Proton-transfer equilibrium constants

Excited-state proton transfer relates to a class of molecules with one or more ionizable proton, whose proton-transfer efficiency is different in the ground and excited states. The works of Forster [2-4] and Weller [5-7] laid the foundation for this area on which much of the subsequent work was based. Forster s work led to the understanding of the thermodynamics of ESPT. He constructed a thermodynamic cycle (Forster cycle) which, under certain acceptable approximations, provides the excited-state proton-transfer equilibrium constant (pK f,) from the corresponding ground-state value (pKa) and electronic transition energies of the acid (protonated) and base (deprotonated) forms of the ESPT molecule ... 

A number of proton-transfer equilibrium constants for reactions similar to those shown in Eq. (3) have been measured by ion cyclotron resonance, high-pressure mass spectroscopy, flowing afterglow, MIKES, and MIKES/CID techniques. These studies allowed the relative proton affinities of a variety of bases to be determined with an accuracy of better than +0.2 kcal mol" and compared with related thermodynamic data measured in solution. 

Mean Initiation Kinetic and Proton Transfer Equilibrium Constants for Ethoxylation (kp, Ke ) and Propoxylation (kp2r (,2) Reactions of Different Starters... 

The proton-transfer equilibrium constants depend on the relative acidity of both the substrate and the growing chain and also on the relative stability of the corresponding ionic couples, and their values strongly influence the oligomer distribution [3,14-17]. Some experimental proton-transfer equilibrium constants are given in Table 15.2. 

The gas-phase basicities of a-trimethylsilylstyrenes were determined by Mishima and coworkers by measurement of proton transfer equilibrium constants. The basicity of a-trimethylsilylstyrene was found to be comparable to that of ot-alkyl styrenes, which was taken to suggest that an a-trimethylsilyl group stabiUzes a carbocation. 

We can also write an equilibrium constant for the proton transfer equilibrium of a base in water. For aqueous ammonia, for instance,... 

To express the relative strengths of an acid and its conjugate base (a conjugate acid-base pair ), we consider the special case of the ammonia proton transfer equilibrium, reaction C, for which the basicity constant was given earlier (Kb = [NH4+l[OH ]/ NH3]). Now let s consider the proton transfer equilibrium of ammonia s conjugate acid, NH4+, in water ... 

Because a proton transfer equilibrium is established as soon as a weak acid is dissolved in water, the concentrations of acid, hydronium ion, and conjugate base of the acid must always satisfy the acidity constant of the acid. We can calculate any of these quantities by setting up an equilibrium table like that in Toolbox 9.1. 

Proton transfer equilibrium is established as soon as a weak base is dissolved in water, and so we can calculate the hydroxide ion concentration from the initial concentration of the base and the value of its basicity constant. Because the hydroxide ions are in equilibrium with the hydronium ions, we can use the pOH and pKw to calculate the pH. 

For each of the following weak acids, write the proton transfer equilibrium equation and the expression for the equilibrium constant Kv Identify the conjugate base, write the appropriate proton transfer equation, and write the expression for the basicity constant Kb. (a) HC102 (b) HCN ... 

One of the most important types of aqueous equilibrium involves proton transfer from an acid to a base. In aqueous soiutions, water can act as an acid or a base. In the presence of an acid, symbolized HA, water acts as a base by accepting a proton. The equilibrium constant for transfer of a proton from an acid to a water molecule is caiied the acid ionization constant (Zg) ... 

Because many species exhibit acid-base properties, it is often possible to write several proton-transfer equilibrium expressions for an aqueous solution. Each such expression is valid if the reactants are species that are actually present in the solution. We have already seen how to focus on the dominant equilibrium consider only those expressions that have major species as reactants, and look for the one with the largest equilibrium constant. 

Acetylene is sufficiently acidic to allow application of the gas-phase proton transfer equilibrium method described in equation l7. For ethylene, the equilibrium constant was determined from the kinetics of reaction in both directions with NH2-8. Since the acidity of ammonia is known accurately, that of ethylene can be determined. This method actually gives A f/ acid at the temperature of the measurement. Use of known entropies allows the calculation of A//ac d from AG = AH — TAS. The value of A//acij found for ethylene is 409.4 0.6 kcal mol 1. But hydrocarbons in general, and ethylene in particular, are so weakly acidic that such equilibria are generally not observable. From net proton transfers that are observed it is possible sometimes to put limits on the acidity range. Thus, ethylene is not deprotonated by hydroxide ion whereas allene and propene are9 consequently, ethylene is less acidic than water and allene and propene (undoubtedly the allylic proton) are more acidic. Unfortunately, the acidity of no other alkene is known as precisely as that of ethylene. 

Complexes between amines and phenols in apolar solvents56 were extensively investigated by several techniques. The equilibrium between molecular complexes and ions was recently investigated57 by 1H NMR techniques for the complexes between phenols and /V./V-dirncthylaniline. The constant of the proton transfer equilibrium (K of equilibrium 7) increases on increasing ApKa (= pKa of protonated base —pKa of phenol) in water, and when the solutions are cooled. 

Acetylene is sufficiently acidic to allow application of the gas-phase proton transfer equilibrium method described in equation 1. For ethylene, the equilibrium constant was... 

Amines are considerably more basic than alcohols, ethers, or water. When an amine is dissolved in water, an equilibrium is established in which water acts as an acid and transfers a proton to the amine. Just as the acid strength of a carboxylic acid can be measured by defining an acidity constant Ka (Section 2.8), the base strength of an amine can be measured by defining an analogous basicity constant K. The larger the value of ifb (and the smaller the value of piTj,), the more favorable the proton-transfer equilibrium and the stronger the base. 

N NMR spectra of a series of A-(R-salicylidene)-alkylamines in CDCI3 solution were measured. Proton transfer equilibrium was estabhshed on the basis of the temperature dependence of the N chemical shift and Vnh coupling constants. 

ABSTRACT. Kinetics of proton transfer photoreactions in simple model systems is analyzed from the point of view of reaction kinetics in microphases. Protolytic photodissociation of some hydroxyaromatic compounds ArOH ( 1- and 2-na-phthol, chlorosubstituted naphthols ) was studied in micellar solutions and phospholipid vesicles by fluorescence spectra and kinetics. Experimental results give evidence of at least two localization sites of naphthols in the microphase of these systems. In lipid bilayer membranes of vesicles there are two comparable fractions of ArOH molecules, one of which undergo photodissociation, but another do not dissociate. In micelles only minor fraction ( few per cent ) of ArOH molecules do not take part in excited-state proton transfer reaction. These phenomena reflect heterogeneous structure and dynamic properties of lipid bilayer membranes and micelles. A correlation between proton transfer rate constants and equilibrium constants in microphases similar to that in homogeneous solutions is observed. Microphase approach give a possibility to discuss reactions in dynamical organized molecular systems in terms of classical chemical kinetics. 

The role of solvent is evaluated quantitatively by the quantity K(s)/K(g), where K,g) is the equilibrium constant for equation 2 and K(s) is the corresponding equilibrium constant for the proton-transfer equilibrium in dilute solution written formally as ... 

A more interesting source of the strong disagreement between the observed and calculated rate constants of electron transfer between Qa and Qb, is the possible absence of proton equilibration on the time scale of electron transfer between the quinones. This is consistent with a key position for AspL213 in providing the path for all proton delivery to the Qb binding domain. On the other hand, equilibration could easily occur on the time scale of electron transfer from Qb to P+, giving the qualitative coincidence between observed and calculated pH dependencies of the electron transfer equilibrium constant. 

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