Use Of Free Energy Relationships Research Articles

The use of free energy relationships to rationalise structural and solvent effects on the photoreduction of cobalt(III) complexes in aqueous organic solvent medium

Quantum yields for the photoreduction of a series of CoIII complex ions, [CoClen2 (RC6H4NH2)]2+ where R=p-OMe, p-OEt, p-Me, m-Me, H and p-F using a low pressure Hg vapour lamp (254 nm) in aqueous ethanol mixtures (15–40% v/v organic cosolvent), increase with increase in mole fraction of the cosolvent in the binary solvent mixture under investigation. The Hammett correlation is linear, affording negative reaction constants, which indicate that the excited state is electron deficient. Correlation of the quantum yield data with Kamlet–Taft's solvatochromic parameters indicates that the specific solvent–solute interactions play a dominant role in governing the reactivity.

Use of free energy relationships to probe the individual steps of hydroxylation of p-hydroxybenzoate hydroxylase: studies with a series of 8-substituted flavins.

We report Hammett correlations, using 8-substituted flavins, to clarify the mechanism of hydroxylation by p-hydroxybenzoate hydroxylase (PHBH). The 8-position of the FAD isoalloxazine ring was chosen for modifications, because in PHBH it has minimal interactions with the protein, and it is accessible to solvent and away from the site of hydroxylation. Although two intermediates, a flavin-C4a-hydroperoxide and a flavin-C4a-hydroxide, are known to participate in hydroxylation, the mechanism of oxygen transfer remains controversial. Mechanisms as diverse as electrophilic aromatic substitution, diradical formation, and isoalloxazine ring opening have been proposed. In the studies reported here, it was possible to monitor spectrally each of the individual steps involved in hydroxylation, because the FAD cofactor acts as a reporter group. Thus, with PHBH, substituted separately with nine derivatives of FAD altered in the 8-position, quantitative structure-reactivity relationships (QSAR) have been applied to probe the mechanisms of formation of the flavin-C4a-hydroperoxide, the conversion to the flavin-C4a-hydroxide with concomitant oxygen transfer to the substrate, and the dehydration of the flavin-C4a-hydroxide to form oxidized FAD. The individual chemical steps in the mechanism of PHBH were not altered when using any of the modified flavins, and normal products were obtained; however, the rates of individual steps were affected, and depended on the electronic properties of the 8-substituent. Increased hydroxylation rates were observed when a more electrophilic flavin-C4a-hydroperoxide (i.e., with an electron-withdrawing substituent at the 8-position) is bound to PHBH. On the basis of QSAR analysis, we conclude that the mechanism of the hydroxylation step is best described by electrophilic aromatic substitution.

The use of free energy relationships to rationalize kinetic data in complex solvent mixtures

The electron transfer reaction [(NH3)4COIII(μ-pzCO2)FeII(CN)5]− → [(NH3)4COII(μ-pzCO2)FeIII(CN)5]− has been studied in water-cosolvent mixtures at ionic strength of I = 0.5 mol dm−3 (NaClO4). A multiparameter regression coefficients have been compared to those obtained for the same reaction to a different ionic strength (I = 2.8 10−3 mol dm−3). The magnitude of these coefficients changes with the ionic strength. An explanation is given for this behavior. © 1996 John Wiley & Sons, Inc.

The use of free energy relationships to rationalize kinetic data in complex solvent mixtures

The electron transfer reaction [(NH3)4COIII(μ-pzCO2)FeII(CN)5]− → [(NH3)4COII(μ-pzCO2)FeIII(CN)5]− has been studied in water-cosolvent mixtures at ionic strength of I = 0.5 mol dm−3 (NaClO4). A multiparameter regression coefficients have been compared to those obtained for the same reaction to a different ionic strength (I = 2.8 10−3 mol dm−3). The magnitude of these coefficients changes with the ionic strength. An explanation is given for this behavior. © 1996 John Wiley & Sons, Inc.

The use of free-energy relationships in the selection of lubricants for high-temperature applications

As a part of a program of developing lubricants and lubrication techniques for high-speed ball bearings operating over the temperature range from room temperature to 1,200°F, candidate lubricants were screened in a high-temperature, high-speed rolling-disk apparatus. Finely divided, inorganic solids suspended in a gas carrier were the principal lubricants selected for the screening evaluations. In order to choose a small number of the most promising compounds from this large group of materials for evaluation, selection criteria were established and applied. These criteria included some of the physical properties which have traditionally been considered important lubricant characteristics. In addition, thermodynamic calculations were used to judge the probability of chemical reaction between prospective lubricants and the substrate materials since it was felt that adherent solid lubricant films may be formed and maintained by chemical bonding with the bearing surfaces. On the basis of the rolling disk experimental results, softness and thermodynamic properties are the most important of the criteria used in the selection of candidate lubricants. With a few exceptions, those compounds which are soft and are thermodynamically favorable for chemical reaction with the substrate provided the lowest friction and wear.