Showing papers in "The Journal of Physical Chemistry in 1985"

Solvent effects on emission yield and lifetime for coumarin laser dyes. Requirements for a rotatory decay mechanism

Abstract: Photophysical parameters have been determined for coumarin laser dyes in a variety of organic solvents, water, and mixed media. The response of fluorescence emission yield and lifetime to changes in solvent polarity was a sensitive function of coumarin substitution pattern. Most important were substituent influences which resulted in enlarged excited-state dipole moments for the fluorescent state. For dyes displaying sharp reductions in emission yield and lifetime with increased solvent polarity, protic media and particularly water were most effective in inhibiting fluorescence. The temperature dependence of emission yield and lifetime was measured for two solvent-sensitive dyes in acetonitrile and in a highly viscous solvent, glycerol. The quenching of coumarin fluorescence by oxygen for dyes with lifetimes > 2 ns was also observed. The dominant photophysical features for coumarin dyes are discussed in terms of emission from an intramolecular charge-transfer (ICT) excited state and an important nonradiative decay path involving rotation of the amine functionality (7-position) leading to a twisted intramolecular CT state (TICT). The role of excited-state bond orders involving the rotating group in determining the importance of interconversions of the type ICT ..-->.. TICT is discussed. 73 references, 1 figure, 3 tables.

Resistance of Polyaniline Films as a Function of Electrochemical Potential and the Fabrication of Polyaniline-Based Microelectronic Devices.

Abstract: : Anodic growth of polyaniline films on a Au microelectrode array has been carried out to add to the characterization of polyaniline and to fabricate polyaniline-based microelectronic devices, diodes and transistors, that function when the polyaniline-functionalized microelectrode array is immersed in an electrolyte solution. The microelectrode array is a set of eight Au electrodes, 0.1 micrometer thick, 4.4 micrometers wide and 50 micrometers long, each individually addressable and separated from each other by 1.7 micrometers. Polyaniline can be deposited in controlled amounts by electrochemical oxidation of aniline. When the polyaniline is deposited in sufficient amounts, two or more of the eight Au microelectrodes can be connected in the electrical sense. Current can pass between two connected microelectrodes when there is an applied potential between them. The magnitude of the current at a given applied potential depends on the electrochemical potential of the polyaniline. In 0.5 M NaHsSO4/H20 the current (at a fixed applied potential) is maximum at an electrochemical potential of approx. +0.4 V vs. SCE and declines by a factor of greater than 1,000,000 upon reduction to +0.1 or oxidation to +0,7 V vs. SCE. Owing to the large change in resistance upon change in electrochemical potential, the functionalized microelectrodes is crucial to device function, owing to the limited conductivity of the polyaniline. The switching time of a transistor-like device is faster than 0.1 s.

Search for stationary points on surfaces

Abstract: Algorithms for finding local minima, maxima, and saddle points on surfaces, starting from an arbitrary point are presented. These algorithms are based on making a local approximation to the surface in the form of a rational function constructed from the local first and second derivatives of the surface. All parameters of these algorithms required for stepping across the surface are determined in nonarbitrary ways. The convergence of these procedures to the desired stationary point is shown to be quadratic. Applications for stationary-point searches on two model surfaces are also given for illustrative purposes. 12 references, 4 figures.

A new model for upper and lower critical solution temperatures in poly(ethylene oxide) solutions

Abstract: Diagramme de phases du systeme PEO-eau calcule en utilisant la theorie de Flory; accord semi-quantitatif avec l'experience

Isomorphous substitution in zeolite frameworks. 1. Acidity of surface hydroxyls in [B]-, [Fe]-, [Ga]-, and [Al]-ZSM-5

Abstract: A series of isomorphously framework-substituted ZSM-5 zeolites has been characterized by Fourier transform infrared spectroscopy, with primary focus on the nu(OH) region, and by temperature programmed NH3 desorption. The protonic form of the zeolites exhibited two characteristic absorptions in the nu(OH) region: a band at 3740 cm assigned to terminal SiOH, and a band at lower frequency assigned to M(OH)Si (bridging hydroxyl), whose frequency depends on acid strength. Based on these techniques, the relative Bronsted acidity is found to increase according to SiOH < B(OH)Si << Fe(OH)Si < Ga (OH)Si < Al(OH)Si.

The absolute potential of the standard hydrogen electrode: a new estimate

Abstract: Essai d'estimation de l'echelle absolue a l'aide d'une methode qui deplace le probleme de la couche dipolaire superficielle hors de l'interface electrolyte-air et le focalise plus sur le solide

Role of solvent reorganization dynamics in electron-transfer processes. Theory-experiment comparisons for electrochemical and homogeneous electron exchange involving metallocene redox couples

Abstract: : Electrochemical Rate Constants and Activation Parameters are reported for the electron exchange of five metallocene couples and dibenzenechromium (I)/ (O) in eight solvents at mercury electrodes. The solvents (acetonitrile, acetone, methylene chloride, formamide, N-methylformamide, N,N'- dimethylformamide, dimethylsulfoxide, and benzonitrile) were chosen so to provide substantial variations in their dynamical as well as dielectric properties. The metallocene couples are of the form M(Cp)2(+/o), where M = Fe, Co, or Mn, and Cp = cyclopentadiene or pentamethylcyclopentadiene. The inner- shell (i.e., bond distortional) barriers are calculated for the metallocene and arene couples from bond-distance and vibrational data to be small (< or approx, 0.25 kcal/mol) yet metal-dependent. Detailed comparisons of the observed solvent-dependent kinetics are made with the rate parameters calculated from contemporary theoretical treatments of outer-sphere electron transfer. Considerably better agreement between the experimental and theoretical kinetic parameters was obtained when the latter take into account the influence of solvent friction upon the barrier-crossing frequency. These results indicate that the conventional transition-state theory may not apply to electron-transfer reactions where the free-energy barrier is due chiefly to solvent reorganization, at least in 'high friction' media where concerted solvent relaxation is slow.

High-temperature stabilities of hydrocarbons

Abstract: Analyse thermodynamique chimique de molecules d'hydrocarbures de 1500° a 3000°K pour les especes C 2n H 2m (n=1-21, m=1-8)

Surface-enhanced Raman spectroscopy as a probe of adsorbate-surface bonding: benzene and monosubstituted benzenes adsorbed at gold electrodes

Abstract: : Surface-enhanced Raman spectra of benzene and eight mono substituted benzenes adsorbed at gold electrodes have been examined in order to probe the nature of adsorbate-surface bonding for simple aromatic molecules. Benzene appears to adsorb flat via pi adsorbate-surface interactions as evidenced by the significant (20-30/cm) downshifts in the symmetric rig breathing mode (v sub 1) and the absence of other ring modes in the SER spectra. Similar results were obtained using gold surfaces prepared by electroplating or anodic-cathodic potential sweeps in chloride, although much more stable SERS was obtained by using the latter procedure. The ring substituents -CHD3, -CH(CH3)2, -C(CH3)3, - Cl, -Br, -CN, -NO2, -COO(-) and -CHO were chosen so to provide steric perturbations or alternative binding modes to the metal surface. With the alkyl, benzoate, and aldehyde substituents, small yet significant (ca. 5-15/cm) decreases in the v sub 12 and v sub 18a characteristic ring modes along with band broadening were observed upon adsorption, consistent with attachment via the benzene ring; For the halogen substituents, smaller such downshifts were obtained along with the appearance of low-frequency modes associated with halogen-surface vibrations, implicating the role of substituent bonding in the adsorption. For the nitrile and nitro substituents, the ring modes were unaltered in frequency or band shape upon adsorption while intense internal substituent modes were observed.

Pulse radiolytic studies of the reaction of perhydroxyl/superoxide O2- with iron(II)/iron(III) ions. The reactivity of HO2/O2- with ferric ions and its implication on the occurrence of the Haber-Weiss reaction

Abstract: Etude des reactions de l'ion Fe 2+ et des complexes FeOH 2+ , FeSO 4 + avec les radicaux superoxyde et perhydroxyle en fonction du pH(1-7) en milieu aqueux sulfate et formiate. Donnees cinetiques. ApH neutre le radical superoxyde est catalytiquement decompose en dioxygene et peroxyde d'hydrogene. Mecanismes

Poly(3-methylthiophene)-coated electrodes: optical and electrical properties as a function of redox potential and amplification of electrical and chemical signals using poly(3-methylthiophene)-based microelectrochemical transistors

Abstract: : Optical and electrical properties of anodically grown poly-3-methythiophene are reported as a function of redox potential in CH3CN/0.1M (n-Bu4N)ClO4. Poly-3-methylthiophene can be grown by the oxidation of 3-methylthiophene and deposited onto Au or Pt electrode surfaces. A pair of poly-3-methylthiophene-connected microelectrodes can function as a transistor where one of the electrodes is regarded as 'source' and the other as 'drain' with the source being referenced to the solution as a gate. The poly-3-methylthiophene is the analogue of the channel of a solid state field effect transistor, since its conductivity changes by 100,000,000 depending on the potential. Large optical (300-800 nm) and electrical changes for the poly-3-methylthiophene occur between approx. +0.3 V and approx. +0.8 V vs. SCE. The reduced material has an absorption maximum at 490 nm and the oxidized material has an absorption maximum at 750 nm. The optical density changes parallel the resistant changes that occur as the potential of the polymer changes between approx. +0.3 and approx. +0.8 V vs. SCE. The charging of a 1.5 micrometer thick film of poly-3-methylthiophene involves approx. 10,000 time more charge per unit of projected area then a smooth Pt electrode, consistent with a large effective internal surface area for the conducting polymer.

Photoluminescence and photocatalytic activity of highly dispersed titanium oxide anchored onto porous Vycor glass

Abstract: Extinction de la photoluminescence par O 2 ou N 2 O associee a un transfert d'electron du catalyseur vers les molecules precitees. La grande dispersion des ions Ti et/ou l'insaturation de leur surface expliquent l'augmentation de l'activite photocatalytique ainsi que celle de la photoluminescence

Making and breaking bonds in the solid state: the ThCr2Si2 structure

Abstract: Among the many examples of the ThCr/sub 2/Si/sub 2/ structural type is a set of AB/sub 2/X/sub 2/ structures, B = transition metal, X = group 14 or 15 element, in which one X...X contact varies over the range of bonding between no bond and a fully formed X...X single bond. The tuning is provided by variation in the transition metal, X...X bonding being promoted by metals on the right-hand side of the periodic table. When two-dimensional layers are stacked, narrow X-X sigma and sigma* bands result. The sigma* level is filled (no X-X bond) or empty (X-X bonding) depending on the Fermi level of the transition-metal B layer. 35 references, 8 figures, 3 tables.

Free Radical and Free Atom Reactions in the Sonolysis of Aqueous Iodide and Formate Solutions

Abstract: On etudie l'influence de l'oxygene sur la formation de peroxyde d'hydrogene en irradiant des solutions de iodure de potassium et de formiate de sodium sous divers melanges d'argon et d'oxygene

Theoretical thermochemistry. 1. Heats of formation of neutral AHn molecules (A = Li to Cl)

Abstract: Ab initio molecular orbital theory (Moller-Plesset theory to full fourth order and a series of extended basis sets) is used to compute total atomization energies for the complete series of AH/sub n/ hydrides (A = Li to Cl). These data are used in combination with generally accepted experimental data on isolated atoms to predict ideal gas heats of formation at 298.15 K for these molecules. Comparisons with high-quality experimental data indicates that, for hydrides, this level of theory has an error level of about +/-2 kcal mol/sup -1/. For several of these hydrides, there are no experimentally determined heats of formation. The authors predict ..delta..H/sub f//sup 0//sub 298/ values (in kcal mol/sup -1/) for BeH/sub 2/ (39.5), MgH/sub 2/ (38.6), AlH/sub 2/ (64.0), PH (59.2), and PH/sub 2/ (34.8). The authors computed heats of formation call into question the experimental heats of formation of BH/sub 2/ and SiH/sub 2/. 23 references, 4 tables.

An infrared study of the influence of carbon monoxide chemisorption on the topology of supported rhodium

Abstract: The adsorption of CO on alumina-supported Rh of different crystallite sizes produced by reduction at 573-1273 K has been investigated by following the development of the infrared bands due to Rh (CO)2 and Rh-CO species. It is concluded that the effect of CO on the state of the dispersed Rh is a two fold one. In harmony with recent EXAFS measurements of Prins et al., CO adsorption at around 300 K leads to disruption of the Rh clusters and to the formation of isolated Rh sites, as indicated by the slow development of the infrared bands of gem-dicarbonyl. Above 423 K, another effect of adsorbed CO comes into prominence, which leads to the formation of Rh crystallites at the expense of isolated Rh sites. It is demonstrated that the formation of isolated Rh sites occurs more slowly under dry conditions. 28 references, 7 figures, 1 table.

Photodissociation of acetylene at 193.3 nm

Abstract: The authors have measured the translational energy release for processes C2H2 193 nm/ C2H + H (I) and C2H 193 nm/ C2 + H (II) using the molecular time-of-flight method. By measuring the maximum release of translational energy for process I it has been determined that the C-H bond energy in acetylene, D0(C2H-H), is 132 +/- 2 kcal/mol. Since the translational energy distribution peaks well away from zero, it is quite unlikely that internal conversion of the electronically excited C2H2 is an important channel for dissociation in channel I. Because of the relatively small amount of energy appearing as product rotation, it is possible to get information on the vibrational structure of C2H radical from the translational energy distribution and it is found that the bending frequency in C2H is 550 +/- 100 cm . This experiment is consistent with earlier work which observed C2( /sub u/) in fluorescence. 21 references, 20 figures, 3 tables.

Flash photolysis of transient radicals. 1. X2- with X = Cl, Br, I, and SCN

Abstract: The radicals Cl/sub 2//sup -/, Br/sub 2//sup -/, I/sub 2//sup -/, and (SCN)/sub 2//sup -/ were prepared by photolysis of appropriate chemical systems with one laser and were subsequently photolyzed with a second laser. The first three species were photolyzed at 355 nm, and (SCN)/sub 2//sup -/ was photolyzed at 532 nm, I/sub 2//sup -/ was also photolyzed at 700 nm. In each case, dissociation into the fragments X and X/sup -/ was detected by a bleach in the absorption of X/sub 2//sup -/. In no case was electron photodetachment observed. Observation of the recovery of the original absorption allowed the corresponding rate constant to be measured. In the case of Cl/sub 2//sup -/ the bleach in absorption does not completely recovery and the loss of absorption is dose dependent. Because the presence of acid allows a more complete recovery, it can be concluded that the product Cl atom is photolyzed by a second photon to produce OH and Cl/sup -/. This photoreaction is direct experimental evidence that the absorption band of Cl involves charge transfer from solvent. Detailed analysis of the bleaching and recovery behavior at high Cl/sup -/ concentrations showed no time lag which could be attributed tomore » the /sup 2/P/sub 1/2/ Cl atom, implying a short lifetime for this species. 13 references, 6 figures, 2 tables.« less

Size quantization in small semiconductor particles

Abstract: Optical effects due to size quantization in three dimensions have been observed for CdS and PbS colloids with particle diameters less than 50 A and 20 to 200 A, respectively. The optical absorption edge is blue shifted approx.0.9 eV for the CdS colloid; for PbS colloids the shift is approx.2 eV for particle diameters 20 to 30 A, and approx.1 eV for particle diameters 50 to 200 A. The results are consistent with perturbation of the semiconductor band structure due to carrier confinement resulting in an increase in the effective band gap. 14 refs., 3 figs.