Rotamers of some radical-ions as evidence by optical and esr spectroscopy

Conformational isomerizations of radical-ions are studied both experimentally and theoretically- The representative systems of stilbenes, polyenes, and non-planar alicyclic compounds will be reviewed first. Then, the result of the study on the radical-cations of 1,4-dithiacyclohexane and ethane-1,4-dithiol will be presented. It is found that the former cation suffers a change from a chair fore to a boat form and the latter from an anti- or a gauche-form to a cis form even in a rigid matrix. The conformational changes are manifested by drastic changes in the electronic absorption and ESR spectra- The strong “through-space” interaction between the lone pair orbitals of the sulfur atoms is responsible for the conformational changes. Ab initio óó'O calculations lend support to the analysis of the experimental results.     

ESR spectra of oxygen radical-ions in kaolinite

V. V. Kuibyshev Novosibirsk Construction Engineering Institute and Institute of Catalysis, Siberian Branch, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 33, No. 1, pp. 171–173, January–February, 1992.     

Rotamers of m-cresol cation studied by mass-analyzed threshold ionization spectroscopy

We have applied two-color resonant two-photon mass-analyzed threshold ionization technique to record the vibrational spectra of the selected rotamers of m-cresol. The adiabatic ionization energies of cis and trans m-cresol are determined to be 66,933+/-5 and 67,084+/-5 cm(-1), respectively. Frequencies of the in-plane ring vibrations 6a, 1, and 9b are measured to be 528, 720, 1167 cm(-1) for the cis and 520, 698, and 1153 cm(-1) for the trans m-cresol cation. This indicates that different orientation of the OH group with respect to the CH(3) group slightly influences these ring vibrations.     

Adsorption of -histidine on copper surface as evidenced by surface-enhanced Raman scattering spectroscopy

The adsorption of -histidine on a copper electrode from H₂O- and D₂O-based solutions is studied by means of surface-enhanced Raman scattering (SERS) spectroscopy. Different adsorption states of histidine are observed depending upon pH, potential, and the presence of the SO²⁻₄ and Cl⁻ ions. In acidic solutions of pH 1.2 the imidazole ring of the adsorbed histidine remains protonated and is not involved in the chemical coordination with the surface. The SO²⁻₄ and Cl⁻ ions compete with histidine for the adsorption sites. In solutions of pH 3.1 three different adsorption states of histidine are observed depending on the potential. Histidine adsorbs with the protonated imidazole ring oriented mainly perpendicularly to the surface at potentials more positive than −0.2 V. Transformation of that adsorption state occurs at more negative potentials. As this takes place, histidine adsorbs through the α-NH₂ group and the neutral imidazole ring. The Cl⁻ ions cause the protonation and detachment of the α-NH₂ group from the surface and the formation of the ion pair NH⁺₃ … Cl⁻ can be observed. In the neutral solution of pH 7.0 histidine adsorbs through the deprotonated nitrogen atom of the imidazole ring and the α-COO⁻ group at E ≥ −0.2 V. However, this adsorption state is transformed into the adsorption state in which the α-NH₂ group and/or neutral imidazole ring participate in the anchoring of histidine to the surface, once the potential becomes more negative. In alkaline solutions of pH 11.9 histidine is adsorbed on the copper surface through the neutral imidazole ring.     

Fast moving nuclei in Li19 clusters as evidenced by molecular beam ESR experiments

ESR spectra of free lithium clusters Li₁₁ and Li₁₉ were measured by a molecular beam method. The spectra show that the observed clusters are Li₁₀ + Li and Li₁₈ + Li, i.e., Li₁₀ and Li₁₈ with an atomic Li attached to them. The host cluster Li₁₈ in Li₁₉ is highly spin-polarized by the attached atom, and gives the spectrum a large number of equally spaced hyperfine-lines indicating 18 equivalent nuclei, which reveal fluid-like behavior of Li₁₈ at an estimated internal temperature of T = 102 K. The host cluster Li₁₀ in Li₁₁ is only weakly spin-polarized probably because Li₁₀ is an ordinary solid-like cluster.     

Determination of the optical purity of some substituted N-phenylpropionamides by NMR spectroscopy

The NMR spectra of a series of chiral propionamides have been determined in D(+)1-phenylethylamine solution. The α-methine protons of the propionic acid moiety in the propionamide enantiomers were found to be anisochronous in this solvent. The sense of non-equivalence of these protons was found to be the same for all members of a homochiral series. The chemical shift difference between their signals was demonstrated to be dependent upon the temperature and concentration of the solution. A method is described whereby the enantiomeric purity of the amides could be calculated from these signals in cases where the position of the baseline in their spectra was unknown.     

Negative effect of Ni on PtHY in n-pentane isomerization evidenced by IR and ESR studies

Ni/PtHY with different Ni loadings was prepared by impregnating HY with hexachloroplatinic acid solution and Ni2+/N,N-dimethylformamide solution. An increase in the Ni loading decreased the crystallinity, specific surface area and meso-micropores of the catalysts. Ni interacted with hydroxyl groups to produce IR absorption bands at 3740-3500 cm-1. Increasing Ni loadings resulted in a decrease in the intensities of the broad bands at 3730-3500 cm-1 and the sharp band at 3740 cm-1 with simultaneous development of new absorbance band at 3700 cm-1 that was attributed to (-OH)Ni. The acidity of the samples did not significantly change with Ni loadings up to 1.0 wt%, which indicated that Ni mostly interacts with non-acidic silanol groups (terminal- and structural-defect OH groups). The presence of Ni decreased the activity of PtHY toward the isomerization of n-pentane because of a decrease in the number of active protonic-acid sites that formed from molecular hydrogen. IR and ESR studies confirmed that Pt facilitated the formation of protonic-acid sites from molecular hydrogen, whereas Ni, even when combined with Pt, didn’t exhibit such ability. The absence of protonic-acid sites from molecular hydrogen significantly decreased the yield of iso-pentane and markedly increased the cracking products.     

Investigation of some oxygen-containing chromium (III) complexes by epr and optical spectroscopy

EPR and electronic absorption spectra of chrome alum solutions in sulphuric and phosphoric acids have been studied. Broadening of the EPR lines and shifts in the positions of the electronic spectra absorption bands were observed at low acid concentrations. In concentrated acids the ⁴T_2g band splits into several well resolved components and the EPR spectra show two resolved areas of fine structure. Each structure may be described by a spinHamiltonian of the form

where D is the zero-field splitting parameter due to the action of the axial crystal fields. The changes in the spectra as the acid concentration is increased are explained by changes in the internal environment of the Cr^III complexes as a result of the substitution of H₂O by anions of the acid residue. The symmetry of the complexes predominating at particular acid concentrations and the parameters D, g and Dt have been determined from an analysis of the EPR and optical absorption spectra. The character of the bond of the Cr^III ion with the surrounding ligands has also been estimated.     

Evidence of polyphenols nitrosation by computer aided ESR spectroscopy

Polyphenols play an important role as model systems in transition metal derivatives for the preparation of macromolecular systems. Among the metal ions ironnitrosyl coordination chemistry has received much attention in the past because of its important role in inorganic and biological processes. In the case of Fe(I)(NO)₂ complexes with polyphenols ligands in solution, difficulties in the interpretation of the ESR spectra arise from complicated patterns due to simultaneous presence of different nitrogen nuclei directly bound to the metal ion or due to the presence of equilibria between species under slow exchange conditions. In order to overcome these difficulties the investigations reported here were carried out using computer simulation of ESR spectra combined with selective isotopic substitution of ¹⁴NO with ¹⁵NO. Resorcinol displays an unexpected nine lines ESR pattern at g=2.018 which can be explained only by considering more than two nitrogen atoms interacting with the unpaired electron delocalized over the metal complex.     

Antioxidant properties of gold nanoparticles studied by ESR spectroscopy

The Au-containing nanocomposites were synthesized by UV irradiation followed by the thermal treatment of chitosan oligomer solutions doped by HAuCl₄. The size of the formed gold nanoparticles depends on the concentration of the dopant, which is proved by UV—Vis absorption spectroscopy and small-angle X-ray scattering (SAXS). The antioxidant activity of the gold nanoparticles with respect to hydroxy radicals significantly depends on the specific surface of the particles, which was found using the secondary radical spin-trapping technique. The change in the ^·OH radical concentration was monitored by the intensity of the ESR signal of the adduct of the spin trap (α-phenyl-N-tert-butylnitrone) with the Me^·radicals formed in the reaction of ^·OH with DMSO. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 509–512, March, 2008.     

Local heterogeneity for a Eu3+-doped glass evidenced by time-resolved fluorescence spectroscopy coupled to scanning near-field optical microscopy

Time-resolved fluorescence spectroscopy (TRFS) was applied to an aluminate glass sample doped with Eu3+ cation as a fluorescent probe of the chemical environment and local symmetry. Conventional far-field experiments revealed the presence of two different phases: an amorphous phase featured by a highly disordered environment surrounding the Eu3+ cation and a more ordered polycrystalline phase that exhibits a significant increase in the Eu3+ fluorescence decay time compared to that of the amorphous phase. Near-field fluorescence spectra and decay kinetics were recorded in the frontier region between the two phases using a home-built scanning near-field optical microscope. SNOM-TRFS experiments confirmed the presence of local heterogeneities in this part of the glass at a sub-micrometric spatial scale. Polycrystalline sites featured an important shear-force interaction with the probing fiber optic tip, a longer fluorescence decay time, and a higher Stark splitting of the 5D0 --> 7FJ (J = 1-4) electronic transitions of the Eu3+ cations.     

Insertion of indigo molecules in the sepiolite structure as evidenced by 1H-29Si heteronuclear correlation spectroscopy

Despite the numerous studies of the famous indigo-based pigment Maya Blue, there are still many questions regarding the elucidation of its structure. Here, two-dimensional (2D) (1)H-(29)Si heteronuclear correlation (HETCOR) spectroscopy with frequency-switched Lee-Goldburg (FSLG) homonuclear decoupling is applied to sepiolite and sepiolite-indigo complexes. Owing to the high resolution in the (1)H dimension of the 2D (1)H-(29)Si HETCOR spectrum obtained by FSLG homonuclear decoupling, the assignment of the (29)Si cross-polarization magic-angle spinning (CPMAS) spectrum of sepiolite is clearly confirmed. Moreover, 2D (1)H-(29)Si FSLG-HETCOR spectroscopy gives the first direct evidence that some indigo molecules are inserted in the sepiolite structure whereas no interaction between indigo and the external side surface (silanol groups) is observed in the (29)Si CPMAS spectra. These results are consistent with the fact that indigo molecules interact with water coordinated to magnesium and suggest that Maya Blue made from sepiolite is not a surface complex.     

Interaction of Sodium Dodecylbenezen Sulfonate with Polymer as Studied by Surface Tension,ESR and UV-vis spectroscopy

The interaction between sodium dodecylbenezen sulfonate(SDBS) and Poly(ethylene oxide) (PEO) has been studied,' using surface tension and dye solubilization to determine the critical micelle concentration(CMC) without PEO, the critical aggregate concentration(cac) and polymer saturation concentration(c₂) in the presence of PEO. The structure of surfactant-polymer complexes is analyzed by ESR and UV-vis spectra. For SDBS-PEO system, the result indicates that ptoymer-surfactant complexes have a more “ open” structure than the ordinary micelles. The polymer molecules may penetrate into the palisade of the micelles. Polyacrylamide(PAm) molecules remain in bulk solution as free molecules, and do not interact with SDBS.     

Structural features of 1,3,5-trifluorobenzene radical anion,as studied by optically detected ESR spectroscopy and quantum chemistry

The ESR spectrum of 1,3,5-trifluorobenzene radical anion was obtained for the first time using optically detected ESR spectroscopy. Experimental data and results of a quantum chemical study of the potential energy surface (density functional theory B3LYP and BHHLYP methods) are indicative of fast interconversions between nonplanar conformations of this radical anion.