Showing papers in "The Journal of Physical Chemistry in 2000"

Theoretical Studies of Excited State Proton Transfer in Small Model Systems

Abstract: Ab initio calculations that address the problem of excited-state proton transfer across an intramolecular hydrogen bond are reviewed. Small molecules, such as malonaldehyde, containing such a H-bond are first examined. This work reveals that in comparison to the ground state, the H-bond is strengthened and the transfer barrier reduced in the1ππ* state; opposite trends are noted in the triplet ππ* as well as nπ* states. Replacement of the H-bonding O atoms of malonaldehyde by N has only a small effect upon these results, as does enlargement or reduction of the malonaldehyde ring, coupled with anionic charge. The transfer barrier is linearly related to the equilibrium length of the H-bond in the various states of each system. Attachment of a phenyl ring to malonaldehyde introduces a fundamental asymmetry into the proton transfer potential, as the enol and keto tautomers are inequivalent. Whereas the enol is more stable in the ground and nπ* states, a reversal occurs in the ππ* states, which may be understoo...

On−Off Blinking and Multiple Bright States of Single Europium Ions in Eu3+:Y2O3 Nanocrystals

Abstract: We show that continuously illuminated single europium ions incorporated in yttrium oxide (Eu3+:Y2O3) nanocrystals (5−15 nm diameter) undergo on−off blinking on a variable time scale ranging from hundreds of milliseconds to several seconds. We observe both a pump-intensity dependent “duty factor” (on-time as a percentage of total measurement time), and quantum jumps between at least three well-defined luminescence intensity levels (bright states) from individual nanoparticles. Interesting switching or oscillation between different bright levels was also observed with a modulation rate that is dependent on pump-laser intensity. These features of single-ion luminescence are not observed for larger particles with multiple chromophores. We propose that these effects derive from pump laser-induced fluctuations between different quasi-stable Eu3+ symmetry sites that effectively modulate the electric dipole transition moment.

Temperature Dependence of the HO2 + ClO Reaction. 1. Reaction Kinetics by Pulsed Photolysis-Ultraviolet Absorption and ab Initio Studies of the Potential Surface

Abstract: The kinetics of the HO_2 + ClO reaction was studied using the flash photolysis/ultraviolet absorption technique over the temperature range 203−364 K and pressure range 50−700 Torr of N_2. In contrast to previous work, the temperature dependence displayed linear Arrhenius behavior over the entire temperature range with the rate constant being described by the expression k(T) = 2.84 × 10^(-12) exp{(312 ± 60)/T} cm^3 molecule^(-1) s^(-1). Ab initio calculations of intermediates and transition states have been carried out on the singlet and triplet potential energy surfaces. These calculations show that the reaction proceeds mainly through the ClO−HO_2 complex on the triplet surface; however, collisionally stabilized HOOOCl formed on the singlet surface will possess an appreciable lifetime due to large barriers toward decomposition to HCl and HOCl. Termolecular rate calculations using ab initio parameters lead to a strong collision rate constant of ∼5 × 10^(-32) cm^6 molecule^(-2) s^(-1) for HOOOCl formation. This intermediate may be important under both laboratory and atmospheric conditions.

The g-Factor Anisotropy of Plant Chlorophyll a•+

Abstract: High-field EPR experiments at 12 T/330 GHz were performed on chlorophyll a radical cations in methylene chloride at low temperatures. Using fully deuterated chlorophyll it was possible to obtain the principal components of the rhombic g-tensor as g{sub {alpha}} = 2.00329 {+-} (5 x 10{sup -5}), g{sub {beta}} = 2.00275 {+-} (8 x 10{sup -5}), and g{sub {gamma}} = 2.00220 {+-} (8 x 10{sup -5}). Protonated chlorophyll radicals did not give enough spectral resolution to yield the g-anisotropy from the EPR spectrum. This was true even at higher field/frequency combinations up to 24 T/670 GHz. Semiempirical calculations were performed using the INDO/S method (ZINDO) which yielded good agreement with the experimental data.

Dye-sensitized Nanocrystalline Ti-oxide-based Solar Cells Prepared by Sputtering: Influence of the Substrate Temperature During Deposition

Abstract: Dye-sensitized Nanocrystalline Ti-oxide-based Solar Cells Prepared by Sputtering: Influence of the Substrate Temperature During Deposition

Characterization and Structure of Molecular Aggregates of a Tetracationic Porphyrin in LB Films with a Lipid Anchor

Abstract: In this paper, the formation of the different aggregates of a water-soluble tetracationic porphyrin, TMPyP, anchored to an anionic phospholipid matrix, DMPA, and assembled in LB films has been investigated. The use of different solid substrates such as hydrophilic glass and supports modified by the transfer of monolayers of several lipids has been employed to assemble the TMPyP molecules in the mixed monolayer with DMPA (in a molar ratio of 1:4) as monomer (TMPyP4+), dimer, diprotonated monomer (TMPyPH26+), and tetramer aggregates. The porphyrin is organized as a monomer (TMPyP4+) when the mixed monolayer is directly transferred onto a hydrophilic glass plate. Also, if the glass and the porphyrin are separated by spacer layers such as a monolayer sequence EA↑/AA↓/EA↑/AA↓, the porphyrin monomer is identified. Porphyrin dimers are formed at the air−water interface in monolayers of 1:4 TMPyP/DMPA20 and may be transferred to solid substrates by the transfer of this system on top of the DMPA monolayer, i.e., (...

Vibrational spectroscopy and ab initio calculation on [N(C2F5SO2)2] and the corresponding superacid HN(C2F5SO2)2.

Abstract: Vibrational spectroscopy and ab initio calculation on [N(C2F5SO2)2] and the corresponding superacid HN(C2F5SO2)2.

Photophysical and Photochemical Processes of 2-Methyl, 2-Ethyl, and 2-tert-Butylanthracenes on Silica Gel. A Substituent Effect Study

Abstract: The photophysics and photochemistry of 2-methylanthracene (2MA), 2-ethylanthracene (2EA), and 2-tert-butylanthracene (2TBA) adsorbed on silica was studied at a silica/air interface to determine the substituent effect on the above processes. In contrast to anthracene (AN), which forms ground state stable pairs at surface coverages as low as 1% of a monolayer, 2MA, 2EA, and 2TBA show no evidence of any ground state pairing even at high surface coverages. Diffuse reflectance and fluorescence data indicate that 2MA, 2EA, and 2TBA crystallize on the surface at higher surface coverages. Photolysis of 2MA, 2EA, and 2TBA at a silica/air interface proceeds more efficiently than photolysis of AN to produce the corresponding 9,10-endoperoxides formed by the addition of singlet molecular oxygen (type II) to the ground state molecules. Thermally unstable endoperoxides slowly decompose on silica surface to give the corresponding 9,10-quinones and other hydroxylated products of 2MA, 2EA, and 2TBA. Although photolyzed AN...

Time-resolved study of acetyl radical in zeolite NaY by step-scan FT-IR spectroscopy

Abstract: Step-scan FT-infrared spectroscopy of 1-naphthyl acetate and pinacolone photodissociation in zeolite NaY revealed a transient at 2125 cm-1 assigned to acetyl radical. Comparison of the intensities of transient and final product absorptions, 2-acetyl-1-naphthol in the case of naphthyl acetate and acetaldehyde for pinacolone, indicates that the acetyl radical represents the main, and probably only, reaction channel. The decay of the radical at room temperature is best described by a single-exponential law with a lifetime of 71 ± 15 μs (1-naphthyl acetate) and 315 ± 30 μs (pinacolone). This constitutes the first detection and kinetic study of a small transient radical in a zeolite. The kinetic result is interpreted in terms of complete separation of the photogenerated pairs from the parent supercage, followed by random walks in subspaces of the zeolite lattice imposed by the much less mobile precursor molecules. These force the geminate radicals to react and thereby contribute to the high selectivity of thes...

A combined temperature-calorimetric study of ion adsorption at the hematite-electrolyte interface: I. Model of a homogeneous oxide surface

Abstract: The two hematite/KNO3 adsorption systems investigated in Part I are now subjected to a more refined quantitative analysis based on the models of an energetically heterogeneous oxide surface. The estimated parameters lead to a much more consistent, simultaneous fit of both titration isotherms and the related directly measured enthalpic effects. That quantitative analysis reveals that the differences in the preparation of these two hematite samples result in substantial differences in the heats accompanying ion adsorption. Generally, the more porous surface is, the lower are the heats of proton and cation adsorption, due probably to a deeper dehydratation of adsorbed ions. Our quantitative analysis also reveals substantial electrostatic contributions to the observed enthalpic effects, as well as contributions caused by surface energetic heterogeneity.

The CO/Pt(111) Puzzle

Abstract: Notwithstanding half a dozen theoretical publications, well-converged density-functional calculations, whether based on a local or generalized-gradient exchange-correlation potential, whether all-electron or employing pseudopotentials underestimate CO's preference for low-coordination binding sites on Pt(111) and vicinals to it. For example, they imply that CO should prefer hollow- to atop-site adsorption on Pt(111), in apparent contradiction to a host of low temperature experimental studies.

Methane chemistry in the hot supersonic nozzle

Abstract: Combination of pyrolysis and expansion to a supersonic molecular beam is shown to be very effective in conversion of pure methane to heavier hydrocarbons. The conversion reaches ∼70% in the supersonic nozzle made of quartz with the diameter of orifice equal 100 μ. The major products are acetylene, benzene and its homologues. Free radicals are also detected in the product distribution. The mechanism consists of the surface generation of the radicals, C−C bond formation, desorption to the gas phase, and expansion to the supersonic beam. It is suggested that relatively short gas−surface contact time (about 50 ms at Tnozzle = 1000 °C) enables simultaneous kinetic and thermodynamic control.