temperature range 150?359 K the rate constants of C₂H with C₂H₄, C₂H₆, and H₂ can be fit to the Arrhenius expressions k_C2H4 = (7.8 ± 0.6) × 10^-11 exp[(134 ± 44)/T], k_C2H6 = (3.5 ± 0.3) × 10^-11 exp[(2.9 ± 16)/T], and k_H2 = (1.2 ± 0.3) × 10^-11 exp[(?998 ± 57)]/T cm³ molecule^-1 s^-1, respectively. The data for C₂H with C₂H₄ and C₂H₆ indicate a negligible activation energy to product formation shown by the mild negative temperature dependence of both reactions. When the H₂ data are plotted together with the most recent high-temperature results from 295 to 854 K, a slight curvature is observed. The H₂ data can be fit to the non-Arrhenius form k_H2 = 9.2 × 10^-18T^2.17±0.50 exp[(?478 ± 165)/T] cm³ molecule^-1 s^-1. The curvature in the Arrhenius plot is discussed in terms of both quantum mechanical tunneling of the H atom from H₂ to the C₂H radical and bending mode contributions to the partition function.

The photodissociation dynamics of four chlorine-containing compounds was studied by the ion imaging technique. Two compounds, ClNO and (CH₃)₃COCl, exhibited strong anisotropy, and the kinetic energy distributions for Cl(²P_3/2) and Cl*(²P_1/2) were indistinguishable. For CCl₄ and SOCl₂ the kinetic energy distributions of Cl and Cl* were very different. In all cases most of the chlorine atoms were released in their ground ²P_3/2 state. A general explanation is given based on these data and those of others. The process is described in terms of three states whose energies are in the following order:? the ground state, a state correlating with Cl*, and a state correlating with Cl. Sooner or later the Cl state must cross the Cl* state. If the crossing is sooner, i.e. in the Franck?Condon region, Cl and Cl* will have different translational energy distributions. If the crossing is later, i.e. at long distances, Cl and Cl* will have descended a repulsive surface and reached their asymptotic and nearly equal kinetic energies.

The kinetics of the gas-phase reactions of SO₃ with H₂O and D₂O were studied over the temperature range 250?360 K in N₂ with a laminar flow reactor coupled to a chemical ionization mass spectrometer. The SO₃ loss is second order in the water concentration, is independent of pressure (20?80 Torr N₂, 300 K), and has a strong negative temperature dependence and a significant H/D isotope effect (k_H2O ≈ 2k_D2O). The yield of sulfuric acid is 1.0 ± 0.5 per SO₃ consumed. These observations are consistent with the rapid association of SO₃ and H₂O to form the adduct H₂OSO₃ which reacts with water to produce sulfuric acid. The first-order rate coefficients for loss of SO₃ by reaction with H₂O and D₂O are given by k^I(s^-1) = (2.26 ± 0.85) × 10^-43T exp((6544 ± 106)/T)[H₂O]² and (9.45 ± 2.68) × 10^-44T exp((6573 ± 82)/T)[D₂O]², where T ≡ K and [H₂O, D₂O] ≡ molecule cm^-3. The errors are the uncertainty at the 95% confidence level for precision only. Analysis of the temperature dependence of the SO₃ loss yields an upper limit for the H₂O?SO₃ bond enthalpy of 13 kcal mol^-1.

Time-resolved spectral hole-burning experiments have been performed to probe the dynamics of the S₁ ← S₀ 0?0 transition of bacteriochlorophyll-a at low concentration (1 × 10^-5M) in four different glasses (2-methyltetrahydrofuran, protonated and deuterated ethanol, diethyl ether, and triethylamine) as a function of delay time t_d (from 10^-6 to 10³ s) and temperature T (1.2?4.2 K). It is shown that spectral diffusion, the broadening of the optical linewidth followed here over nine orders of magnitude in time, increases with temperature as T^1.3±0.1 and strongly depends on the glass structure. The functional dependence, however, is not influenced by the specific glass. The variation of the “effective” homogeneous linewidth (Γ‘_hom) with T and t_d is described by a function Γ‘_hom(T,t_d) derived by modifying the standard model of two-level systems (TLS). This revised TLS model, in which the distribution functions of the TLS tunneling parameters are different from those in the standard model, takes into account the common origin of the dependence of Γ‘_hom on t_d and T. It is shown that other hole-burning and photon-echo data reported in the literature can also be fitted by the same function Γ‘_hom(T,t_d). In ethanol glass, the number of TLSs and the amount of spectral diffusion appear to be independent of the probe molecule.

The water-induced disproportionation of the electrogenerated superoxide ion (O₂^-) in acetonitrile, dimethylformamide, and dimethyl sulfoxide media containing various concentrations of water as a Br?nsted acid has been examined by UV?vis spectroscopy. Analysis of the kinetics as a function of O₂^- and water concentrations and of the measurement time demonstrated that the disproportionation of O₂^- by water in these media obeys a common mechanism:? O₂^- + H₂O ? HO₂^? + OH^- (k₁, k_-1) HO₂^? + O₂^- → HO₂^- + O₂ (k₂) (HO₂^?, hydroperoxyl radical; HO₂^-, hydrogen peroxide anion). The solvent dependence of the obtained kinetic parameters of (k₁/k_-1)k₂, k₁ and k_-1/k₂ is discussed in terms of the solvation of O₂^- and H₂O as well as the effective acidities of H₂O in different aprotic solvents.

Mn²⁺-doped CdS nanocrystals (1?2 nm in size) dispersed in organic?inorganic silica xerogels are prepared by combining a controlled precipitation of nanocrystals in inverted micelles, a separation as a pure capped cluster powder, and a dispersion in hydrolyzed silicon alkoxide. ESR study identified two Mn²⁺ sites, one bulklike, and another one which is attributed to ions located near the surface and which represents the main contribution. The average number of Mn²⁺ per particle is in the 0.2?0.8 range. The luminescence is characteristic of a Mn²⁺ internal transition, with energy and lifetime as in bulk materials. This bright emission (quantum yield of 7%) corresponds to an energy transfer from surface trapped carriers to Mn²⁺ ions.

Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) has been applied to characterize amorphous silicon monohydride films before and after reaction with deuterium atoms. The hydride films were grown by chemical vapor deposition on oxide-covered silicon substrates, and the data suggest that the film is terminated by a homogeneous monolayer of primarily dimerized silicon monohydride. Exposure of this film to atomic deuterium causes the replacement of silicon hydride with adsorbed deuterium. Only the monodeuteride is formed by reaction at 200 °C. Reaction at ?110 °C produces mono-, di-, and trideuteride, demonstrating that the isolation of insertion products is temperature-dependent.

We have extended Monte Carlo procedures for computing statistical averages over protonation states of a protein to include conformational states of the titrating amino acid side chains. This computational method couples side chain motion and protonation with changes in solution pH. Using a continuum electrostatic model for protein titration, we applied this sampling method to calculate titration curves for lysozyme, myoglobin, and hemoglobin. In addition to the X-ray conformation, each titrating site was allowed to reorient to a conformation with maximum solvent accessibility. For all proteins considered, inclusion of these additional conformations improved agreement with experimental measurements for both overall titration and individual pK_as. The results suggest that well-solvated orientations of amino acid side chains are an important factor in determining proton binding characteristics of proteins.

Previously, the scanning tunneling microscope has been employed to deposit single nanoscopic silver structures from aqueous silver solutions onto the graphite basal plane surface. In this paper, the mechanism of this electrochemical lithography is elucidated. The deposition of metal occurs via a two-step mechanism following the application of a sample negative pulse to the graphite surface. First, within 5 μs of the application of a bias pulse having an amplitude greater than +4 V (tip positive), the formation of a shallow, circular pit in the surface is observed. If the pulse is sustained for longer durations, the reductive deposition of metal begins to occur at ～10 μs and the volume of the nascent silver nanostructure saturates at 30?50 μs. The resulting metal nanostructure has a disk geometry with typical dimensions of 200?400 ? in diameter and 20?50 ? in height. STM data coupled with electrochemical measurements and computer simulations of the deposition process demonstrate that the silver metal involved in nanostructure formation is initially present as an underpotentially deposited (UPD) monolayer of silver on the surface of the platinum STM tip. After application of a bias pulse, this adsorbed silver is oxidatively desorbed and silver ions migrate across the tip?sample gap and are deposited in the shallow nucleation site in the graphite surface. This adsorbed silver is susceptible to depletion when silver nanostructures are deposited in rapid succession from dilute silver electrolytes.

The contribution of a redox mechanism involving lattice oxygen in the catalytic combustion of methane over PdO/ZrO₂ catalysts, prepared from amorphous Pd?Zr alloys, has been studied by means of gas pulse methods, including a novel technique “pulse thermal analysis”, and using labeled catalysts containing Pd¹⁸O. Special emphasis was devoted to the influence of the isotope exchange (scrambling) of reactants and products, especially O₂ and CO₂, with the catalyst on the quantity of ¹⁸O-containing reaction products. Substantial amounts of H₂¹⁸O and C¹⁸O¹⁶O were detected during pulses of a reactant mixture consisting of methane and ¹⁶O₂ in a ratio 1:4 at 300 and 500 °C. The effect of the oxygen exchange of molecular oxygen with the solid phase proved to be negligible due to its low extent. At 300 °C, at least 20% of the CO₂ formed originated from the redox mechanism involving lattice oxygen. At 500 °C, oxygen exchange of CO₂ with the catalyst became predominant and precluded determining reliably the proportion of CO₂ formed by the redox process. The results indicate that a substantial part of methane is oxidized via a redox process. Consequently, this reaction has to be taken into account when interpreting the catalytic behavior of palladium-based catalysts and explaining the structure?activity relations previously observed.