Showing papers in "The Journal of Physical Chemistry in 1999"

Pump-Probe Spectroscopy and the Exciton Delocalization Length in Molecular Aggregates

Abstract: Using numerical simulations, we investigate to what extent the pump−probe spectrum can be used as a tool to determine the exciton delocalization length in disordered molecular (J) aggregates. We compare the delocalization length obtained through heuristic arguments from the spectral separation between the bleaching and the one- to two-exciton induced absorption features in this spectrum to the delocalization length obtained from the participation ratio at the J band center. We find that up to a certain saturation length these two delocalization lengths are indeed proportional. In the case of long-pulse two-color pump−probe spectra, the slope of this linear scaling is insensitive to the pump frequency; it is sensitive to the width of the probe pulse. Beyond the saturation length, which is determined by the homogeneous line width of the exciton transitions, both the two-color and the short-pulse pump−probe spectra saturate and no longer can be used as a measure for the delocalization length. The latter appl...

Sum Frequency Generation of CO on (111) and Polycrystalline Platinum Electrode Surfaces: Evidence for SFG Invisible Surface CO

Abstract: The vibrational spectroscopy sum frequency generation (SFG) is used to investigate the adsorption of carbon monoxide on the single crystal (111) and polycrystalline platinum surfaces. By varying the frequency and polarization of the light beams, different surface species of CO species are probed. SFG signal intensities for different polarization indicate that adsorbed CO polarizability is significantly perturbed from the gas-phase molecule. The SFG signal of CO disappears well below the main oxidation potential of CO to CO2. The disappearance of the CO signal is interpreted as a transformation in the CO layer to a state which is invisible to SFG. The invisible state is suggested to be CO with the bond axis nearly parallel to the platinum surface.

Strength of Solvent-Exposed Salt-Bridges†

Abstract: This paper uses a recently developed computer model to study the energetics of solvent-exposed salt-bridges. The model uses the “mining minima” method to compute conformational free energies with the CHARMm empirical force and the generalized Born solvation model. Satisfactory agreement is obtained in comparison with the measured binding affinities of ion pairs in solution and with the salt-bridge energetics deduced from studies of salt-bridges in helical peptides. The calculations suggest that stabilizing charge−charge interactions in helical peptides do not require well-defined salt-bridge conformations. This is in agreement with crystallographic studies of charge pairs added to T4 lysozyme by site-directed mutagenesis. The computer model is also used to make a testable prediction that arginine and phosphotyrosine residues in an (i, i + 4) relationship will form a particularly strong salt-bridge in helical peptides. The biological implications of these results are discussed.

129Xe NMR of Mesoporous Silicas

Abstract: The porosities of three mesoporous silica materials were characterized with {sup 129}Xe NMR spectroscopy. The materials were synthesized by a sol-gel process with r = 0, 25, and 70% methanol by weight in an aqueous cetyltrimethylammonium bromide solution. Temperature dependent chemical shifts and spin lattice relaxation times reveal that xenon does not penetrate the pores of the largely disordered (r= 70%) silica. For both r = 0 and 25%, temperature dependent resonances corresponding to physisorbed xenon were observed. An additional resonance for the r = 25% sample was attributed to xenon between the disordered cylindrical pores. 2D NMR exchange experiments corroborate the spin lattice relaxation data which show that xenon is in rapid exchange between the adsorbed and the gas phase.

Molecular thermodynamics for protein precipitation with a polyelectrolyte

Abstract: A molecular-thermodynamic framework is developed for phase equilibria in an aqueous system containing a charged globular protein and an oppositely charged linear polyelectrolyte. The globular protein is represented by a spherical macroion with its coins; the linear polyelectrolyte is represented by a charged hard-sphere chain (polyion) with corresponding counterions. The potential of mean force contains Coulombic interactions between macroions, polyions, and small ions; long-range dispersion attractions between protein macroions; and hydrophobic macroion-polyion and macroion-macroion associations. Analytic expressions for thermodynamic properties are obtained, and liquid-liquid phase equilibria (precipitation) are calculated for model systems. Adding polyelectrolyte to a protein solution leads to precipitation, but further addition of polyelectrolyte leads to redissolution of the protein. This destabilization-restabilization phenomenon follows from electrostatic interactions with coupled polymer adsorption. The effects on phase equilibria of protein charge, protein size, association energy between protein-polyion, polyion chain length, and polyion charge density are investigated for model systems and compared with experimental data. Brief consideration is given to fractional precipitation for binary aqueous mixtures of proteins with different charge densities.

The Iron(III)-Catalyzed Oxidation of Ethanol by Hydrogen Peroxide: A Thermokinetic Oscillator

Abstract: We present an experimental and theoretical study of a laboratory scale continuous flow stirred tank reactor (CSTR) in which the exothermic iron(III)-nitrate-catalyzed oxidation of ethanol with hydrogen peroxide to ethanal and acetic acid takes place. This reaction can display temperature and concentration oscillations when it is performed in a CSTR. A model is known in the literature, which is derived from a more detailed mechanism. We investigate the behavior of the system under different conditions using the volumetric flow of the cooling water as an experimental bifurcation parameter. The model is analyzed by one and two parameter continuation of stationary and periodic solutions. We characterize period doubling sequences to chaos, homoclinic orbits, and cross-shaped diagrams, which separate regions of oscillations and bistability.

Generalization of the linearized approximation to thesemiclassical initial value representation for reactive flux correlationfunctions

Abstract: The semiclassical (SC) initial value representation (IVR) provides a general and practical approach for including quantum effects in classical molecular dynamics simulations. The linearized approximation (LA) to the SC-IVR simplifies the description much further, reducing it to the well-known classical Wigner model (i.e., a classical trajectory calculation with a Wigner distribution of initial conditions); the LA is able to describe quantum effects well for short times (t {approx{underscore}lt} {Dirac{underscore}h}{beta}) but not so for longer times. It is shown here how the full SC-IVR approach, which is able to describe quantum effects for long times, can be cast in a form very similar in structure to the LA, with specific application to flux correlation functions relevant to chemical reaction rates. This formulation may thus make it possible to carry out full SC-IVR calculations while still retaining much of the simplifying aspects of its linearized approximation.

The Structure and Density of Mo and Acid Sites in Mo-ExchangedH-ZSMZ Catalysts

Abstract: Mo/H-ZSM5 (1.0-6.3 wt percent Mo; Mo/Al = 0.11-0.68) catalysts for CH4 aromatization were prepared from physical mixtures of MoO3 and H-ZSM5 (Si/Al= 14.3). X-ray diffraction and elemental analysis of physical mixtures treated in air indicate that MoOx species migrate onto the external ZSM5 surface at about 623 K. Between 773 and 973 K, MoOx species migrate inside zeolite channels via surface and gas phase transport, exchange at acid sites, and react to form H2O. The amount of H2O evolved during exchange and the amount of residual OH groups detected by isotopic equilibration with D2 showed that each Mo atom replaces one H+ during exchange. This stoichiometry and the requirement for charge compensation suggest that exchanged species consist of (Mo2O5)2+ditetrahedral structures interacting with two cation exchange sites. The proposed mechanism may provide a general framework to describe the exchange of multivalent cations onto Al sites in zeolites. As the Mo concentration exceeds that required to form a MoOx monolayer on the external zeolite surface ({approx}4 wt percent Mo for the H-ZSM5 used), Mo species sublime as (MoO3)n oligomers or extract Al from the zeolite framework to form inactive Al2(MoO4)3 domains detectable by 27Al NMR. These (Mo2O5)2+ species reduce to form themore » active MoCx species during the initial stages of CH4 conversion reactions. Optimum CH4 aromatization rates were obtained on catalysts with intermediate Mo contents ({approx}0.4Mo/Al), because both MoCx and acid sites are required to activate CH4 and to convert the initial C2H4 products into C6+ aromatics favored by thermodynamics.« less

The Heterogeneous Chemistry of Acetone in Sulfuric Acid Solutions: Implications for the Upper Troposphere

Abstract: The uptake of acetone vapor by liquid sulfuric acid has been investigated over the range of 40-87 wt. %H(sub 2)SO(sub 4) and between the temperatures 198 to 300K.

Alignment and Orientation Effects in Sr Energy Pooling

Abstract: then the total energy transfer cross section for a particular initial state alignment can be expressed as a sum of the so-called fundamental cross sections, Um1m2 and Um1m2;m1′m2′, that describe collisions between the various mj states. Here Um1m2 represents the cross section for energy transfer when an atom in state m1 collides with an atom in state m2. The cross section Um1m2;m1′m2′ represents the contribution to the total cross section from interference when the colliding system is in a superposition of the state m1 colliding with m2 and the state m1′ colliding with m2′. It is found that the cross sections U1-1, U00, and U10 as well as the interference terms Re(U00;1-1) and U01;10 have relatively large values while U11 and U1-1;-11 are small. Coupled with future theoretical work, these results may provide new insights into the dynamics of the curve crossings that lead to energy transfer.