- Kinetics of homolysis of substituted benzyl complexes of pentaaquachromium(III) and product variation with substituent and scavenger
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The kinetics of homolysis have been studied for (H2O)5Cr-CH2C6H5 2+ and derivatives with the following substituents: 4-CH3, 4-fluoro, 2,4-difluoro, 3,5-difluoro, 2-cyano, 3-cyano, and 4-cyano. The reactions have been studied in the general temperature range 20-35°C in 0.01-0.90 M HClO4 at ionic strength 1.00 M (NaClO4/ HClO4). The reactions were done in the presence of several oxidizing scavengers, dioxygen, aqueous iron(III), (NH3)5CoCl2+, and (NH3)5CoBr2+. The homolysis rate is independent of the nature and concentration of the scavenger, and the rate constant (25°C) changes from 4 × 10-3 to 0.23 × 10-3 s-1 from the most reactive 4-methyl to the least reactive 3-cyano. This change is primarily due to a change in ΔH* from 26 to 30 kcal mol-1, while ΔS* is typically in the range 22-24 cal mol-1 K-1. The organic products, identified by 1H NMR, vary considerably with the nature of the scavenger and the substituent on the benzyl ligand. With aqueous iron(III) in several systems, the products show competition between oxidation to form the alcohol and radical dimerization to give the bibenzyl derivative. From the dependence of the product distribution on the iron(III) concentration, the rate constant for oxidation of ·CH2C6H5 by Fe(OH2)63+ is calculated to be 1.8 × 104 M-1 s-1. With (NH3)5CoBr2+, the more easily oxidized radicals give the alcohol, while others give a mixture of alcohol and bromide or exclusively bromide for the least oxidizable systems. However, (NH3)5CoCl2+ is different and gives only the bibenzyl derivative for all the systems.
- Zhang, Zhongsheng,Jordan, Robert B.
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p. 680 - 686
(2008/10/08)
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- Kinetics of heterolysis of pentaaquachromium(III) complexes of benzyl and 4-methyl-, cyano-, and fluoro-substituted benzyl ligands
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The kinetics of the heterolysis have been studied for (H2O)5Cr-CH2C6H5 2+ and the 2-, 3-, and 4-cyano derivatives; the 4-fluoro and 2,4- and 3,5-difluoro derivatives, and the 4-CH3 complex. The reactions were studied in the general temperature range 60-75°C in 0.01-0.95 M HClO4 at an ionic strength of 1.00 M (NaClO4/HClO4). Chromium(II) was added to suppress homolysis. The reaction rate has terms first order and independent of [H+] and pathways involving catalysis by sulfate ion have been characterized for all but the more reactive 2- and 4-cyano systems. The latter is the most reactive (studied at 25-43°C) and is also unusual in showing a saturation effect with [H+] which is attributed to protonation of the ligand to give (H2O)5Cr-CH2C6H 4(4-CNH)3+ with pKa = 1.50 (25°C, 1.00 M NaClO4/HClO4). The results are discussed in terms of substituent effects on the general reactivity and the activation parameters, ΔH* and ΔS*.
- Zhang, Zhongsheng,Jordan
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p. 5472 - 5476
(2008/10/08)
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- Kinetics of the heterolysis of benzyl and cyanobenzyl complexes of pentaaquachromium(III) in acetate and phosphate buffers
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The kinetics of the heterolysis of the o-CN, m-CN, and p-CN benzyl complexes of pentaaquachromium(III) have been studied in phosphate buffers, and both heterolysis and homolysis have been studied in acetate buffers. Previous work on the parent benzyl system in acetate has been extended to heterolysis in phosphate and methyl phosphate buffers. All of the results are at 25°C at a total ionic strength of 1.00 M, adjusted with NaClO4. The pH total buffer dependence of the rate constants for heterolysis indicate that the conjugate bases (Hn-xAx-) of the buffer species form mono complexes, [(Hn-xA)(H2O)4Cr-(CH2C 6H4X)]2-x, whose formation constants correlate with the pKa of the parent acid (Hn-x+1A1-x). The effectiveness of the buffer components in promoting heterolysis generally depends on the basicity of Hn-xAx-, but species with an ionizable proton are much more reactive than the fully deprotonated species OAc- and MeOPO32-. Acetate ion causes the rate of homolysis to increase by modest factors of 1.5-2.5 for the benzyl and m-CN and p-CN systems but causes a reduction in rate for the o-CN complex.
- Zhang, Zhongsheng,Jordan
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p. 3412 - 3417
(2008/10/08)
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- Kinetic study of the modes of decomposition of pentaaquabenzylchromium(III)
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The preparation of moderately concentrated solutions of (H2O)5CrCH2C6H5 2+ has allowed its visible spectrum to be fully characterized with maxima (nm (molar absorptivities, M-1 cm-1)) at 533 (31.9) and 358 (2.15 × 103). The kinetics of acidolysis of (H2O)5CrCH2C6H5 2+ have been studied between 56 and 74°C in 1 M NaClO4/HClO4 and in the presence of chromium(II) to suppress homolysis. The rate law has terms independent of (k0) and first order in [H+] (ka) with ΔH0* and ΔHa* values (kcal mol-1) of 21.4 ± 0.4 and 20.3 ± 2.1 and ΔS0* and ΔSa* values (cal mol-1 deg-1) of -11.7 ± 1.9 and -16.5 ± 8.7. Calculated values at 25°C are k0 = 3.8 × 10-6 s-1 and ka = 2.0 × 10-6 M-1 s-1. The homolysis reaction was studied at 41°C and found to have apparent half-order kinetics in [(H2O)5CrCH2C6H5 2+] with an inverse dependence on the hydrogen ion concentration. The products for the heterolysis, homolysis, and decomposition in the presence of ethanol and dioxygen have been characterized by ion-exchange separation and spectrophotometry for the inorganic species and by extraction and proton NMR spectroscopy for the organic products.
- Kita,Jordan
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p. 3489 - 3494
(2008/10/08)
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- Kinetic study of the reaction of aquochromium(II) ions with benzyl radicals in aqueous solutions: Thermodynamics of the chromium-carbon bond
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Flash-photolytic determinations based on both the photohomolysis of CrCH2Ph2+ (product ratio) and on the photodecomposition of (PhCH2)2CO in the presence of Cr2+ (kinetic spectrophotometry) yielded a consistent value for kCr, the second-order rate constant for the reaction Cr2+ + PhCH2· → CrCH2Ph2+, kCr = (8.5 ± 0.6) × 107 M-1 s-1 (23 ± 2°C, 0-2 M CH3CN in H2O, 0.05 M HClO4 at μ= 0.10 M). This value, together with literature values or estimates for other quantities, affords ΔG° = -59.9 kJ mol-1 (ΔGCr≠ = 27.8, ΔG-1≠ = 87.7), ΔH° = -123 ± 10 kJ mol-1 (ΔH-1≠ = 133 ± 3), and ΔS° = -211 ± 34 J mol-1 K-1 (ΔSCr≠ = -60 ± 35).
- Blau, Reed J.,Espenson, James H.,Bakac, Andreja
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p. 3526 - 3528
(2008/10/08)
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