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Hammett Constant (hammett + constant)
Selected AbstractsKinetics of (Porphyrin)manganese(III)-Catalyzed Olefin Epoxidation with a Soluble Iodosylbenzene DerivativeEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 12 2006James P. Collman Abstract We examined the kinetics of a well-behaved system for homogeneous porphyrin-catalyzed olefin epoxidation with a soluble iodosylbenzene derivative 1 as the terminal oxidant and Mn(TPFPP)Cl (2) as the catalyst. The epoxidation rates were measured by using the initial rate method, and the epoxidation products were determined by gas chromatography. The epoxidation rate was found to be first order with respect to the porphyrin catalyst and zero order on the terminal oxidant. In addition, we found the rate law to be sensitive to the nature and concentration of olefin substrates. Saturation kinetics were observed with all olefin substrates at high olefin concentrations, and the kinetic data are consistent with a Michaelis,Menten kinetic model. According to the observed saturation kinetic results, we propose that there is a complexation between the active oxidant and the substrate, and the rate-determining step is thought to be the breakdown of this putative substrate,oxidant complex that generates the epoxidation products and the resting state porphyrin catalyst. Competitive epoxidations further indicate a reversible complexation of the active oxidant and the olefin substrate. The activation parameters ,H, and ,S, for the oxygen-transfer process (k2) in the cis -cyclooctene epoxidation were determined to be 12.3,±,0.9 kcal,mol,1 and,15.6,±,3.2 cal,mol,1,K,1, respectively. In addition, the Hammett constant ,+ was measured for the epoxidation of para -substituted styrenes, and the value of ,0.27,±,0.04 is too low to be consistent with the involvement of a discrete carbocation in the transition state. We also prepared a (porphyrin)manganese catalyst immobilized on silica support, and found the epoxidation of cis -cyclooctene catalyzed by this heterogeneous catalyst proceeds at virtually the same turnover frequency as by the homogeneous porphyrin catalyst. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] Long-range polar and steric effects in propionate-SG1-type alkoxyamines (SG1-CHMeCOOX): a multiparameter analysis,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 4 2006Gennady Ananchenko Abstract The effects of the substituent X on the homolysis rate constants (kd) of SG1-propionate type alkoxyamines (SG1-CHMeCOOX) are analyzed by a multiparametric equation with ,, the steric constant and ,I, the polar inductive/field Hammett constant of X. An influence of long-range polar and steric effects on kd was observed, that is, decrease in kd with increasing size of the X group and increase in kd with increasing polarity of the X group. Copyright © 2006 John Wiley & Sons, Ltd. [source] Semi-empirical studies of substituent effects on the ionization of bicyclooctane carboxylic acids and quinuclidinesJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 2 2003Helena Diez y Riega Abstract Semiempirical AM1 calculations were performed for a representative series of 4-substituted bicyclooctane carboxylic acids and quinuclidines. It was found that the Hammett constant, ,I, and the Swain and Lupton field constant, F, correlate linearly with the differences in the heat of formation of isodesmic reactions. These constants also correlate with the charges on the acid moiety of the bicyclooctane acids and their anions, and with the hydrogen net charge on the protonated quinuclidines. For all cases, the NO2 was the poorest correlated substituent. Copyright © 2003 John Wiley & Sons, Ltd. [source] Effective Manipulation of the Electronic Effects and Its Influence on the Emission of 5-Substituted Tris(8-quinolinolate) Aluminum(III) ComplexesCHEMISTRY - A EUROPEAN JOURNAL, Issue 17 2006Victor A. Montes Abstract The unique electron-transport and emissive properties of tris(8-quinolinolate) aluminum(III) (Alq3) have resulted in extensive use of this material for small molecular organic light-emitting diode (OLED) fabrication. So far, efforts to prepare stable and easy-to-process red/green/blue (RGB)-emitting Alq3 derivatives have met with only a limited success. In this paper, we describe how the electronic nature of various substituents, projected via an arylethynyl or aryl spacer to the position of the highest HOMO density (C5), may be used for effective emission tuning to obtain blue-, green-, and red-emitting materials. The synthetic strategy consists of four different pathways for the attachment of electron-donating and electron-withdrawing aryl or arylethynyl substituents to the 5-position of the quinolinolate ring. Successful tuning of the emission color covering the whole visible spectrum (,=450,800 nm) was achieved. In addition, the photophysical properties of the luminophores were found to correlate with the Hammett constant of the respective substituents, providing a powerful strategy with which to predict the optical properties of new materials. We also demonstrate that the electronic nature of the substituent affects the emission properties of the resulting complex through effective modification of the HOMO levels of the quinolinolate ligand. [source] On the Tacticity of Polynorbornenes with 5,6- endo Pendant Groups That Contain Substituted Aryl ChromophoresCHEMISTRY - AN ASIAN JOURNAL, Issue 6 2007Wei-Yu Lin Abstract Two dimers and a series of polymers with 5,6- endo pendant aryl groups that contain different substituents at the para positions were synthesized. The conformation and stereochemistry of the dimers and polymers were determined by nonlinear optical analysis (EFISH) as well as UV/Vis and 13C,NMR spectroscopy. The chemical shifts of C7 for the polymers appeared as two peaks in the 13C,NMR spectra when the substituents are electron-withdrawing groups. The percentage decrease in the relative extinction coefficient of the polymers, ,d, was linearly related to the Hammett constant ,. Polynorbornenes with electron-withdrawing substituents may adopt isotactic stereochemistry with all pendant groups aligned in one direction. The nature of the interactions between neighboring chromophores may be one of the most important factors in directing the stereoregularity and conformation of these polymers. The corresponding polymers derived from the exo isomers appeared to be less stereoregular. [source] Substituent effects on the physical properties and pKa of phenolINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4-5 2001Kevin C. Gross Abstract Substituent effects on the physical properties and pKa of phenol were studied using density functional theory [B3LYP/6-311G(d,p)] calculations. Substituents alter the physical properties of phenol such as the hydroxyl-group CO and OH bond lengths, the C OH bond angle, and the energy barrier to rotation about the C O bond, and also influence the hydroxyl-group pKa. Except for the rotational barrier, Hammett , constants showed strong correlation with these property changes. Several quantum chemical parameters, including the natural charge on the phenolic hydrogen Qn(H) and the natural charge on the phenoxide oxygen Qn(O,), the HF/6-311G(d,p) HOMO energy Ehomo, and the proton-transfer energy ,Eprot, outperformed the empirical Hammett constants in modeling changes in the pKa. All of these latter parameters yielded correlation coefficients ,r,>0.94 for the pKa. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001 [source] Substituent effects in reductions of heteroaromatic cationsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 10 2002David Heyes Abstract A set of 11 each of 2,4,6-triphenylpyrylium, -thiopyrylium and - N -methylpyridinium tetrafluoroborates carrying a range of substituents in the phenyl rings were prepared. First and second wave reduction potentials were determined. For the thiopyrylium series there are linear correlations between scaled potentials (E°/0.05915) and summed Hammett constants for substituents in the pendant phenyl groups (,,=,2.29 and 3.38 for first and second waves respectively). For the pyrylium series, a good linear relationship (,,=,2.79) is obtained for all substituent patterns for the first wave reduction potentials, but for the second wave there are separate correlations for salts carrying substituents in the 4-phenyl and for those carrying substituents in 2- and 6-phenyls. For the pyridinium series, the first wave potentials show separate correlations for salts carrying substituents in the 4-phenyl and for those carrying substituents in 2-and 6-phenyls, but a single linear relationship for the second wave potentials. These are related to particular structural features in the cations, radicals and anions in each series. Rates and products were determined for reductions of the pyrylium and thiopyrylium cations by sodium cyanoborohydride and of all cations by sodium borohydride in acetonitrile solution. Reactions are first order in reducing agent and cation. Primary kinetic isotope effects were determined for borohydride reduction of the least reactive of each of the series of cations. Plots of logarithms of second-order rate constants against summed Hammett constants for substituents in the pendant phenyl groups are linear for all combinations of reagent and cation with 0.91 < , < 1.50 across all substituent patterns. For parent pyrylium and thiopyryliums, kBH4/kCNBH3,=,8.4,×,104 and 1.5,×,104, respectively, and for reductions by borohydride the reactivities of the pyrylium, thiopyrylium and pyridinium, series decrease in the order 1.4,×,105:8.8,×,103:1. Constant selectivities are not observed. Comparison of the correlations for electrochemical reduction and for hydride addition leads to the conclusion that charge neutralization in the hydride addition transition states runs ahead of bonding changes at the originating B,H bond. Copyright © 2002 John Wiley & Sons, Ltd. [source] Current trends in QSAR on NO donors and inhibitors of nitric oxide synthase (NOS),,MEDICINAL RESEARCH REVIEWS, Issue 4 2002Christos A. Kontogiorgis Abstract This article evaluates the quantitative structure-activity relationships (QSAR) of nitric oxide (NO) radical donors and nitric oxide synthases (NOS) inhibitors, using the C-QSAR program of Biobyte. Furoxans, triazines, amidoximes, tetrazoles, imidazoles and N,,2-nitroarylamino acid analogues were included in this survey. In nine out of seventeen cases, the clog P plays a significant part in the QSAR of the NO radical donors and of the NOS inhibition. Many of the compounds must be interacting with a hydrophobic space in a non-specific way. In some cases molecular refractivity CMR/MR as well as sterimol parameters (B1 and L) are important. Electronic effects, with the exception of the Hammett's constant , and the Swain,Lupton parameter F, are not found to govern the biological activity. Stereochemical and electronic features are also found to be important. Indicator variables were used after the best model was found to account for the usual structural features. © 2002 Wiley Periodicals, Inc. Med Res Rev, 22, No. 4, 385,418, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/med.10012 [source] | ||||||||||