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September 25, 2009
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The self-association (dimerization) due to hydrogen bonding for some alcohols with bulky side chains was investigated by nuclear magnetic resonance spectroscopy. The systems studied include 2,4-dimethyl-3-pentanol or 3-methyl-3-pentanol in [ 2 H 12 ]cyclohexane, carbon tetrachloride or [ 2 H 1 ]chloroform, and 2,3,4-trimethyl-3-pentanol in carbon tetrachloride. The dilution shift data of hydroxyl proton were measured and employed to deduce the monomer shift, dimer shift and dimerization constant with the help of a graphic method. Use of van´t Hoff plot for dimerization constants thus obtained at different temperature gave the enthalpy and entropy of dimerization. Also discussed herein is the effect of substituent hindrance and dielectric constant of solvent on the self-association.
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September 25, 2009
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The exciplex formation in the system 1,2-benzanthracene (electron acceptor) and 1,2,4-trimethoxybenzene (electron donor) in n -hexane and diethyl ether was studied by fluorescence measurements up to 5 kbar. Whereas the equilibrium between the exciplex and the excited electron acceptor and donor in n -hexane is established at even higher pressures this is not the case in more polar diethyl ether already at atmospheric pressure. By means of a van´t Hoff type equation the volume change of exciplex formation, Δ V o * , can be obtained from the pressure dependence of the equilibrium. For the solvent diethyl ether, the measured fluorescence intensities could be adequately adjusted to equilibrium conditions. Δ V o * (-22.4 cm 3 mole -1 ) in more polar diethyl ether is clearly greater than in n -hexane (-15.4 cm 3 mole -1 ). It can be shown that the difference of 7 cm 3 mole -1 essentially originates from stronger electrostriction of the solvating diethyl ether envelope of the exciplex. The possibility of formation of a nonfluorescent radical ion pair directly from an encounter complex or via exciplex is discussed.
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September 25, 2009
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The thermally induced isomerization of neutral silicon clusters has been studied by a molecular beam experiment, investigating velocity slip distributions with a time-of-flight method. The clusters dispersed in helium flow through a heated tube and, at its end, expand into vacuum. The observed velocity distributions can be explained by the existence of a cylindrical and a spherical class of isomers, whose relative intensities can be examined in dependence of the tube temperature. Assuming that the isomerization can be described by a unimolecular gas-phase reaction, one is able to derive a rate constant for the isomerization. The results are analyzed within the activated complex theory and give evidence, that entropic effects play a crucial role for the cluster isomerization reaction.
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September 25, 2009
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Using the stochastic model of n -stage first-order reversible chemical reactions, the lifetimes of equilibrium fluctuations about the most probable equilibrium microstate and the times to reach the macroscopic chemical equilibrium are derived. In the latter case, a semistochastic approach (compatible with Schrödinger´s √ N – rule) and a first-passage time approach are compared. It is shown that for n > 2 and large number of reacting particles in the system, the formula for first-passage time can be divided into two parts having a simple physical interpretation: the composition of the system reaches first the boundary of the region of equilibrium fluctuations demarcated by their mean amplitudes (the first term which is identical with the semistochastic formula) and then, after a long random wandering among microstates inside and outside this region, it finally reaches the most probable equilibrium microstate (the second term which is approximately equal to the mean lifetime of system composition fluctuations about the most probable equilibrium microstate). It is concluded that in the case of n -stage reactions, n > 4, the first-passage time approach leads to values of time to reach the equilibrium which correspond from the physical point of view to infinity.
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September 25, 2009
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Ultrasonic speeds in and densities of mixtures of 1-butanol with glycerol have been measured at 298.15 K. The excess values of the molar volume and molar adiabatic compressibility (as well as adiabatic compressibility coefficient) were estimated. Also deviations of the ultrasonic speeds from the values calculated for ideal mixtures were determined. All the excess quantities are expressed by the Redlich-Kister polynomials and disscused in terms of molecular interactions.
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September 25, 2009
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New spectroscopic data are reported for the intermolecular and intramolecular hydrogen bonding in 2-methoxyethanol + n -hexane and in 2-ethoxyethanol + n -hexane mixtures. The solutions were studied at 30, 40, and 50°C and at concentrations where either only intramolecular association is observed or where, both, intermolecular and intramolecular associations are present. The data are treated with a new statistical thermodynamic model, which accounts for both types of hydrogen bonds. The energy and entropy change upon formation of the above hydrogen bonds were calculated and were used subsequently for the calculation of heats of mixing of alkoxyethanol + hydrocarbon mixtures. These calculations show that the intramolecular hydrogen bonding contribution to the heats of mixing is exothermic and significant.
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September 25, 2009
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With the combination of redox reaction and co-precipitation, a series of Sn-Fe-Cr composite metal oxide catalysts with various compositions were prepared, which exhibit higher specific surface areas, thermal stability and CH 4 deep oxidation activity than the single component SnO 2 , Fe 2 O 3 and Cr 2 O 3 . Fe and Cr cations were found to dope into the crystal lattice of rutile SnO 2 (Cassiterite) to form solid solution structure. As a consequence, more active oxygen species were formed in Sn-Fe-Cr composite oxides. The high surface area and the presence of more reactive oxygen species were considered as the two predominant reasons leading to the enhanced activity of Sn-Fe-Cr composite oxide catalysts.
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September 25, 2009
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New acidic sites were generated on finely dispersed ZnO particles supported on silica prepared by impregnation method. Ammonia-TPD, XPS, XRD, TEM and catalytic measurements revealed that the acidic properties on the nano-sized amorphous particles of ZnO were quite different from homogeneous ZnO-SiO 2 prepared by co-precipitation method.