- HYDROGEN BONDING AND PROTON TRANSFER IN HYDRIDO-BIS-PHENOLATE COMPLEXES IN ACETONE
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The homoconjugation, (ArO)2H-, and heteroconjugation, Ar'O- HOAr, (where Ar is aromatic) with proton transfer have been determined in acetone at 298 K.Tetra-alkylammonium phenolates were titrated with a variety of phenols to given ho
- Pawlak, Zenon,Nowak, Boguslaw,Fox, Malcolm F.
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- Heterolysis and homolysis energies for some carbon-oxygen bonds
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Methods described previously for obtaining heterolysis (ΔHhet) and homolysis (ΔHhomo) enthalpies for bonds that can be cleaved to produce resonance-stabilized carbenium ions, anions, and radicals are extended to the study of carbon-oxygen bonds through the reactions of resonance-stabilized carbenium ions with substituted phenoxide ions. Titration calorimetry was used to obtain the heat of heterolysis, and the second-harmonic ac voltammetry (SHACV) method was used to obtain reversible oxidation potentials for the anions. In several cases, the electrode reactions were so fast that reversible potentials were obtained only with the greatest difficulty. Nonetheless, there is remarkably good agreement between these oxidation potentials for phenoxide ions obtained by electrochemical methods in sulfolane solution and those reported by others using entirely different techniques in different media. Such agreement provides unprecedented evidence for the soundness of the various methods used to study redox potentials of organic ions and radicals. As before, a wide variety of correlations was tested between ΔHhet and ΔHhomo. These two properties showed little correlation with each other, but ΔHhet gave good correlations between many properties for which neutral species are converted into ions or vice versa, such as redox potentials of both types of ions, the pKas of the anions, or the free energies of electron transfer. In contrast to the earlier study of cleavage to carbanions and carbenium ions, the present ΔHhet values are predicted well by a general equation that employs the pKR+ of the carbenium ion (without modification) and the pKa of the phenol. The improvement is consistent with the fact that the cleavage of carbon-oxygen bonds of the triarylcarbinols used to establish the pKR+ stability scale is a more appropriate model for the heterolysis of carbon-oxygen bonds in sulfolane at 25°C than it is for the cleavage of carbon-carbon bonds under the same conditions.
- Arnett, Edward M.,Amarnath, Kalyani,Harvey, Noel G.,Venimadhavan, Sampath
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p. 7346 - 7353
(2007/10/02)
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- Thermodynamics of Proton Transfer in Phenol-Acetate Hydrogen Bonds with Large Proton Polarizability and the Conversion of Light Energy into Chemical Energy in Bacteriorhodopsin
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Phenol-acetate solutions in CCl4 are studied by IR spectroscopy as a function of the pKa of the phenols.The (I) Ar-OH...-OC Ar-O-...HOC (II) hydrogen bonds formed show large proton polarizability as indicated by continua in the IR spectra.The percent proton transfer (PT) increases from the p-cresol-acetate to the pentachlorophenol-acetate system from 0percent to 56percent.The Gibbs free energy, ΔG0PT, values of the PT equilibria at 295 K are determined as well as the standard enthalpy values, ΔH0PT, and the standard entropy values, ΔS0PT.These data are given in Table II.The shape of the intensity of the continuum as a function of the ΔH0PT value changes.In the classical approximation the average difference between the two minima of the proton double-minimum potential is given by ΔH0PT.With the decreasing amount of ΔH0PT, i.e., decreasing degree of asymmetry, the intensity of the continua decreases at higher and increases at lower wavenumbers.This result is in good agreement with the predictions from calculated line spectra.In the photocycle of bacteriorhodopsin a tyrosine-aspartate hydrogen bond is probably of importance for the conversion of light energy into chemical energy.On the basis of the obtained data it is shown that 9.5 kJ/mol can be converted into chemical energy due to a proton transfer induced by a local electrical field in a Tyr-Asp hydrogen bond.Furthermore, if the Ar-O-...HOC structure is broken afterwards by a conformational change, at least 25 kJ/mol of conformational energy is converted.Thus, altogether 34.5 kJ/mol of Gibbs free energy may be converted into chemical energy and stored by these processes.
- Merz, Helmut,Tangermann, Ulrike,Zundel, Georg
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p. 6535 - 6541
(2007/10/02)
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