Showing papers in "The Journal of Physical Chemistry in 2002"

Near Infrared Optical Absorption of Gold Nanoparticle Aggregates

Abstract: The reduction of HAuCl4 by Na2S has been reported to produce gold nanoparticles with an optical absorption in the near-infrared along with its characteristic absorption in the visible. The optical resonances in the visible are due to the gold surface plasma, which are a function of the geometry of the particles. The near-infrared absorption had been attributed to the formation of Au2S/Au core/shell structures. In this report we present new electronic absorption, electron microscopy, and X-ray absorption data in several systems to show that the near-infrared absorption does not involve core/shell structures. We further suggest that the near-infrared adsorption is most likely the result of the formation of aggregates of gold nanoparticles. The identification of the origin of the near-infrared resonance is critical in understanding the optical properties of metal nanoparticle systems.

Binding Mode of meso-Tetrakis(N-methylpyridinium-4-yl)porphyrin to Poly[d(I−C)2]: Effect of Amino Group at the Minor Groove of Poly[d(G−C)2] on the Porphyrin−DNA Interaction

Abstract: The spectral properties of meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (TMPyP) complexed with poly[d(I−C)2] were compared with that bound to poly[d(G−C)2] and poly[d(A−T)2]. At low TMPyP/DNA base ratios, the TMPyP-poly[d(I−C)2] complex exhibits a similar NaCl dependent absorption and circular dichroism spectrum to that of the TMPyP-poly[d(A−T)2] complex. The negative CD band in the Soret absorption region of these complexes disappears as the NaCl concentration reaches around 10 mM. At a NaCl concentration higher than 10 mM, the resulting positive CD band is pronounced. In contrast, the change in negative CD band of the TMPyP-poly[d(G−C)2] complex, which is believed to be intercalated, is small. The molecular plane of porphyrin in the TMPyP-poly[d(I−C)2] complex for both species associated with negative and positive CD is perpendicular relative to the polynucleotide helix axis. Therefore, it is indicative that TMPyP (partially) intercalates between the base pairs of poly[d(I−C)2] at a very low mixing r...

Modeling the NO+H2 reaction on a Pt field emitter tip : Mean-field analysis and Monte Carlo simulations

Abstract: A minmwlmodel for the NO + 112 reaction on a ]'t cmitter tip is proposed, with emphasis placcd on surtnc.:: explosions observed in fleld ion microscopy experiments. The model is first studied in the mean-neld approximation, where it is shown 10 exhibit bistability and associated explosive phenomcna. Using kinetic Monte Carlo simulalions, irreversihle phase translIions, fluctuation induced dynamics and reaclion front prop.1gallon arc observed, which arc not predicted by the mean- field approach, This comparative study allow; to shed some light on the orgins of the surface explosions. and to ffivestigate lhe influcnce of the discrete nature of the suppon on the dynamics.

Red versus Blue-Shifting Hydrogen Bonds: Are There Fundamental Distinctions?

Abstract: The OH covalent bond of conventional hydrogen bonds of the OH···O type undergoes a lengthening as a result of the complex formation, and the OH stretching frequency shifts to the red; the same is t...

Geometric investigation of association/dissociation kinetics with an application to the master equation for CH{sub 3} + CH{sub 3}{leftrightarrow}C{sub 2}H{sub 6}.

Abstract: The dynamics of association/dissociation kinetics is studied, with an application to the titled reaction. The focus is the geometry of the phase space of the phenomenological rate law, a Lindemann mechanism, and the master equation of this reversible reaction, all of which are nonlinear. It is shown that all three systems possess similar phase space structure, including a 1-D manifold. This 1-D manifold describes asymptotic motion for either dissociation or recombination and is the analogue of the corresponding eigenvector for linear master equations describing dissociation without recombination. The 1-D manifold allows for the separation of asymptotic motion from transient behavior and together with other manifolds in phase space allows a better understanding of the dissociation and recombination processes. The 1-D manifold also allows us to test various approximations that have been used in the past to calculate association rate constants from the master equation and Lindemann mechanism and develop new methods for calculating association rate constants and generating rate laws.

Correlation between Acoustic Caviation Noise, Bubble Population, and Sonochemsitry

Abstract: Quantitative studies of the correlation between sonochemical activity and acoustical noise spectra have been performed. The width of the second harmonic (fwhm2) of the acoustical signal in the frequency domain shows a sensitive dependence to the presence of small amounts (mM range) of an anionic surfactant in water. This sensitive dependence is also observed for other characteristics of the cavitation noise spectrum and in the sonochemical production of peroxides and correlates well with the sonoluminescence intensity observed by other researchers. Analysis of the experimental data shows that SDS probably modifies the coalescence phenomena.