Dynamics Of Exchange Reactions Research Articles

Ultrafast Proton Transfer Pathways Mediated by Amphoteric Imidazole

Imidazole, being an amphoteric molecule, can act both as an acid and as a base. This property enables imidazole, as an essential building block, to effectively facilitate proton transport in high-temperature proton exchange membrane fuel cells and in proton channel transmembrane proteins, enabling those systems to exhibit high energy conversion yields and optimal biological function. We explore the amphoteric properties of imidazole by following the proton transfer exchange reaction dynamics with the bifunctional photoacid 7-hydroxyquinoline (7HQ). We show with ultrafast ultraviolet-mid-infrared pump-probe spectroscopy how for imidazole, in contrast to expectations based on textbook knowledge of acid-base reactivity, the preferential reaction pathway is that of an initial proton transfer from 7HQ to imidazole, and only at a later stage a transfer from imidazole to 7HQ, completing the 7HQ tautomerization reaction. An assessment of the molecular distribution functions and first-principles calculations of proton transfer reaction barriers reveal the underlying reasons for our observations.

Influence of applied bias for A-site and X-site ion exchange reaction dynamics in perovskite quantum dots

Due to the excellent photophysical properties of perovskite quantum dots, their introduction into solar driven chemistry and optoelectronic devices has been taken up as an attractive alternative. Despite the outstanding photophysical properties and successful application to photovoltaic devices, applied electrical field induced phenomena could induce various component change and time dependent variations in the perovskite quantum dots. In this study, we investigated the behavior of X-site halide and A-site cation exchange processes in between the perovskite quantum dots. Through in-situ photoluminescence spectroscopy under the applied bias, we figured out the role of applied bias and current to the exchange process to the exchange process. Through these studies, we were able to track the enhanced migration rate of ions in the batch solution could facilitate the ion interdot exchange reaction dynamics in between the perovskite quantum dots. This study can be used to develop the practical strategy to apply the perovskite quantum dots to the solar driven chemistry and the optoelectronic devices.

Crystal structure dependent cation exchange reactions in Cu2-xS nanoparticles.

Because of high mobility of Cu+ in crystal lattice, Cu2-xS nanoparticles (NPs) utilized as cation exchange (CE) templates to produce complicated nanomaterials has been extensively investigated. Nevertheless, the structural similarity of commonly used Cu2-xS somewhat limits the exploration of crystal structure dependent CE reactions, since it may dramatically affect the reaction dynamics and pathways. Herein, we select djurleite Cu1.94S and covellite CuS nanodisks (NDs) as starting templates and show that the crystal structure has a strong effect on their CE reactions. In the case of djurleite Cu1.94S NDs, the Cu+ was immediately substituted by Cd2+ and solid wurtzite CdS NDs were produced. At a lower reaction temperature, these NDs were partially substituted, giving rise to the formation of Janus-type Cu1.94S/CdS NDs, and this process is kinetically and thermodynamically favorable. For covellite CuS NDs, they were transformed into hollow CdS NDs under a more aggressive reaction condition due to the unique disulfide covalent bonds. These disulfide bonds distributed along [0 0 1] direction were gradually ruptured/reduced and CuS@CdS core-shell NDs could be obtained. Our findings suggest that not only the CE reaction kinetics and thermodynamics, but also the intermediates and final products are intimately correlated to the crystal structure of the host material.

Localized and delocalized bound states of the main isotopologue 48O3 and of 18O-enriched 50O3 isotopomers of the ozone molecule near the dissociation threshold.

Knowledge of highly excited rovibrational states of ozone isotopologues is of key importance for modelling the dynamics of exchange reactions, for understanding longstanding problems related to isotopic anomalies of the ozone formation, and for analyses of extra-sensitive laser spectral experiments currently in progress. This work is devoted to new theoretical study of high-energy states for the main isotopologue 48O3 = 16O16O16O and for the family of 18O-enriched isotopomers 50O3 = {16O16O18O, 16O18O16O, 18O16O16O} of the ozone molecule considered using a full-symmetry approach. Energies and wave functions of bound states near the dissociation threshold are computed in hyperspherical coordinates accounting for the permutation symmetry of three identical nuclei in 48O3 and of two identical nuclei in 50O3, using the most accurate potential energy surface available now. The obtained vibrational band centers agree with observed ones with the root-mean-squares deviation of about 1 cm-1, making the results appropriate for assignments and analyses of future experimental spectra. The levels delocalized between the three potential wells of ozone isomers are computed and analyzed. The states situated deep in the three (for 48O3) or two (for 50O3) equivalent potential wells have similar energies with negligible splitting. However, the states situated just below the potential barriers separating the wells, are split due to the tunneling between the wells resulting in the splitting of rovibrational sub-bands. We evaluate the amplitudes of the corresponding effects and consider possible perturbations in vibration-rotation bands due to interactions between three potential wells. Theoretical predictions for the splitting of observable band centers are provided for the first time.

Study of the Gas-Phase Oxygen–Hydrogen Exchange Reaction of O(3P) + i-C3H7 → H(2S) + CH3COCH3

The gas-phase oxygen-hydrogen exchange reaction dynamics of O((3)P) + i-C3H7 (isopropyl) → H((2)S) + CH3OCH3 (acetone) was first investigated by the vacuum-ultraviolet laser-induced fluorescence (VUV-LIF) spectroscopy in a crossed beam configuration. The nascent H-atom Doppler-profile analysis shows that the average translation energy of the products and the fraction of the total available energy released as the transitional energy were determined to be 33.3 ± 1.64 kcal mol(-1) and 0.38, respectively. With the aid of the CBS-QB3 level of ab initio theory and statistical calculations, it was found that the title reaction is one of the major reaction pathways and proceeds through the formation of dynamical, short-lived addition complexes. On the basis of a systematic comparison with several exchange reactions of hydrocarbon radicals, the large variation in the fractions of translational energy release can be understood in terms of the unique geometrical features of the transition states along the reaction coordinates on the doublet potential energy surfaces.

Reflection high energy electron diffraction observation of exchange reaction dynamics on InAs surfaces

We have used time-resolved reflection high energy electron diffraction (RHEED) measurements to study the dynamics of a surface, anion exchange reaction. In the experiment, InAs surfaces are exposed to Sbx fluxes and subsequent changes in the crystals’ RHEED patterns are examined. We find that when an InAs surface is initially exposed to an Sb flux the specular spot intensity first decreases, then recovers back toward its initial value. The shape of the intensity versus time curves is extremely reproducible if the absolute Sb flux and the Sb species are kept constant. The length of time required for the RHEED pattern to stabilize is much shorter for cracked Sb than for uncracked Sb. The RHEED dynamics are also faster if the total Sb flux increases. The behavior of the RHEED dynamics as a function of Sb flux and Sb species is consistent with the changes in the RHEED pattern being due to an Sb/As exchange reaction on the crystals’ surface. The RHEED data are compared to previously published x-ray photoelectron spectroscopy (XPS) data which studied exchange reactions on InAs surfaces exposed to Sb fluxes. The XPS study confirmed that the incident Sb did indeed exchange with As in the epilayer and estimated the exposure time needed to complete the Sb/As exchange reaction. The time scales for exchange associated with the RHEED and XPS data are in good agreement, which further indicates that the observed RHEED dynamics are due to the Sb/As exchange reaction. Preliminary results from exposing GaSb surfaces to As fluxes show similar RHEED and XPS behavior. This suggests that RHEED could be generally applicable to the study of surface exchange reaction dynamics.

Cl′+HCl(v=1, j=3)→Cl′H(v′, j′)+Cl reaction dynamics over an extended collision energy range

Quasiclassical trajectory calculations have been used to model the Cl′+HCl(v=1, j=3) →Cl′H(v′, j′)+Cl exchange reaction dynamics over the 0–1.5 eV collision energy range. A kinematic constraint, which results from the H+LH mass combination, forces the reaction to strongly conserve initial vibrational excitation, independent of collision energy. This kinematic constraint is weaker than forces induced by the reaction potential anisotropy however, for the reaction does not strongly conserve initial rotational excitation. Reaction proceeds via two mechanisms which are classified as direct (single H atom exchange) and indirect (multiple H atom exchanges). Approximately 30% of the reaction proceeds via indirect exchange, independent of collision energy.

Effect of vibrational excitation of HBr on the H+HBr abstraction and exchange reactions

The effect of vibrational excitation of HBr on the H+HBr→H2+Br and H+H′Br→H′+HBr reactions has been investigated on the extended LEPS surface (ELEPS) constructed on the basis of quantum chemically calculated points of PES. Together with this surface the LEPS surface of Sudhakaran and Raff [1] was used for comparison at two relative translational energies. A quasiclassical trajectory method was used to study the abstraction and exchange reaction dynamics. The reactive cross section was calculated as a function of the relative collision energy and the vibrational state of HBr. The following conclusions can be drawn from the results of the study: (i) vibrational excitation v=0→ v=2 more than doubles the reaction cross section, (ii) the increase in the collision energy is most effectively channelled into the product translational energy.

Quasiclassical trajectory studies of H(D)+HBr(DBr) abstraction and exchange reactions

Abstract The abstraction and exchange reaction dynamics for H(D)+HBr(DBr) systems have been investigated on three LEPS potential-energy surfaces whose features are in accord with the surface topography suggested by recent molecular-beam and thermal experiments (abstraction barrier less than 1.0 kcal/mole, exchange reaction barriers of =5.0 kcal/mole, and no attractive wells with a depth greater than 0.209 kcal/mole). The surfaces differ primarily in the magnitude of the abstraction barrier which varies from 0.19 to 1.01 kcal/mole. Reaction cross sections have been computed on each surface as a function of relative collision energy from the results of 139000 quasiclassical trajectoris. Comparison of these results with measured relative abstraction cross sections suggests that the true abstraction barrier is very small, perhaps between 0.0 and 0.25 kcal/mole. However, thermal rate coefficients computed on the - best- surface at 300 K are about a factor of 2 larger than the most recently measured values. The calculated (H,D)/(D,H) isotope ratio at 300 K lies between the two reported experimental results. The computed thermal activation energy for abstraction is 835 cal/mole, which is in good agreement with a very early measurements but a factor of 2.5 less than the most recently reported experimental result. These results suggest that the molecular-beam and thermal rate measurements are inconsistent. The average fraction of the available energy which is partitioned into internal product modes E > is found to be nearly independent of relative collision energy and the small topographical differences present in the potential surfaces used in these calculations. We find E > = 0.40. In all reactions, the differential scattering cross sections are peaked in the backward direction for the molecular products, indicating a rebound mechanism.

Location of Energy Barriers. II. Correlation with Barrier Height

In Paper I of this series a hypothetical potential-energy surface was used in order to examine the effect on the dynamics of exchange reactions A + BC→AB + C of moving the energy barrier from an “early” to a “late” position along the reaction coordinate (i.e., from the entry valley to the exit valley of the energy surface). In the present work an attempt has been made to correlate barrier location with other properties of the energy surface, as a step toward the application of the generalizations of Paper I to real cases. Related families of reactions have been examined in the London–Eyring–Polanyi–Sato (LEPS) and bond-energy bond-order (BEBO) approximations. The principal generalizations may be summarized as follows: (1) For substantially exothermic reactions the barrier is in the entry valley, and for substantially endothermic reactions the barrier is in the exit valley. In the light of Paper I this implies that the cross sections for these exothermic reactions will rise most steeply with increasing translational energy in the reagents, whereas the cross sections for the endothermic reactions will rise most steeply with increasing vibrational energy in the bond under attack. (2) For decreasing barrier height in related exothermic reactions the barrier moves to successively earlier positions along the entry valley, with increasing percentage “attractive energy release.” (3) In general [i.e., if (4), below, is obeyed even qualitatively], for increasing barrier height in related endothermic reactions the barrier moves to successively later positions along the exit valley. (4) The correlation between the classical barrier height EC and reaction energy qc, written ΔEC = −α Δqc by Ogg and Polanyi (OP) was found to be a weaker correlation than (2), above. For families where barrier height did decrease with increasing reaction energy, a logarithmic relationship, Δ logEC = −αl Δqc, was preferable for the exothermic reactions. The same relationship encompassed members of the family with qc < 0 (endothermic reactions). With the proviso that (4) shall be applicable, (1)–(3) are summarized in the proposition that the barrier moves to successively later positions along the reaction coordinate with increasing barrier height. This progressive shift of the barrier is embodied in the following approximate relationships: Δ logEC = −β1lΔr1‡,0, Δ log r2‡,0 = −aΔr1‡,0 + cΔ[(r1‡,0)−1], where r1‡,0 and r2‡,0 are the extensions of AB and BC, from their equilibrium separation, at the crest of the barrier. In view of (4), Δqc = (β1l / αl)Δr1‡,0, a relationship which encompasses both exothermic and endothermic reactions. The fraction of attractive energy release alters from one exothermic reaction to the next by approximately Δ(α⊥) = γ1Δr1‡,0; hence, Δ logEC = −(β1l / γ1) Δ (α⊥).

Location of Energy Barriers. I. Effect on the Dynamics of Reactions A + BC

The dynamics of exchange reactions A+BC→AB+C have been examined on two types of potential-energy hypersurfaces that differed in the location of the energy barrier along the reaction coordinate. On “surface I” the barrier was in the entry valley of the energy surface, along the approach coordinate. On “surface II” the barrier was in the exit valley of the energy surface, along the retreat coordinate. The classical barrier height was Ec = 7.0 kcal mole−1 on both surfaces, and was displaced from the corner of the energy surface by the same amount; on surface I, r1‡ = 1.20 Å, r2‡ = 0.80 Å; on surface II, r1‡ = 0.80 Å, r2‡ = 1.20 Å (r1 ≡ rAB, r2 ≡ rBC, and the superscript ‡ refers to the location of the crest of the barrier). Three-dimensional (3D) classical trajectory calculations were performed for the mass combination mA = mB = mC at several reagent energies. The reagent energy took the form of translation, vibration or an equilibrium distribution of the two. The main findings were that translation was markedly more effective than vibration in promoting reaction on surface I, and vibration markedly more effective than translation in promoting reaction on surface II. The total reactive cross section with the entire reagent energy vested in translation was symbolized ST, with the reagent energy (but for 1.5 kcal) in vibration, SV, and with an equilibrium distribution over reagent translation and vibration, Seq. On surface I ST ≫ SV: on surface II SV ≫ ST. Close to the threshold for ST on surface I, ST / Seq ∼ 10; close to the threshold for SV, on surface II, SV / Seq ∼ 10. At high reagent energies (2 × threshold) on surface I ST / Seq fell to 2, whereas on surface II SV / Seq increased to extremely large values. Product energy and angular distributions were recorded for two reagent energies. On surface I with low translational energy in the reagents a major part of the available energy appeared as vibration in the molecular product. At higher collision energy this fraction decreased. On surface II with low vibrational energy in the reagents only a small part of the available energy appeared as vibration in the product. At higher vibrational energy this fraction increased. The product angular distribution at low reagent translational energy on surfaces I and II corresponded to backward-peaked scattering of the molecular product. At increased reagent energy on both surfaces the distribution shifted forward (this is a novel phenomenon in the case of increased reagent vibration; surface II).