Donald M. Friedrich

Characterization of Salicylate-Alumina Surface Complexes by Polarized Fluorescence Spectroscopy

Speciation of salicylate anions (o-hydroxybenzoate) adsorbed on aqueous colloidal alumina (δ-Al2O3) was determined by polarized fluorescence excitation spectroscopy. Adsorption isotherms and pH and ionic strength edges suggest the existence of both inner-sphere and outer-sphere salicylate surface complexes. Spectroscopic characteristics of inner-sphere surface-salicylate complexes (one bidentate and two monodentate) were identified through comparison of suspension spectra with remarkably similar fluorescence and excitation spectra of solution phase Al-salicylate complexes. The large fluorescence Stokes gap of the aqueous salicylate anion is highly sensitive to complexation, resulting in spectral shifts characteristic of aluminum binding in the three inner-sphere salicylate complexes. These species appear to be present even at extremely low surface coverage, and the relative distributions are dependent on pH, ionic strength, and the relative concentrations of alumina and salicylate. The bidentate complex, however, is the predominant species at low surface coverages. Fluorescence anisotropy measurements, both steady-state and time-resolved, demonstrate that the bidentate and monodentate surface complexes do not undergo rotational reorientation on the time-scale of the fluorescence (τf = 4.0 ns), consistent with inner-sphere, polar covalent binding of these salicylate complexes to alumina surface sites. At high surface coverage, time-resolved anisotropy measurements suggest the existence of a surface salicylate species that is rotationally hindered (τr = 31 ps ) relative to free solution phase salicylate ions (τ r = 20 ps ). This behavior is consistent with an electrostatically bound outer-sphere complex suggested by the pH and ionic-strength sorption edges.

Kinetics and mechanism of surface reaction of salicylate on alumina in colloidal aqueous suspension

The reaction kinetics of salicylate with Al(III) in aqueous solution and at the colloidal alumina–water interface was studied by stopped-flow laser fluorescence spectroscopy. Temporal evolution of the fluorescence spectra suggests that formation of a carboxylate monodentate complex was the reaction intermediate that occurs transiently at the beginning of the reaction in aqueous salicylate–Al(III) solution. However, by lowering the pH to 2.0, the formation of such an intermediate can be directly observed as it is the only species formed. The reaction of salicylate with aqueous Al3+ is completed within 10 min at pH 3.3 but is significantly slower at pH 2.0. At both pH the aqueous reaction follows a single pseudo-first order rate law. In alumina suspension the reaction was initially fast but slowed down after ∼30 s. Completion of the reaction took up to 12 h, depending on pH and ionic strength. The formation of a carboxylate monodentate surface complex as a transient species is clearly observed in alumina suspensions at near neutral pH. The initial rapid reaction ( 4, suggesting that the slow reaction pathway involves ligand substitution reactions between salicylate and the hydroxyl groups for which the Al–O binding and activation energy are affected by site heterogeneity or site density to a lesser degree than Al–OH2+ sites.

Resonance Raman studies of benzene derivatives with strong conjugation: nitrile substitution

Abstract Raman spectra in resonance with the L a state of benzene derivatives exhibit activity in both the fundamental and two-quantum overtones of modes that are of e 2g symmetry in benzene. The Raman spectra can be used to test proposals of Petruska that result in the conclusion that 1,2,3-trisubstituted benzenes should have L a transitions with very small allowed transition moments. We find that nitrile substitution results in absorption and resonance Raman spectra that show deviations from Petruskas rules. The 1,2,3-tricyano derivative exhibits Raman spectra that demonstrate allowed transition character.

Performance of a Spatial-Filter-Equipped Single Monochromator for Raman Spectroscopy

A technique for reducing stray-light artifacts in Raman spectroscopy is described. The performance of a spatial filter at the output of a single monochromator is reported. The filter reduces both the diffuse background and the scattered-light artifacts encountered in single monochromators while maintaining high luminous throughput. Unlike other wavelength-selective methods for rejecting laser-excitation light (such as use of holographic edge or notch filters), spatial filtering of scattering artifacts may be used with any excitation wavelength. This is an advantage for Raman spectroscopy in the quartz ultraviolet region where holographic filters are not yet available. The enhanced ability to observe low-wavenumber scattering with an optical multichannel detector on a single monochromator is a particular advantage of the method.

Resonance Raman Studies of Benzene Derivatives with Methoxy Substitution: Conformational Symmetry‐Breaking Effects

Author Institution: Department of Chemistry, Syracuse University; Department of Chemistry, University of Oregon; Pacific Northwest, National Laboratory; Steacie Institute for Molecular Science, National Research Council of Canada

Vibrational Spectra And Chemical Bonding In Phosphazenes

The polarity and polarizability of -P=N and P-X bonds in phosphazenes (NPX2)n are influenced by the structure and effective electronegativities of the ligands (X) attached to the phosphorus atoms. The nature of chemical bonding in phosphazenes is revealed through responses of vibrational spectra to perturbations such as external pressure and solvent-hydrogen bonding, as well as chemical and isotopic substitution of the ligands. Raman spectra of a series of cyclic phosphazenes are used to identify and classify the vibrational modes. Changes in vibrational frequencies and Raman intensities are interpreted in terms of the polarized chemical bonding model suggested by recent calculations.