“…Considering the reaction scheme (Scheme ), we expect that the proton (H + ), NS - , and NS can contribute to the species grating signal. The diffusion constant of the fast component is remarkably larger than that expected for the anion (NS - ) or the neutral form (NS) of the o -nitrosobenzoic acid but rather close to the reported value of proton ( D H + ) in water ((9.0−10) × 10 -9 m 2 s -1 at 25 °C). ,,− Considering the magnitude of D , we can assign the chemical species of the fast-decay component to H + definitely and the slow one to NS - . 2 Plots of the decay rate constant ( k ) of the TG signal against q 2 in pH = 6.5 and 8.2 solutions and the least-squares fit by k = Dq 2 (solid line: fast component; dotted line: slow component).…”
A proton-releasing photodissociation reaction of o-nitrobenzaldehyde is investigated by using the time-resolved transient grating method. The grating signal is very sensitive to the pH of the solution. The chemical species existing in the solution are identified from the diffusion constant of the species. For low-pH solutions, the grating signal due to the proton is observed clearly without any pH indicator. The origin of the signal is attributed to a pure volume grating, from which the partial molar volume of the proton can be determined accurately. The diffusion constant of the proton decreases with increasing the pH. This change is explained by the electrostatic interaction between the proton and the anion product, o-nitrosobenzoic anion. In an alkali solution, the hydroxyl ion is observed as the main species of the grating signal. By the quantitative measurement of the grating signal and the photoacoustic signal, the enthalpy change and the volume change of the reaction and the partial molar volumes of the proton are determined. This proton signal can be used to quantitatively measure the proton concentration change without any pH indicator.
“…Considering the reaction scheme (Scheme ), we expect that the proton (H + ), NS - , and NS can contribute to the species grating signal. The diffusion constant of the fast component is remarkably larger than that expected for the anion (NS - ) or the neutral form (NS) of the o -nitrosobenzoic acid but rather close to the reported value of proton ( D H + ) in water ((9.0−10) × 10 -9 m 2 s -1 at 25 °C). ,,− Considering the magnitude of D , we can assign the chemical species of the fast-decay component to H + definitely and the slow one to NS - . 2 Plots of the decay rate constant ( k ) of the TG signal against q 2 in pH = 6.5 and 8.2 solutions and the least-squares fit by k = Dq 2 (solid line: fast component; dotted line: slow component).…”
A proton-releasing photodissociation reaction of o-nitrobenzaldehyde is investigated by using the time-resolved transient grating method. The grating signal is very sensitive to the pH of the solution. The chemical species existing in the solution are identified from the diffusion constant of the species. For low-pH solutions, the grating signal due to the proton is observed clearly without any pH indicator. The origin of the signal is attributed to a pure volume grating, from which the partial molar volume of the proton can be determined accurately. The diffusion constant of the proton decreases with increasing the pH. This change is explained by the electrostatic interaction between the proton and the anion product, o-nitrosobenzoic anion. In an alkali solution, the hydroxyl ion is observed as the main species of the grating signal. By the quantitative measurement of the grating signal and the photoacoustic signal, the enthalpy change and the volume change of the reaction and the partial molar volumes of the proton are determined. This proton signal can be used to quantitatively measure the proton concentration change without any pH indicator.
“…It was proposed that the excess bonds generate entropic effects reflected in a larger entropic cost 19 for solvating molecules in D 2 O and bond rearrangements in H 2 O-D 2 O solutions in comparison to those in pure water. [20][21][22][23][24][25][26][27] It is reasonable to expect that the excess bond number and bond rearrangements (if existing) should also be reflected in the enthalpy of mixing of H 2 O-D 2 O solutions with different D/H ratios. To be more specific, H 2 O-D 2 O solutions are composed of three components, H 2 O, D 2 O, and HDO.…”
Section: Introductionmentioning
confidence: 99%
“…Previous studies investigated the effect of the lower volume packing density of D 2 O compared to that in H 2 O on the physicochemical properties of nonpolar and polar solutes in H 2 O−D 2 O solutions. , The results were related to the higher average number of hydrogen bonds per molecule (10% more) in D 2 O compared to the number of bonds in H 2 O. It was proposed that the excess bonds generate entropic effects reflected in a larger entropic cost for solvating molecules in D 2 O and bond rearrangements in H 2 O−D 2 O solutions in comparison to those in pure water. – …”
There is much renewed interest in the arrangement and kinetic of hydrogen bonds in water and heavy water. D 2 O forms a higher average number of hydrogen bonds per molecule (10% more) compared to the case for H 2 O, which cause a larger entropic cost for solvating molecules in D 2 O. Here we used isothermal titration calorimetry (ITC) to investigate the enthalpy of titration of D 2 O-H 2 O solutions with different D/H isotope ratios. We found significant enthalpy deviations (exothermic contributions) relative to the computed enthalpy for the limit of ideal mixing both for dilution titration and for concentration titration (injection of solutions with lower D/H ratios into solutions with higher ratios and vice versa). We propose that the observed exothermic deviations might be connected to entropic effects associated with differences in the H and D arrangements that depend on the D/H ratio of the solutions. This ratio varies during the titration processes, leading to the entropy production beyond that of ideal mixing. We also used the ITC in the nonstirring mode to measure the titration kinetics and found long relaxation times of up to tens of minutes for the concentration titrations (but not for the dilution titrations). These observations are consistent with slow propagation of the reaction H 2 O + D 2 O T 2HDO that involves hopping of deuterium and rearrangements of the H and D bonding.
“…The structures of molecules in PFSA were analyzed by RDF expressed in Eq. [5], the n B is the number of B particles situated at the distance of r in a shell of thickness dr from particle A, and N B and V are the total number of B particles and total volume of the system respectively. As shown in Fig.…”
Protons transfer in polyelectrolyte membrane by both Vehicle and Grotthus mechanisms. This study describes the property of proton and water transfer in perfluorosulfonic acid (PFSA) membrane using molecular dynamics (MD) simulation in view of both Vehicle and Grotthus mechanisms. To treat Grotthus mechanism, Empirical Valence Bond (EVB) method was introduced to MD simulation. The potential energy barrier of proton hopping in EVB method was adjusted to the computational result of Density Functional Theory (DFT) to replicate the experimental value of diffusion coefficient of oxonium ions in water. The transport property of water and oxonium ions in PFSA membrane was analyzed by such as Radial Distribution Function (RDF) and Mean Square Displacement (MSD) at different water contents. From this analysis, some perception of the structure of water cluster and the diffusivity of water and proton in PFSA membrane was derived.
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