Effect on rate constant for

PEDRIELLI p, PEDULLi G F and SKIBSTED L H (2001a) Antioxidant mechanism of flavonoids. Solvent effect on rate constant for chain-braining reaction of quercetin and epicatechin in autoxidation of methyl linoleate, JAgric Food Chem, 49, 3034-40. 

Structures have been determined for [Fe(gmi)3](BF4)2 (gmi = MeN=CHCF[=NMe), the iron(II) tris-diazabutadiene-cage complex of (79) generated from cyclohexanedione rather than from biacetyl, and [Fe(apmi)3][Fe(CN)5(N0)] 4F[20, where apmi is the Schiff base from 2-acetylpyridine and methylamine. Rate constants for mer fac isomerization of [Fe(apmi)3] " were estimated indirectly from base hydrolysis kinetics, studied for this and other Schiff base complexes in methanol-water mixtures. The attenuation by the —CH2— spacer of substituent effects on rate constants for base hydrolysis of complexes [Fe(sb)3] has been assessed for pairs of Schiff base complexes derived from substituted benzylamines and their aniline analogues. It is generally believed that iron(II) Schiff base complexes are formed by a template mechanism on the Fe " ", but isolation of a precursor in which two molecules of Schiff base and one molecule of 2-acetylpyridine are coordinated to Fe + suggests that Schiff base formation in the presence of this ion probably occurs by attack of the amine at coordinated, and thereby activated, ketone rather than by a true template reaction. ... 

Reports on solvent effects on rate constants for aquation of diimine complexes include those on [Fe(5Brphen)3] + and [Fe(4,7-Me2phen)3] " " in methanol- and ethanol-water, [Fe(bipy)3] +, [Fe(phen)3] +, and [Fe(5N02phen)3] + in aqueous methyl D-glycopyranosides, and... 

Temperature and pressure effects on rate constants for [Fe(phen)3] +/[Fe(phen)3] + electron transfer in water and in acetonitrile have yielded activation parameters AF was discussed in relation to possible nonadiabaticity and solvation contributions. Solvation effects on AF° for [Fe(diimine)3] " " " " half-cells, related diimine/cyanide ternary systems (diimine = phen, bipy), and also [Fe(CN)6] and Fe aq/Fe aq, have been assessed. Initial state-transition state analyses for base hydrolysis and for peroxodisulfate oxidation for [Fe(diimine)3] +, [Fe(tsb)2] ", [Fe(cage)] " " in DMSO-water mixtures suggest that base hydrolysis is generally controlled by hydroxide (de)hydration, but that in peroxodisulfate oxidation solvation changes for both reactants are significant in determining the overall reactivity pattern. ... 

The final application of solvatochromic solvent strength scales is the correlation of reaction rate and equilibrium constants in SCF solvents. Solvatochromic scales are often quantitative indicators of the solvent effect on rate constants for a variety of reaction mechanismsU) In a SCF, this solvent effect may be achieved conveniently with a single solvent using pressure. Based on solvatochromic data, it was predicted that an activation volume can reach thousands of mL/mol in a SCF(8). This prediction was confirmed for various types of reactionsClSbZl). For example, the solvatochromic parameter Ex for phenol blue... 

Substituent Effects on Rate Constants for Proton Transfer at Carbon... 

TABLE 8.8 Solvent Effect on Rate Constants for the Sn2 Reaction of CH3I with Cl ... 

Theoretical study of kinetic isotope effects on rate constants for the H2+C2H—>H+C2H2 reaction and its isotopic variants ... 

Micellar effects on rate constants for the hydrolysis of the adduct EtgNjSOs have been studied. Both anionic, e.g. sodium hexadecylsul-phonate, and cationic, e.g. dodecyltrimethylammonium bromide, surfactants have a marked accelerating effect. ... 

Table 52.2 compares the effects of ring substituents on rate constants for triplet-state hydrogen abstraction by valerophenone, together with intrinsic k , values that reflect independently measured substituent effects on rate constants for bimolecular hydrogen abstraction (p = 0.5) and derived values for the... 

A good number of compounds have been studied whose triplets undergo both the type II reaction and one or more other processes, either physical or chemical. Because structural effects on rate constants for y-hydrogen abstraction are well understood, the type II reaction can serve as a clock for measuring rates of the other reactions, with quantum yield measurements allowing triplet hfetimes to be divided into separate rates for the competing reactions. 

A kinetic study for the polymerization of styrene, initiated with n BuLi, was designed to explore the Trommsdorff effect on rate constants of initiation and propagation and polystyryl anion association. Initiator association, initiation rate and propagation rates are essentially independent of solution viscosity, Polystyryl anion association is dependent on media viscosity. Temperature dependency correlates as an Arrhenius relationship. Observations were restricted to viscosities less than 200 centipoise. Population density distribution analysis indicates that rate constants are also independent of degree of polymerization, which is consistent with Flory s principle of equal reactivity. 

It is assumed in these experiments that the modification closest to the electron-transfer site will have most effect on rate constants. Rate constants are enhanced for 3+ and retarded for 3- redox partners. With Co(phen)33+ and Fe(CN)53 as oxidants it has been demonstrated that both react at the exposed heme edge of cytochrome c (23). The exposed heme edge is also relevant with PCu(II) as oxidant, Table III. With all three oxidants... 

Figure 10. View of cytochrome c with positions of exposed heme edge (block) and lysine modifications (sequence numbers for a-carbons) shown. The smaller circles indicate the relative effectiveness of modifications on rate constants for the reaction of Cyt c(II) + PCu(II) ( see Table III). Preferential interaction with PCu(II) in the direction 25,27,13,87 is indicated.

The experimental and simulation results presented here indicate that the system viscosity has an important effect on the overall rate of the photosensitization of diary liodonium salts by anthracene. These studies reveal that as the viscosity of the solvent is increased from 1 to 1000 cP, the overall rate of the photosensitization reaction decreases by an order of magnitude. This decrease in reaction rate is qualitatively explained using the Smoluchowski-Stokes-Einstein model for the rate constants of the bimolecular, diffusion-controlled elementary reactions in the numerical solution of the kinetic photophysical equations. A more quantitative fit between the experimental data and the simulation results was obtained by scaling the bimolecular rate constants by rj"07 rather than the rf1 as suggested by the Smoluchowski-Stokes-Einstein analysis. These simulation results provide a semi-empirical correlation which may be used to estimate the effective photosensitization rate constant for viscosities ranging from 1 to 1000 cP. 

Note, from Table 4, that kH for PI1CH2CCI is 6 x 107 s 1, about 3 times greater than kH for PI12CHCCI. This underscores the previously noted nonadditivity of bystander effects on rate constants in going from 10a to 66, the second Ph bystander actually slows the 1,2-H shift. The KGH analysis provides a solution to this conundrum. Steric effects in 66 cause the second Ph to adopt... 

It is important to point out here, in an early chapter, that the Born-Oppenheimer approximation leads to several of the major applications of isotope effect theory. For example the measurement of isotope effects on vapor pressures of isotopomers leads to an understanding of the differences in the isotope independent force fields of liquids (or solids) and the corresponding vapor molecules with which they are in equilibrium through use of statistical mechanical theories which involve vibrational motions on isotope independent potential functions. Similarly, when one goes on to the consideration of isotope effects on rate constants, one can obtain information about the isotope independent force constants which characterize the transition state, and how they compare with those of the reactants. 

In earlier sections of this chapter we learned that the calculation of isotope effects on equilibrium constants of isotope exchange reactions as well as isotope effects on rate constants using transition state theory, TST, requires the evaluation of reduced isotopic partition function ratios, RPFR s, for ordinary molecular species, and for transition states. Since the procedure for transition states is basically the same as that for normal molecular species, it is the former which will be discussed first. 

Table 6.3 Effect of Metal Coordination on Rate Constants for Base Hydrolysis of Amino Acid Esters at 25 °C...

Salt effects (chlorides and bromides of alkali metal, alkaline earth, and ammonium cations) on rate constants for aquation of [Fe(bipy3] " " and [Fe(phen)3] " " in aqueous solution have been interpreted in terms of added cation effects on the activity of the water. 

Figure 2 Diagrammatic summary of selected structural, substituent, and solvent effects on rate constants (kj, at 298 K) for base hydrolysis of low spin iron(II)-diimine complexes. Ligand abbreviations not appearing in the list at the end of this chapter are apmi = (73) with = Me BOH cage = (78) with X = OH ...

Chemical reactions at supercritical conditions are good examples of solvation effects on rate constants. While the most compelling reason to carry out reactions at (near) supercritical conditions is the abihty to tune the solvation conditions of the medium (chemical potentials) and attenuate transport limitations by adjustment of the system pressure and/or temperature, there has been considerable speculation on explanations for the unusual behavior (occasionally referred to as anomalies) in reaction kinetics at near and supercritical conditions. True near-critical anomalies in reaction equilibrium, if any, will only appear within an extremely small neighborhood of the system s critical point, which is unattainable for all practical purposes. This is because the near-critical anomaly in the equilibrium extent of the reaction has the same near-critical behavior as the internal energy. However, it is not as clear that the kinetics of reactions should be free of anomalies in the near-critical region. Therefore, a more accurate description of solvent effect on the kinetic rate constant of reactions conducted in or near supercritical media is desirable (Chialvo et al., 1998). 

Fig. 16. Structural effects on rate constants (25°C) for base hydrolysis of some...

Pyrazoles161 and Imidazoles (see Sections VI,A, B, D, and E). Effects of A-aryl substituents on rate constants for the quaternization of pyrazoles (60)168 and imidazoles (61)169 are correlated using a Hammett equation. It is not possible to decide whether the greater sensitivity (p... 

The isobutane-1 -butene alkylation was studied in dense CO2 in both fixed-bed and slurry reactors.165-167 Both Nafion SAC-13 and Nation SAC-25 exhibited steady-state conversions and selectivities for 50 h. Enhanced Cg alkylate selectivity could be achieved at near total butene conversion. The maximum value attained, however, was only about 40%. The higher effective alkylation rate constant for SAC-25 compared to SAC-13 indicates improved accessibility of the acid sites. Nafion SAC-13 and SAC-25 applied in a study to test the effect of supercritical fluids on alkylation exhibited only modest activities.168... 

Solvatochromic shift data have been obtained for phenol blue in supercritical fluid carbon dioxide both with and without a co-solvent over a wide range in temperature and pressure. At 45°C, SF CO2 must be compressed to a pressure of over 2 kbar in order to obtain a transition energy, E, and likewise a polarizability per unit volume which is comparable to that of liquid n-hexane. The E,j, data can be used to predict that the solvent effect on rate constants of certain reactions is extremely pronounced in the near critical region where the magnitude of the activation volume approaches several liters/mole. 

Solvent effect on rate constants. In this section, the rate constant will be predicted qualitatively in CO2 for the Diels-Alder cycloaddition of isoprene and maleic anhydride, a reaction which has been well-characterized in the liquid state (23,24). In a previous paper, we used E data for phenol blue in ethylene to predict the rate constant of the Menschutkin reaction of tripropylamine and methyliodide (19). The reaction mechanisms are quite different, yet the solvent effect on the rate constant of both reactions can be correlated with E of phenol blue in liquid solvents. The dipole moment increases in the Menschutkin reaction going from the reactant state to the transition state and in phenol blue during electronic excitation, so that the two phenomena are correlated. In the above Diels-Alder reaction, the reaction coordinate is isopolar with a negative activation volume (8,23),... 

Substituent effects on rate constants of base-promoted ionisation of ketones have led to the conclusion that an electron-withdrawing substituent increases the rate of ionisation, in agreement with the anionic character of the transition state. This is based chiefly on studies on halogenation and isotope exchange of aromatic ketones. Data on p-values observed by plotting ionisation rate constants versus Hammett -parameters (Table 3) for substituted... 

The quaternization of 1-substituted imidazoles is a facile reaction which leads to a stable quaternary salt via an 5n2 reaction (Scheme 16) which may be affected by steric factors. It has been shown (78AHC(22)7l) that the effects of fV-aryl substituents on rate constants for the quaternization of imidazoles can be correlated using a Hammett equation. The value of p (-0.45) for the ethylation of imidazoles shows little sensitivity to substituent effects, but this is to be expected since the basic pK values of the fV-aryl compounds do not vary widely, and the rate of alkylation must depend on the basicity of the nitrogen being quaternized. Certainly the expected sequence of reactivities, 1-methyl > 1-benzyl > 1-phenyl, is observed in the ethylation of 1-substituted imidazoles with iodoethane in ethanol or acetone, and although only qualitative observations are available, l-methyl-4-and -5-chloroimidazoles react less readily than 1-methylimidazole. Considerable experimental difficulty is experienced in quaternizing nitro-substituted imidazoles. 

Marcus has introduced a model for 5n2 reactions of the ET type based on two interacting states which takes into account the relevant bond energies, standard electrode potentials, solvent contributions, and steric effects. The rate constant for intramolecular electron transfer between reduced and oxidized hydrazine units in the radical cation of the tetraazahexacyclotetradecane derivative (43) and its analogues has been determined by simulation of their variable temperature ESR spectra. The same researchers also reported their studies of the SET processes of other polycyclic dihydrazine systems. ... 

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