The Journal of Physical Chemistry 最新文献

Laser Raman spectroscopy was used to study the dissolution of volatile hydrocarbon molecules in thin films of poly-α-olefin oil. Above the boiling point of the hydrocarbon, or when no liquid hydrocarbon was present, the dissolution was driven by Henry's law solubilities in line with previous models. However, below the boiling point and when significant liquid hydrocarbon was present, dissolution was entropically driven. Under these conditions, where equilibrium solubilities were not achievable, the Raman spectra were used to determine dissolution rates. Over the range of conditions studied, and particularly for a given class of volatile hydrocarbons, pressure and temperature influenced the dissolution rate primarily by affecting the vapor pressure and concentration of the volatile hydrocarbon. The rate-limiting step for the dissolution involved mass transport at the vapor/film interface.

The protonation of CF₃C₆H₅ and deprotonation of [CF₃C₆H₅]H⁺ ions have been studied by FT-ICR with the kinetic bracketing technique. The protonation by weak BH⁺ acids is dissociative, with an onset for C₆H₅CF₂⁺ and HF formation at a gas phase basicity of B equal to 172 kcal mol^-1. Stronger acids, CH₅⁺, SO₂H⁺ and C₂H₅⁺ yield persistent [CF₃C₆H₅]H⁺ ions. These results have been interpreted with the aid of ab initio MO calculations showing that the fluorine atoms have a higher proton affinity than the ring carbons. The dissociative proton transfer appears to be entropically driven, surmounting a C₆H₅CF₂⁺/HF binding energy of 10.3 kcal mol^-1. The kinetics of proton transfer from [CF₃C₆H₅]H⁺ to benzene and to (C₂H₅O)₂CO, a fairly strong base in the gas phase, show similar rate ratios in FT-ICR and in radiolytic systems at atmospheric pressure.

The structures and vibrational frequencies of the acetate ion interacting with a metal ion (Na⁺, Mg²⁺, and Ca²⁺) in the unidentate, bidentate, bridging, and pseudobridging forms are studied by ab initio molecular orbital calculations. Effects of a water molecule coordinating to either the acetate ion or the metal ion are also examined. The calculations are carried out by using the self-consistent reaction field method at the Hartree?Fock level with the 6-31+G** basis set. For the species interacting with a divalent metal cation, the lengths of the two CO bonds of the acetate ion are nearly equal in the bidentate form but are significantly different in the unidentate form. The frequency of the COO^- antisymmetric stretch of the unidentate species is higher than that of the ionic species, which is in turn higher than that of the bidentate species. The reverse is the case for the COO^- symmetric stretch. As a result, the frequency separations (Δν_a-s) between the COO^- antisymmetric and symmetric stretches for the unidentate, bidentate, and ionic species are in the following order:? Δν_a-s (unidentate) > Δν_a-s (ionic) > Δν_a-s (bidentate). It is demonstrated that such a correlation between the vibrational frequencies of the COO^- group and the types of its coordination to a divalent metal cation is related to changes in the CO bond lengths and the OCO angle. The results of the present study clarify the physical basis of the empirical structure?frequency correlation, which has been used in the analysis of the infrared spectra of Ca²⁺-binding proteins.

We have employed subpicosecond transient absorption spectroscopy to study the rate of electron injection following optical excitation of the ruthenium dye Ru^II(2,2‘-bipyridyl-4,4‘-dicarboxylate)₂(NCS)₂ (1) adsorbed onto the surface of nanocrystalline titanium dioxide (TiO₂) films. This sensitizer dye is of particular interest as it is the most efficient sensitizer dye reported to date and is receiving considerable attention for applications in photoelectrochemical solar energy conversion. Transient data collected for 1 adsorbed onto TiO₂ films were compared with those obtained for control dye-coated ZrO₂ films, as the high conduction band edge of ZrO₂ prevents electron injection. Adsorption of the dye onto the TiO₂ film was found to result in a rapid (<500 ps) quenching of the dye excited-state luminescence. Absorption difference spectra collected for the two dye-coated films were assigned by comparison with the spectroscopy of the dye excited and cation states in solution. These transient absorption data indicated that electron injection in these films occurs in ≤10^-12 s. Detailed analysis indicates the injection is at least biphasic, with ～50% occurring in <150 fs (instrument response limited) and 50% in 1.2 ± 0.2 ps. These ultrafast electron injection kinetics are contrasted with the charge recombination reaction, which occurs on the microsecond?millisecond time scales. The ultrafast rate of electron injection observed here is critical both for the high energy conversion efficiencies obtained with this sensitizer dye, and for the excellent long-term stability of this dye in photoelectrochemical solar cells.

Lead iodide (PbI₂) clusters were synthesized from the chemical reaction of NaI (or KI) with Pb(NO₃)₂ in H₂O, D₂O, CH₃OH, and C₃H₇OH solvents. The observation of absorption features between the 550 and 350 nm region obtained with an integrating sphere strongly suggests PbI₂ quantum dot formation in solution. Comparison of spectra of PbI₂ clusters in solution with PbI₂ clusters formed by impregnation of PbI₂ in four different pore-sized porous silica substrates indicates that the PbI₂ cluster size in solution is less than 2.5 nm in the lateral dimension. Atomic force microscopy (AFM) measurements of PbI₂ solutions deposited on mica and highly oriented pyrolytic graphite surfaces indicate that the clusters are single layered. The measured height is 1.0 ± 0.1 nm, which is ～0.3 nm larger than the layer thickness observed for the bulk materials. The swollen layer thickness can be attributed to the intralayer contraction from the strong lateral interaction among PbI₂ molecules, which is supported by ab initio calculations. Raman scattering measurements of the LO and TO modes of PbI₂ in bulk and in the confined state were also conducted in 50?150 cm^-1 region. Three bands observed at 74, 96, and 116 cm^-1 for the confined materials are assigned to the TO₂, LO₂, and LO₁ modes, respectively. The relatively small red shift in the LO modes for PbI₂ in the porous hosts may be caused by the surface phonon of PbI₂ nanoparticles confined in the porous silica.

Development of molecular diffusion to convective fingering is theoretically studied. Linear stability analysis of the Boussinesq perturbation equation including cross-diffusion terms was carried out for a ternary system. The theory describes a fluctuation growth process where the gravitationally stabilized fluid layers at initial are developed to the convective motion by the diffusion of solute. Onset time as well as onset condition of the fluctuation growth is derived. A theory to treat the steady fingering is also presented. A condition to stabilize the fingering flow is derived for the case that the intermolecular diffusion coefficients cannot be neglected.

The titled compounds are investigated using large one-particle basis sets in conjunction with the singles and doubles coupled-cluster method that includes a perturbational estimate of the effects of connected triple excitations, denoted CCSD(T). Accurate geometries, dipole moments, harmonic vibrational frequencies, and IR intensities are determined. Agreement with the available experimental data (which is limited to fundamental vibrational frequencies) is very good except for the Br?N stretching mode in BrNO₂. Convincing theoretical and experimental arguments are presented which indicate that the experimental assignment for the Br?N fundamental is in error. Accurate isomerization energies are determined at the CCSD(T) level using spdfg one-particle basis sets. Comparison of ab initio results for the titled compounds with previously published ab initio results from the fluorine and chlorine analogs elucidates several trends involving equilibrium geometries, bonding characteristics, and relative energies. Heats of formation of all three titled compounds are evaluated and used to show that BrNO₂ and cis-BrONO are sufficiently thermally stable to exist in the Earth's stratosphere. Possible stratospheric formation and destruction mechanisms are discussed.

Addition of Lewis acid cations such as Mg²⁺, Ca²⁺, Ba²⁺, Li⁺ and Na⁺ considerably improves the catalysis of CO₂ reduction by iron(0) tetraphenylporphyrins, in terms of both catalytic efficiency and lifetime of the catalyst. Carbon monoxide is the main product, while formate is formed to a lesser extent (30% with Mg²⁺ and only 10% with Li⁺). Cyclic voltammetry indicates that the order of reactivity of these Lewis acid synergists is Mg²⁺ ? Ca²⁺ > Ba²⁺ > Li⁺ > Na⁺. Systematic analysis of the kinetics allows the proposal of a mechanism for the effect of both the divalent and monovalent cations. In many instances, the assisted catalysis is so strong that the current is affected by product inhibition as observed previously with Br?nsted acid synergists. Self-inhibition is more irreversible in the present case and required a different approach to be analyzed. The importance of push?pull mechanisms in the catalysis of CO₂ reduction is thus confirmed; electrons are pushed into the CO₂ molecule by the electron-rich catalyst and the cleavage of one of the C?O bonds is helped by the presence of an electron deficient synergist.