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Microwave-specific chemical rate enhancement originates from the selective heating and accumulation of energy by solvated dipolar molecules in solution.

The Boudouard reaction, which is the reaction of carbon and carbon dioxide to produce carbon monoxide, represents a simple and straightforward method for the remediation of carbon dioxide in the environment through reduction: CO 2 (g) + C(s) ⇌ 2CO. However, due to the large positive enthalpy, typically reported to be 172 kJ/mol under standard conditions at 298 K, the equilibrium does not favor CO production until temperatures >700 °C, when the entropic term, −TΔS, begins to dominate and the free energy becomes negative. We have found that, under microwave irradiation to selectively heat the carbon, dramatically different thermodynamics for the reaction are observed. During kinetic studies of the reaction under conditions of flowing CO 2 , the apparent activation energy dropped from 118.4 kJ/mol under conventional convective heating to 38.5 kJ/mol under microwave irradiation. From measurement of the equilibrium constants as a function of temperature, the enthalpy of the reaction dropped from 183.3 kJ/mol at ∼1100 K to 33.4 kJ/mol at the same temperature under microwave irradiation. This changes the position of the equilibrium so that the temperature at which CO becomes the major product drops from 643 °C in the conventional thermal reaction to 213 °C in the microwave. The observed reduction in the apparent enthalpy of the microwave driven reaction, compared to what is determined for the thermal reaction from standard heats of formation, can be thought of as arising from additional energy being put into the carbon by the microwaves, effectively increasing its apparent standard enthalpy. Mechanistically, it is hypothesized that the enhanced reactivity arises from the interaction of CO 2 with the steady-state concentration of electron−hole pairs that are present at the surface of the carbon due to the space-charge mechanism, by which microwaves are known to heat carbon. Such a mechanism is unique to microwave-induced heating and, given the effect it has on the thermodynamics of the Boudouard reaction, suggests that its use may yield energy savings in driving the general class of gas−carbon reactions.

Chromium(VI) sites homogeneously dispersed in a transparent silica xerogel matrix have been investigated to determine the coordination environment and rationalize the Raman spectra. X-ray absorption fine structure (EXAFS) analysis gives a structure that is consistent with Cr containing two terminal oxygens and is bound to the silica by two Cr-O-Si linkages. The structure was refined to an R factor of 1.28%. The terminal CrdO bonds were found to have a bond length of 1.60 Å and bridging Cr-O bonds of 1.80 Å. The Raman spectrum, collected with 785 nm excitation above the absorption edge of the chromium, shows a strong band at 986 cm -1 and a resolved shoulder at 1001 cm -1 . Isotopic labeling and polarization studies of low concentrations of Cr (0.5 mol %) indicate that the strong 986 cm -1 band is the totally symmetric Cr(dO) 2 mode; however, the isotopic shift and strong polarization of the 1001 cm -1 mode preclude it from being the antisymmetric component of the terminal dioxo stretch. At higher concentrations (e5.0 mol %) the high-energy shoulder becomes a resolved peak at 1004 cm -1 . While isotopic labeling shifts the peak to a position predicted for the antisymmetric stretch, the polarization ratio increases but does not reach a value that is unambiguous for an antisymmetric mode.

transition appears at progressively lower energy as n increases. An interesting additional case is Rh"("+2,+ with n odd, in particular, the Rh35+ unit characterized by Balch and Olmstead12 in the complex [(C6H5CH2NC)12Rh3I2]3+. The MO diagram for Rh35+ is shown in Figure 9, where the orbitals are energetically ranked according to the number of nodes (just as for Rh46+). The Rh-Rh bond order is 1 /V2 (neglecting overlap), which is consistent with the observed12 ¿(Rh-Rh) = 2.796 Á. The only allowed -* transition is a2u -*• 2a lg, a nonbonding-to-antibonding transition that can also be described as outerinner rhodium charge transfer. The observed wavelength (525 nm)12 for the

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There has been considerable interest in molecular magnets based on the Prussian blue class of transition-metal cyanide complexes.[1] Pioneering work on these materials has realized a broad range of ferro-and ferrimagnetic solids with Curie points ranging from cryogenic to above room temperature. [2,3] A parallel interest also exists in the development of nanocomposite structures containing nanoscale magnetic particles exhibiting single-domain magnetic behavior. [4][5][6] Investigations of magnetic nanocomposites is driven by their novel properties and their potential as new magnetic, optical, and electronic materials. With one exception, [7] however, the Prussian blue class of magnetic materials has not been produced as nanoparticles, and, to the best of our knowledge, there have been no reports of their incorporation into composite structures. Herein we report the fabrication of a new nanocomposite material containingferromagnetic analogue of Prussian blue, in a porous silica matrix. This material is made by a controlled multicomponent sol-gel synthesis in which the precipitation of the Prussian blue analogue is arrested at nanoscale dimensions by gelation of the silica network. The resulting materials are homogeneous, optically transparent, and exhibit superparamagnetic and tunable photomagnetic behavior. We believe that this study suggests a new approach for utilizing the Prussian blue class of magnetic materials in advanced optical and magnetic applications.Homogeneous silica xerogels containing cyanide-bridged Co II /Fe III centers were made by incorporation of both transition-metal components during the solution phase of the synthesis. To obtain homogeneous materials reproducibly, conditions were optimized for the amount of water and the concentration and molar ratio of Co II and ferricyanide by following previously developed procedures. [8,9] In the optimized preparation, Co II nitrate was dissolved in methanol and added to tetramethylorthosilicate. Aqueous potassium ferricyanide was added to this solution to give a 1:1 Co:Fe molar ratio. Upon mixing, the solution turns dark purple, which suggests the formation of the mixed-valence complex. At concentrations up to 0.03 mol % total-metal to silicon (Fe + Co/Si), the solution remained transparent through gelation, aging, and drying, and ultimately yielded a homogeneous, optically transparent xerogel (Figure 1, inset). The spectrum of this glass (Figure 1) is qualitatively similar to that of the bulk materials, with a broad intervalence charge-transfer band in the visible region between 450 and 650 nm and a sharp, higher energy peak around 400 nm. However, the maximum of the intervalence band lies at 452 nm (22 124 cm À1 ) which is blue-shifted by approximately 2900 cm À1 from that of the bulk materials. To determine whether the magnetic behavior was singular, the magnetic susceptibility of these materials was measured as a function of temperature and field strength.[10] Bulk M

The self-initiation of the thiol–ene coupling reaction of tetravinyl monomers containing main group elements and trivinyl heterocycles with alkyl and aryl dithiols resulted in the formation of highly cross-linked prepolymer gels which upon final curing at 120 °C yielded hard, monolithic polymeric materials. Because of the presence of highly polarizable main group elements such as Si, Ge, Sn, and S and the relative absence of highly electronegative elements, the resulting polymers exhibited high refractive indices ranging from 1.590 to 1.703 and Abbe numbers between 24.3 and 45.0. All of the polymers were highly transparent over the UV–vis region of the spectrum. Moreover, due to the high cross-linked density achievable in specific compositions, very hard materials capable of being ground and polished could be produced. The range of compositions produced yields important structure–property relationships, indicating the effect of monomer structure on mechanical and optical properties.