We have investigated a wide variety of surfactants for their efficiency in dissolving isolated single‐walled carbon nanotubes (SWNTs) in water. In doing so, we have completely avoided the harsh chemical or mechanical conditions, such as acid or ultrasonic treatments, that are known to damage SWNTs. Bile salts in particular are found to be exceptionally effective in dissolving individual tubes, as evidenced by highly resolved optical absorption spectra, bright bandgap fluorescence, and the unprecedented resolution (∼ 2.5 cm–1) of the radial breathing modes in Raman spectra. This is attributed to the formation of very regular and stable micelles around the nanotubes providing an unusually homogeneous environment. Quantitative information concerning the degree of solubilization is obtained from absorption spectroscopy.
Water accessibility of single wall carbon nanotubes (SWCNTs) can be monitored by resonant Raman spectroscopy (RRS) (see figure and cover) of the radial breathing modes of bile salt‐solubilized CNTs. The RRS features of empty and water‐filled CNTs can be very well resolved, and vibrational and electronic shifts and broadenings have been accurately determined. The results also lead to a simple ratiometric method to quantify tube opening under various chemical and mechanical treatments.
Light-induced electron paramagnetic resonance ͑LEPR͒ measurements are reported in composites of poly͑2methoxy-5-͑3-,7-dimethyloctyloxy͒-1,4-phenylenevinylene͒ ͑MDMO-PPV͒ and ͓6,6͔-phenyl-C 61-butyric acid methyl ester ͑PCBM͒, a soluble derivative of C 60. Under illumination of the sample, two paramagnetic species are formed due to photoinduced charge transfer between conjugated polymer and fullerene. One is the positive polaron P ϩ on the polymer backbone and the other is the radical anion on the methanofullerene. Using high-frequency ͑95 GHz͒ LEPR it was possible to separate these two contributions to the spectrum on the basis of their g factors, and moreover to resolve the g anisotropy for both radicals. The positive polaron on the conjugated polymer chain possesses axial symmetry with g values g ʈ ϭ2.0034(1) and g Ќ ϭ2.0024(1). EPR on low doped polymer gave extra proof for the assignment to the positive polaron. The negatively charged methanofullerene has a lower, rhombic symmetry with g x ϭ2.0003(1), g y ϭ2.0001(1), and g z ϭ1.9982(1). Different spin-lattice relaxation of both species gives rise to a rapid passage effect for the positive polaron spectrum.
A high-frequency (95 GHz) electron paramagnetic resonance (EPR) study is reported on single crystals of the planar tetranuclear complex Fe 4 (OCH 3 ) 6 (dpm) 6 (where Hdpm ) dipivaloylmethane), which has been previously shown to present typical single-molecule magnet behavior. The spectra, all originating from the S ) 5 ground state, possess quasi-axial symmetry along the normal to the plane defined by the four Fe(III) ions. The measured spectra are shown to belong to three different structural variations of the compound, resulting from disorder in the ligands around two of the Fe(III) ions. Accurate values could be obtained for the second-and fourth-order crystal field parameters related to the parallel EPR spectra, while the other parameters could be determined only for the dominant species. The separation between individual lines is decreasing and vanishing with increasing temperature. This effect is attributed to the contribution of fast relaxing excited states, whose population is varying with temperature.
In spite of the tremendous progress in the field of pulse electron paramagnetic resonance (EPR) in recent years, these techniques have been scarcely used to investigate high-spin (HS) ferric heme proteins. Several technical and spin-system-specific reasons can be identified for this. Additional problems arise when no single crystals of the heme protein are available. In this work, we use the example of a frozen solution of aquometmyoglobin (metMb) to show how a multi-frequency pulse EPR approach can overcome these problems. In particular, the performance of the following pulse EPR techniques are tested: Davies electron nuclear double resonance (ENDOR), hyperfine correlated ENDOR (HYEND), electron-electron double resonance (ELDOR)-detected NMR, and several variants of hyperfine sublevel correlation (HYSCORE) spectroscopy including matched and SMART HYSCORE. The pulse EPR experiments are performed at X-, Q- and W-band microwave frequencies. The advantages and drawbacks of the different methods are discussed in relation to the nuclear interaction that they intend to reveal. The analysis of the spectra is supported by several simulation procedures, which are discussed. This work focuses on the analysis of the hyperfine and nuclear-quadrupole tensors of the strongly coupled nuclei of the first coordination sphere, namely, the directly coordinating heme and histidine nitrogens and the 17O nucleus of the distal water ligand. For the latter, 17O-isotope labeling was used. The accuracy of our results and the spectral resolution are compared in detail to an earlier single-crystal continuous-wave ENDOR study on metMb, and it will be shown how additional information can be obtained from the multi-frequency approach. The current work is therefore prone to become a template for future EPR/ENDOR investigations of HS ferric heme proteins for which no single crystals are available.
Silica-supported molybdenum oxide catalysts have been prepared by liquid and gas phase deposition, followed by calcination of the deposited molybdenyl acetylacetonato complex. Fourier-transform infrared spectroscopy indicated a hydrogen bonding anchorage mechanism for the liquid phase deposition and a two step reaction mechanism for the gas phase deposition. After calcination of the absorbed molybdenum complexes, the supported molybdenum oxides were characterized by combining Fourier-transform infrared and Raman spectroscopy. X-ray di †raction was used to probe the possible clustering towards crystallites. An evaluation of the molecular designed dispersion method has been made by comparing the deposited molybdena structures obtained by the designed dispersion of with catalysts prepared by the conventional impregnation MoO 2 (acac) 2 method using ammonium heptamolybdate. It is concluded that the molecular designed dispersion method results in a better grafting (more SiÈOÈMo bonds) and thus a stronger metal oxideÈsupport interaction than the conventional impregnation methods.
Single-crystal W-band (95 GHz) electron paramagnetic resonance (EPR) studies have been performed at 20 K and at room temperature on a tetragonal Mn(III) compound with potential application as a building block for high-spin clusters. The observed EPR spectra correspond to an anisotropic high-spin S = 2 ground state and have been attributed to equivalent centers related by fourfold symmetry. Accurate values for the spin Hamiltonian parameters were obtained from the analysis of the data at both temperatures. At 20 K the contribution of fourth-order zero-field splitting terms was shown to be significant, with parameter values Bá = 0.0009(3) cm -', B = 0.0006(2) cm-' and B = 0.0017(3) cm -', to be considered together with the second-order parameters D = -1.1677(7) cm -' and E = -0.0135(6) cm -'.
Three different paramagnetic [RhCl6]4− complexes were detected in x-ray irradiated solution-grown NaCl single crystals: RhCl64−⋅nVac, n=2, 1, 0. These complexes all have a 4d7 ground state, with the unpaired electron spin mainly in a 4dz2 orbital and differ only by the presence of two, one, or none next-nearest cation vacancy (Vac). The RhCl64−⋅2Vac is formed at 77 K and partially converts into RhCl64−⋅1Vac at about 190 K. At room temperature the RhCl64−⋅0Vac is dominant, but traces of the RhCl64−⋅1Vac and RhCl64−⋅2Vac centers remain. A thermally induced reorientation motion of the 4dz2 molecular orbital is used to explain the temperature dependence of the RhCl64−⋅1Vac electron spin resonance spectrum.
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