The Journal of Physical Chemistry 最新文献

Unfettered anodic dissolution occurs in halide-free sulfuric acid solutions for Pd electrodes pretreated with a single chemisorbed layer of iodine atoms; no dissolution takes place in the absence of iodine. Tandem cyclic voltammetry and in situ scanning tunneling microscopy have been employed to investigate the mechanism of this type of corrosion under low-current conditions. The ordered adlattices studied were those spontaneously formed upon immersion of the Pd single-crystal surface to a dilute solution of iodide:? Pd(111)-(√3 × √3)R30°-I; Pd(100)-c(2 × 2)-I; Pd(110)-pseudohexagonal-I. It has been found that (i) adsorbed-iodine-catalyzed corrosion of Pd is a structure-sensitive reaction; it decreases in the order Pd(110)?I > Pd(111)?I ≥ Pd(100)?I. (ii) At Pd(111)?I, dissolution occurs exclusively at step-edges in a layer-by-layer sequence without deterioration of the iodine adlattice structure. (iii) At Pd(100)?I, dissolution takes place anisotropically along a step aligned in the {100} direction but in a layer-by-layer process without disruption of the iodine adlattice structure. (iv) At Pd(110)?I, dissolution transpires predominantly at a step-edge that runs parallel to the {100} direction; pit formation at terraces precluded layer-by-layer dissolution and led to progressive disorder of the substrate structure. Heuristic models are presented to account for these observations.

The effects of a dielectric medium on the magnetizabilities and nuclear shielding constants of the fluoromethanes are calculated in a gauge-origin independent approach. It is shown that in order to model the effects of a dielectric medium properly, we have to go beyond the dipole approximation and the geometry has to be optimized for each value of the dielectric constant. Geometrical distortions play an important role, as is clearly demonstrated for the magnetizability of the CH₃F molecule where the geometrical distortions alter the sign of the dielectric contribution to the solvent shift. The effects on the nuclear shielding constants from a dielectric medium are interpreted in terms of the polarization of the charge distribution and the change in geometry. Other medium effects on nuclear shielding constants are discussed, and it is demonstrated that they must be included in order to reproduce the experimental gas-to-liquid chemical shift of the CH₃F and CHF₃ molecules. Basis set convergence and electron correlation effects of the magnetizabilities and the nuclear shielding constants in the gas phase are also investigated.

The chemical equilibrium Br + CH₃OH ? HBr + CH₂OH (1, ?1) has been studied by investigating the kinetics of the forward and reverse reactions. Excimer laser photolysis coupled with Br atom resonance fluorescence detection was used over the temperature range 439?713 K to obtain k₁ = (3.41 ± 0.89) × 10⁹T^1.5 exp[?(29.93 ± 1.47) kJ mol^-1/RT] cm³ mol^-1 s^-1. The reverse reaction was studied with the fast flow technique, in the temperature range 220?473 K, using laser magnetic resonance for monitoring the CH₂OH radicals. Thus, k_-1 = (1.20 ± 0.25) × 10¹² exp[(3.24 ± 0.44) kJ mol^-1/RT] was obtained. The kinetic results were compared with available literature data and possible causes of the deviations were discussed. Kinetic information on the foward and back reactions was combined to obtain the heat of formation for CH₂OH. Both second-law and third-law procedures were used in the derivations, giving a recommended value of Δ_fH°₂₉₈(CH₂OH) = ?16.6 ± 1.3 kJ mol^-1, which corresponds to the C?H bond dissociation energy of DH°₂₉₈(H?CH₂OH) = 402.3 ± 1.3 kJ mol^-1. These thermochemical data obtained from kinetic equilibrium studies agree within the error limits with current photoionization mass spectrometric and ab initio theoretical results.

Geometry optimization and harmonic vibrational frequency calculations were carried out on the X²B₁ and the lowest ²B₂ states of PF₂ at the MP2/6-31G* and MP2/6-311G(2df) levels. CCSD(T)/6-311G(2df)//MP2/6-31lG(2df) calculations were also performed to obtain improved adiabatic and vertical transition energies for the two combining electronic states. In addition, Franck?Condon analyses were carried out employing the ab initio data obtained to simulate emission spectra of the PF₂ (²B₂ → X²B₁) transition. Both the computed relative energies and the theoretical spectra confirm that the observed emission spectra by Zhao and Setser were due to the ²B₂ → X²B₁ transition. Furthermore, the geometry of PF₂ in the lowest ²B₂ state was also varied in the vibrational intensity calculations to give the best agreement between the theoretical and observed spectra. The bond length and bond angle thus deduced for PF₂ in the ²B₂ state are 1.628 ± 0.008 ? and 84.9 ± 0.2°, respectively. Spectra involving excited vibrational levels of the ²B₂ state were also generated by assuming the Boltzmann distribution at selected temperatures. Comparison of these spectra with the observed one suggested that the vibrational population distribution in the upper state does not follow the Boltzmann rule. The potential surface of the upper ²B₂ state under study may be perturbed vibronically by that of the lower, near-linear, ²A₁ state via the asymmetric stretching mode. This may explain why the theoretical spectra as obtained using the harmonic oscillator model are different from the experimental one.

An attempt is made to estimate the polarizability of an ion in an ionic diatomic molecule. The method is applied to alkali halide and hydride diatoms. In this connection, the applicability of Rittner's model to these test molecules is examined critically. A new phenomenological form of the repulsive part of Rittner's model is suggested. It is argued that the polarizability of an ion can be used as an index of the gas-phase chemical hardness of an ion in a molecule.

The adsorption behavior of 1,1,2,2-tetrahydrohenicosafluorododecanol, CF₃(CF₂)₉(CH₂)₂OH (FC₁₂OH), at the hexane/water interface was investigated in both the absence and presence of the deposit by measuring the interfacial tension γ of its hexane solution against water as a function of pressure p and molality m₁ at 298.15 K. It was concluded that a breakpoint on the γ vs p and γ vs m₁ curves in a very low concentration region shows a first-order phase transition from the gaseous to the condensed state in the adsorbed film. It was found that the volume change of adsorption per mole of FC₁₂OH is larger than that of 1,1,2,2-tetrahydroheptadecafluorodecanol, CF₃(CF₂)₇(CH₂)₂OH (FC₁₀OH). Then it was suggested that the increase in the partial molar volume of alcohol with increasing fluorocarbon chain length is larger in the solution than in the interface because of the weak mutual interaction of the fluorocarbon chain with hexane molecules. Furthermore, another breakpoint due to the solubility of FC₁₂OH in hexane was observed on the γ vs p and γ vs m₁ curves in a high concentration and pressure region. A small value of partial molar volume change accompanied by the adsorption from the solid substantiates our view that the condensed state of the adsorbed film resembles the solid state.

ZEKE spectroscopy is based on delayed detection by pulsed field ionization. It is thereby possible to monitor the time evolution at a given excitation frequency. Moreover, by varying the depth of detection, one can harvest different Rydberg series. The qualitative features expected for such a spectrum are discussed. The quantitative theory required to compute spectra is outlined and applied to the realistic example of Na₂⁺. The computed spectrum is found to very accurately exhibit two time scales, just as has been observed, with the shorter decay time being faster for lower Rydberg states. Extensive interseries coupling is noted.

At room temperature, extremely long-lived (>8 h) charge-separated (CS) states following photoinduced electron transfer were found for poly(N-vinylcarbazole) (PVCz) coadsorbed with 1,2,4,5-tetracyanobenzene as an electron acceptor on a macroreticular resin, Amberlite XAD-8. The formation of the long-lived CS states was ascribed to both a hole-migration process along the polymer chain and a hole-trapping process resulting in the large interionic distance. The temperature dependence on the decay profiles of the long-lived CS states indicated that the charge recombination due to the long-distance electron tunneling was responsible for the disappearance of the CS states in the long time region. The simulation curve taking account of both the long-distance electron transfer and the distribution of the interionic distance reproduced the decay profiles of the CS states fairly well. The effects of both tacticity of PVCz and the modification of the adsorbents on the formation and the deactivation processes of the long-lived CS states were examined. The results revealed that the deep hole-trap sites resulting in the large interionic distance were not simply related to the carbazolyl dimer cation sites but to the carbazolyl moieties adsorbed in the vicinity of the polar ester groups in the adsorbent.

We present the results of a computational study on both WH₆ and WF₆ using a relativistic pseudopotential on tungsten, comparing results at the HF and MP2 levels with density functional calculations using local (SVWN), nonlocal (BLYP), and hybrid HF-DFT (ACM) functionals. In agreement with the earlier all-electron HF study of Schaefer and co-workers (J. Chem. Phys. 1993, 98, 508), we find that the ground state geometry of WH₆ is far from octahedral; instead, there are four low-lying structures of C_3v (2) and C_5v (2) symmetry. Barrier heights for interconversion of these isomers are low, indicating a highly fluxional molecule. In contrast, the octahedral structure appears to be the only stable species on the ground state potential energy surface of WF₆. It derives its stability from the greater ionic character of the W?F bond compared to that of W?H.

Two-dimensional aggregation of a long-chain thiacarbocyanine dye 5,5‘-dichloro-3,3‘-dioctadecyl-9-ethylthiacarbocyanine perchlorate in the monolayers on subphases containing different synthetic polyanions were studied. The aggregation process of the dye in the monolayers was followed by the in situ measurement of the absorption spectra of the monolayers. Two J aggregates were observed at 648 and 666 nm in the monolayer on a plain water surface at all the surface pressures. On the subphase containing poly(acrylic acid), sodium (PAA), and poly(vinyl sulfonic acid), potassium salt (PVS) subphases, the J aggregates were formed at relative high surface pressures (15 and 17.5 mN/m, respectively). On the PVS subphase, an H aggregate at 472 nm was also observed at higher surface pressure (>40 mN/m). On the poly(styrenesulfonic acid), sodium salt (PSS) subphase, neither J nor H aggregates were observed even at higher surface pressures. Models were proposed to explain these interactions between the positively charged dye monolayers and polyanions.