Acids proton fractionation factor

The value of the fractionation factor for any site will be determined by the shape of the potential well. If it is assumed that the potential well for the hydrogen-bonded proton in (2) is broader, with a lower force constant, than that for the proton in the monocarboxylic acid (Fig. 8), the value of the fractionation factor will be lower for the hydrogen-bonded proton than for the proton in the monocarboxylic acid. It follows that the equilibrium isotope effect on (2) will be less than unity. As a consequence, the isotope-exchange equilibrium will lie towards the left, and the heavier isotope (deuterium in this case) will fractionate into the monocarboxylic acid, where the bond has the larger force constant. 

The equilibrium constant for the isotope-exchange equilibrium can be expressed (6) in terms of the solvent isotope effects on the acid-dissociation constants and of the monocarboxylic acid and dicarboxylic acid monoanion, respectively. It follows that a lower value for the fractionation factor of the hydrogen-bonded proton means that the solvent isotope effect on the acid-dissociation constant will be lower for the dicarboxylic acid monoanion than for the monocarboxylic acid. 

The values of the fractionation factors in structures [15]-[21] are not strictly comparable since they are defined relative to the fractionation in different solvent standards. However, in aqueous solution, fractionation factors for alcohols and carboxylic acids relative to water are similar and close to unity (Schowen, 1972 Albery, 1975 More O Ferrall, 1975), and it seems clear that the species [15]-[21] involving intermolecular hydrogen bonds with solvent have values of cp consistently below unity. These observations mean that fractionation of deuterium into the solvent rather than the hydrogen-bonded site is preferred, and this is compatible with a broader potential well for the hydrogen-bonded proton than for the protons of the solvents water, alcohol and acetic acid. 

For a reaction involving slow proton transfer from an acid to carbon (A—Se2 mechanism) shown in (126) the dependence of the rate coefficient upon the fraction of deuterium (n) in the solvent (127) can be derived by fractionation factor theory [122, 123, 204, 211(a)]. In eqn. (127) kn is the observed rate coefficient in a solvent of composition n and k0 is the observed rate coefficient in pure H20. The fractionation factors 0, and 02 are given by (129) and (130) and represent the fractional... 

Fractionation factors for proton transfer from cyanocarbon acids ... 

The protonation equilibria for nine hydroxamic acids in solutions have been studied pH-potentiometrically via a modified Irving and Rossotti technique. The dissociation constants (p/fa values) of hydroxamic acids and the thermodynamic functions (AG°, AH°, AS°, and 5) for the successive and overall protonation processes of hydroxamic acids have been derived at different temperatures in water and in three different mixtures of water and dioxane (the mole fractions of dioxane were 0.083, 0.174, and 0.33). Titrations were also carried out in water ionic strengths of (0.15, 0.20, and 0.25) mol dm NaNOg, and the resulting dissociation constants are reported. A detailed thermodynamic analysis of the effects of organic solvent (dioxane), temperature, and ionic strength on the protonation processes of hydroxamic acids is presented and discussed to determine the factors which control these processes. 

Chromatography by ion exchange on a sulfonated poly(styrene-co-divinyl benzene) phase has been proposed as a replacement for titrimetry.57 Eluted by a dilute solution of a neutral salt such as sodium ethanesulfonate, the conductance of the protons can be measured in the absence of a suppressor from sub-millimolar to molar concentration. The response factors of mono-, di-, and trichloroacetic acid and of o-phthalic acid were large and essentially equivalent to ethanesulfonic acid, while the response factor of acetic acid was far smaller. A syringe pump has generated pressures as high as 72,000 psi (5000 bar) in a capillary column packed with 1 p particles, generating a fraction capacity of 300 peaks in 30 minutes.58... 

In order to clarify the complexatlon reaction mechanisms, HjS transport experiments were performed using tetramethyl EDA [chemical formula (CHjljNCCHjlaNlCHjlj] as a carrier In an lEM. The HaS complexatlon reaction with EDA as well as most primary amines Is postulated to be an acid base reaction (10). Since TMEDA Is also a strong base, It should accept protons from the HaS and act as a carrier In an lEM environment. A facilitation factor of 1.93 was measured for a TMEDA lEM at an HaS feed mole fraction of 0.05. The degree of facilitation with the TMEDA membrane was much smaller than the EDA membrane ( EDA 15.8), but the data for the TMEDA lEM does support the acid-base complexatlon mechanism for HjS facilitated transport. The smaller F value for the TMEDA membrane may be due to the very low mobility of the TMEDA or because the binding between HaS and TMEDA Is so strong that the rate of the decomplexatlon reaction Is very slow. 

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