Acidic dissociation constant, probe

Acid centers, structure, sulfate-supported metal oxides, 37 192-196 Acidic catalysis, 6 241 montmorillonite, 38 266-268 Acidic dissociation constant, probe molecules, 38 210... 

Prins reaction, heteropolyacid catalysis, 41 156 Probe molecules, 42 119 acidic dissociation constant, 38 210 NMR solid acidity studies, 42 139-140 acylium ions, 42 139, 160 aldehydes, 42 162-163 alkyl carbenium ions, 42 154-157 allyl cation, 42 143-144 ammonia, 42 172-174 arenium ions, 42 150-154 carbonium ions, 42 157-160 chalcogenenonium ions, 42 161-162 cyclopentenyl cations, 42 140-143 indanyl cations, 42 144-147 ketones, 42 162,163-165 nitrogen-containing compounds, 42 165-170... 

Proton Affinities, Vertical Ionization Potentials, and Acidic Dissociation Constants of Selected Basic Probe Molecules"... 

Metadynamics was used to probe thermodynamics of hydrolysis by calculating the first acidity constant, pKa, for all three oxidation states of aqueous uranium (U (aq), UO faq), and U02 (aq)). The collective variable employed to describe the deprotonation reactions is the coordination number of an arbitrary first-shell water oxygen atom with respect to all protons, no-H- The coordination number parameters were chosen to be k =10A and rau = 1.38A. Starting with well-equilibrated NVT AIMD simulations, standard metadynamics simulations were carried out at 300 K with H = 0.063 kcal/mol and The time between the addition of Gaussians was r = 100(it for U (aq) and t = 20dt for U02(aq) and U02 (aq), where 6t is the simulation time step. Once the free energy difference, AF, for the reaction was known, the first acid dissociation constant, pKa, was computed as... 

The cell in Figure 14-9 acts as a probe to measure [H+] in the left half-cell. Using this type of cell, we could determine the equilibrium constant for acid dissociation or base hydrolysis in the left half-cell. 

However, acidity or basicity of a gas-phase adsorbate is not a sole criterion for its choice as a probe molecule. Firstly, the strength of an acidic or basic probe should be distinguished accordingly to its acid- or base-dissociation constant Ka or Kb). In addition, very important feature of probe molecule is its radius. If there is a need to locate all active sites in the structure of microporous solid material, the radius of probe molecule has to be smaller then the diameter of pore(s) opening(s). In other words, probe molecules have to be of appropriate size, so the entrance in the micropores of the solid and the access of adsorbate to each active site become possible. For example, ammonia, which is frequently used to reveal the acidic property of solids, is selected as a probe due to its basicity and due to the size of the molecule. Its molecule is smaller than the diameter of the pores in the zeolites structures, and also in many other solids. The other probe often used for investigation of solids acidity is pyridine however, the application of other chemical species is also possible. 

Thus pH = —log(CH,b), and Cb and Chb. the concentrations of the dissociated and protonated species of an acid-base pair, respectively (1, 12, 25). The quantity pK — F( )s/(RT hilO) can be considered as an apparent pK for proton binding of an acid-base pair on the membrane surface. Eqn. 8 shows that the protonation state Cb/chb of the probe is altered if ( )s changes at constant pH, which results in a change in the fluorescence yield ie the protonated form is much less fluorescent than the deprotonated form. 

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