The Journal of Physical Chemistry 最新文献

Electron attachment to BrCN and ClCN is studied in a crossed beam experiment. Relative cross sections for the formation of negative ions in the energy range 0?15 eV are reported. The kinetic energy release of fragment ions is studied by means of a time-of-flight (TOF) analysis. Both target molecules effectively capture low-energy electrons (<0.5 eV), leading to the complementary dissociative attachment (DA) channels X^- + CN (a) and X + CN^- (b). From the shape of the ion yield curve, the temperature behavior, and ab initio calculations, it is concluded that in both BrCN and ClCN the DA channels a and b originate from precursor ions with a different electronic configuration:? channel a correlates with a ²Σ state leading to direct electronic dissociation and channel b to a ²Π state associated with vibrational predissociation. The ions X^- and CN^- also appear from further, comparatively weak resonances at higher energies. The TOF analysis reveals that only the products Br^- + CN appear with appreciable kinetic energy (2.25 eV) from a resonance between 4.5 and 8.5 eV. For the three other channels (Br + CN^-, Cl^- + CN, Cl + CN^-), however, most of the total excess energy (amounting up to 7?8 eV) appears as internal energy of CN or CN^-. In the system BrCN, the Br^- intensity strongly increases with the gas temperature while the CN^- intensity strongly decreases as expected from the endothermicity of reaction a and the exothermicity of reaction b. In ClCN both DA channels are endothermic. While the intensity of Cl^- increases with the gas temperature, the CN^- intensity decreases above 600 K. This behavior is explained by the (temperature-dependent) competition between autodetachment and the comparatively slow vibrational predissociation process in the transient molecular anion.

Microcalorimetry experiments were carried out on aqueous micellar solutions of hexadecyltrimethylammonium bromide (CTAB) in the presence of potassium bromide (KBr). We have investigated the effect of surfactant concentration and of the salt content upon the specific heat. The curves of the specific heat versus temperature exhibit a maximum resulting from the transition between spherical and elongated micelles. The temperature T_max of these maxima depends on surfactant and salt concentration as expected from simple theoretical arguments. Static and dynamic light scattering experiments were performed on the same systems. The results show that light scattering and microcalorimetry experiments yield a different kind of information:? blob size and variation of the micellar length. Microcalorimetry provides an interesting tool for determining the micellar growth.

Channel electrode methodology is used to study the photoelectrocatalytic reduction of 4-chlorobiphenyl (CBP) in acetonitrile solution using 9,10-diphenylanthracene (DPA) as a mediator. The latter is reduced at a lower potential than is required for the direct reduction of the substrate, and the resulting radical anion, DPA^?-, when photoexcited irreversibly transfers an electron to CBP. The mechanism of the reaction is explored, and it is shown that in addition to the catalytic reduction of the biphenyl by excited state DPA^?- the latter also undergoes quenching by DPA and photoinduced disproportionation leading to the dianion, DPA^2-, which undergoes irreversible chemical transformation. Kinetic parameters are reported.

Small-angle neutron scattering (SANS) was measured for aqueous solutions of amphiphiles with organic additives or counterions such as 2-indenecarboxylic acid, cinnamic acid, and salicylic acid or their ions. The effect of organic species on the supramolecular assembly structure was investigated. Equimolar mixtures of dodecyldimethylamine oxides and aromatic carboxylic acids constructed vesicles. The vesicular bilayer thicknesses were 29?30 ?, independent of organic species. On the other hand, hexadecyltrimethylammoniums with aromatic carboxylate counterions always formed rodlike micelles with axial ratios 8?10. Aqueous solutions of tetradecyltrimethylammonium salicylate displayed a strong interparticle interference effect on SANS profiles at concentrations above 0.8 × 10^-2 g cm^-3, although such an effect disappeared with addition of more than 5/mM sodium salicylate.

Structures, energies, and harmonic vibrational frequencies of CN₂O₂ isomers have been investigated theoretically at the ab initio CCSD(T)/TZ2P//MBPT(2)/6-31G* level in search of new high-energy molecules and in a study of the mechanism of the reaction between NCO and NO radicals. Nitrosoisocyanate, ONNCO (1), earlier studied as a collision complex in the reaction of NCO and NO (Lin, M. C.; Melius, C. F. J. Phys. Chem. 1993, 97, 9124) is the most energetically favorable CN₂O₂ isomer, but its 18 kcal/mol unimolecular dissociation barrier is very low. Thus 1 can only be observed as a short-lived intermediate. However, nitrosofulminate, ONCNO (8), and nitryl cyanide, NCNO₂ (12), higher energy isomers (69 and 38 kcal/mol above trans-1a, respectively), are more stable than 1 toward decomposition. This offers species 8 and 12 as interesting molecules for experimental study. Moreover, 12 can be a reasonably stable molecule as its C?N bond dissociation energy (59 kcal/mol) and the barrier to decomposition into N₂ and CO₂ (54 kcal/mol) are rather high, being comparable to those of nitromethane. The estimated large values of the heat of formation (ΔH_f°₃₀₀ = 60 kcal/mol) and of the decomposition energy of 12 (12 → N₂ + CO₂; ΔE = 150 kcal/mol) make this species potentially interesting as a high-energy molecule. Our study also includes four- (2) and three-membered (17) cyclic and bicyclic (3) isomers. The C_s cyclic isomers, 2 and 17, are extremely unstable, but the bicyclic C_2v form (3) has a 29 kcal/mol dissociation barrier and should be observable.

In situ FTIR results on the adsorption of phosphate species on Pt(111) and Pt(100) single crystal surfaces are presented and discussed. In weak acid solutions (fluoride electrolyte pH = 2.8), the adsorption of phosphate species on Pt(111) starts at about 0.4 V vs Pd/H₂ and the maximum adsorption occurs at about 0.6 V. At low potentials, two spectral features due to adsorbed H₂PO₄^- are observed at 1110 and 1000 cm^-1. With increasing potentials these two original bands are replaced by a new band located between 1150 and 1180 cm^-1, which is assigned to adsorbed HPO₄^2- species. In strongly acidic solution (pH = 0.23), undissociated H₃PO₄ molecules are adsorbed at low potentials. This species is characterized by a band between 1035 and 1050 cm^-1. With increasing adsorption potential adsorbed H₃PO₄ dissociates, generating H₂PO₄^-. For both solution pHs the deprotonation of adsorbed species is observed after the maximum of the anomalous voltammetric wave of the Pt(111) electrode. On Pt(100) and Pt(111) the adsorption of phosphate species shows almost identical behavior in mildly acidic solutions. Transformation from H₂PO₄^- to HPO₄^2- occurs on both single crystal surfaces at the same potentials.

This paper reports “self-similarity” in the time domain for the neutron scattering functions from the microphases which develop in the solid state at room temperature when binary paraffin mixtures of C₃₀/C₃₆ alkanes are quenched from the melt. Data for a number of compositions and at several temperatures inside the microphase separation regions have been taken using small-angle neutron scattering and isotopic contrast of the components. Remarkably, the structure factor in all cases agrees well with theoretical predictions of time-dependent scaling. This shows that the kinetic parameters which control the formation of the microphase are applicable over the whole time regime studied and points the way to eventual computer simulation of the dynamics in such systems.