Kinetics Of Isomerization Research Articles

Intrinsic kinetic study of 1-butene isomerization over magnesium oxide catalyst via a Berty stationary catalyst basket reactor.

In this work, we studied the intrinsic kinetics of 1-butene isomerization over a commercial MgO catalyst using a Berty-type reactor (gradient-less recycle reactor). The Berty-type reactor has a behavior equivalent to a continuous stirred tank reactor (CSTR) with perfect mixing. The experimental results from this reactor were selected in a range in the absence of external and internal mass- and heat-transfer resistances, thereby representing the intrinsic kinetics. The 1-butene isomerization was carried out under atmospheric pressure, a WHSV of 1.05–5.47 h−1, and the reaction temperature from 350 °C to 450 °C. The kinetic models were established on the basis of different mechanisms for 1-butene isomerization over MgO. The results showed that the LHHW kinetic model provided the best agreement with the experimental data. The reactions involve three steps, including (1) adsorption of 1-butene on the active site, (2) the chemical surface reactions of 1-butene to trans- and cis-2-butene, and (3) the desorption of trans- and cis-2-butene. The surface reactions were assumed to be the rate-determining step. The percentage error of all predicted values was less than 5% under the studied operating conditions.

π-Extended push-pull azo-pyrrole photoswitches: synthesis, solvatochromism and optical band gaps.

A new family of push-pull biphenyl-azopyrrole compounds 3b-g and 4b-d was efficiently obtained via a Suzuki cross-coupling reaction between 2-(4'-iodophenyl-azo)-N-methyl pyrrole (1a) or 3-(4'-iodophenyl-azo)-1,2,5-trimethyl pyrrole (2a) and 4'-substituted phenyl boronic acids in excellent yields. The influence of the π-biphenyl backbone and pyrrole pattern substitution was correlated with their optical properties. Solvatochromic studies via UV-visible spectrophotometry revealed that the inclusion of a 4'-nitro-biphenyl fragment favors a red-shift of the main absorption band in these azo compounds compared with their non-substituted analogues. Likewise, optical band-gaps were estimated by means of electronic absorption spectra and correlated with TD-DFT studies. The pyrrole pattern substitution and the π-conjugated backbone exhibit a clear influence on their thermal isomerization kinetics at room temperature. In all cases, biphenylazo-pyrrole compounds lead to the formation of J-type aggregates in binary MeOH : H2O solvents. Under these conditions, compounds 3b-c undergo a water-assisted cis-to-trans isomerization at room temperature.

A family of azoquinoline derivatives: Effect of the substituent at azo linkage on thermal cis-trans isomerization based on an experimental and computational approach

The subject of the research is a series of azo-dyes based on the quinoline structure, which are practically omitted as materials which use photoinduced optical anisotropy for applications. The work presents the kinetics of thermal cis-trans isomerization reaction of 5-azo-8-hydroxy-2-methylquinoline and 5,5'-((1E,1′E)-[1,1′-biphenyl]-4,4′-diylbis(diazene-2,1-diyl))bis(2-methylquinolin-8-ol) compounds. The impact of the structure of the substituent at the azo linkage on the cis-trans isomerization kinetics was investigated experimentally and theoretically using the UV–Vis spectroscopy and the density-functional theory (DFT). The kinetics of the dark-return of the cis-to trans-isomer of azoquinolines was investigated after UV-irradiation. It was found that attaching the nitro substituent to the azoquinolines structure retards the photoisomerization process in comparison to the other substituents (i.e. F, Cl, Br, N atoms or CH3, OH, COOH, OCH3 groups). The calculated rate constants were in qualitative agreement with the experimental values. To the best of our knowledge, the cis-trans isomerization kinetics for azoquinolines has not been studied yet.

Studying tautomerism in an important pharmaceutical glibenclamide confined in the thin nanometric layers

The uniform thin films with variable thicknesses (d = 49, 120, 220 nm) of active pharmaceutical ingredient (API) glibenclamide (GCM) was spin-coated and investigated using broadband dielectric, grazing incident FTIR spectroscopies, atomic force microscopy, and ellipsometry. Data analysis revealed that nanoconfined systems consist of a mixture of amide and imidic acid forms of this pharmaceutical, wherein the ratios of both tautomeric forms in the thin films were different with respect to the molten supercooled bulk system. Moreover, changes in the populations of glibenclamide tautomers, i.e. higher amide to imides ratio in the spatially restricted API with respect to the bulk sample, had a strong impact on the character of the proton transfer reaction. In this context, the kinetic curves constructed on the base of infrared data for the bulk system follow the sigmoidal shape, characteristic for the autocatalytic reaction, while results obtained for the confined samples provide exponential character and indicate first-order transformation. This allows hypothesizing that the autocatalytic nature of the tautomerism in the bulk sample is most likely related to the formation of the amide tautomers which further catalyze the progress of imide-amide transformation. Our results are the first studies showing that the change in the thickness of the film may affect the properties and isomerization kinetics in a pharmaceutical systems. Finally, our data open a new perspective for developing new drug delivery systems.

Picosecond Switchable Azo Dyes.

Azo dyes that combine electron-withdrawing thiazole/benzothiazole heterocycles and electron-donating amino groups within the very same covalent skeleton exhibit relaxation times for their thermal isomerization kinetics within milli- and microsecond timescales at room temperature. Notably, the thermal back reaction of the corresponding benzothiazolium and thiazolium salts occurred much faster, within the picosecond temporal domain. In fact, these new light-sensitive platforms are the first molecular azo derivatives capable of reversible switching between their trans and cis isomers in a subnanosecond timescale under ambient conditions. In addition, theoretical calculations revealed very low activation energies for the isomerization process, in accordance with the fast subnanosecond kinetics that were observed experimentally.

Long-Distance Rate Acceleration by Bulk Gold.

We report on a very unusual case of surface catalysis involving azobenzenes in contact with a Au(111) surface. A rate acceleration of the cis-trans isomerization on gold up to a factor of 1300 compared to solution is observed. By using carefully designed molecular frameworks, the electronic coupling to the surface can be systematically tuned. The isomerization kinetics of molecules with very weak coupling to the metal is similar to that found in solution. For their counterparts with strong coupling, the relaxation rate is shown to depend on the spin-density distribution in the triplet states of the molecules. This suggests that an intersystem crossing is involved in the relaxation process. Aside from their impact on catalytic processes, these effects could be used to trigger reactions over long distances.

Long‐Distance Rate Acceleration by Bulk Gold

AbstractWe report on a very unusual case of surface catalysis involving azobenzenes in contact with a Au(111) surface. A rate acceleration of the cis–trans isomerization on gold up to a factor of 1300 compared to solution is observed. By using carefully designed molecular frameworks, the electronic coupling to the surface can be systematically tuned. The isomerization kinetics of molecules with very weak coupling to the metal is similar to that found in solution. For their counterparts with strong coupling, the relaxation rate is shown to depend on the spin‐density distribution in the triplet states of the molecules. This suggests that an intersystem crossing is involved in the relaxation process. Aside from their impact on catalytic processes, these effects could be used to trigger reactions over long distances.

Nanodiscs bounded by styrene-maleic acid allow trans-cis isomerization of enclosed photoswitches of azobenzene labeled lipids.

Styrene-and-maleic acid (SMA) copolymers behave as amphipathic belts encircling lipids in the form of nanodiscs. It is unclear to what extent the SMA belt affects the order and dynamics of the enclosed lipids. We aimed to obtain insight into this by making use of synthetic azobenzene-labeled phospholipids incorporated into di-16:0 PC nanodiscs. Azobenzene lipids undergo geometric isomerization upon exposure to light at 365 nm, resulting in the formation of cis-isomers that possess a larger cross-sectional area than the trans-isomers. The influence of the lipid properties on the kinetics and extent of isomerization of the azobenzene groups was first tested in large unilamellar vesicles constituted by lipid mixtures with different packing properties of the acyl chains. Fastest isomerization kinetics were found when azolipids were present in membranes supplemented with lysolipids and slowest in those supplemented with di-unsaturated lipids, suggesting that the isomerization rate is sensitive to the lateral pressure profile in the lipid bilayer and hence may be considered a convenient tool to monitor packing properties of lipids enclosed in nanodiscs. When azolipids were incorporated in SMA-bounded nanodiscs, azolipid isomerization was found to take place readily, indicating that SMA polymers behave as rather flexible belts and allow expansion of the enclosed lipid material.

Preparation, characterization, X-ray crystal structures, and isomerization equilibria and kinetics of cis- and trans-bis(5-imino-2,2-dimethyl-3-hexanonato-N,O)palladium(II)

The palladium(II) β-ketoenaminate complexes cis- and trans-bis(5-imino-2,2-dimethyl-3-hexanonato-N,O)palladium(II), Pd(admh)2, have been prepared by reaction of PdCl2 and 5-amino-2,2-dimethylhex-4-en-3-one, Hadmh, in aqueous NaOH and have been separated by column chromatography on Florisil. The square-planar complexes were characterized in solution by 1H NMR chemical shifts, the nuclear Overhauser effect, and electric dipole moments, which showed that the first-eluted isomer from Florisil is trans and the second-eluted isomer is cis. X-ray crystal structures of the solid complexes confirm the isomer assignments and reveal close NH ⋯ HN contacts between the two ligands in the cis isomer and close NH ⋯ O contacts in the trans isomer. Isomerization equilibria favor the trans isomer in chlorobenzene and favor the cis isomer in pyridine. The kinetics of trans-to-cis isomerization in pyridine was studied in the range 55.8–100.6 °C: ΔH‡ = 100 ± 2 kJ/mol and ΔS‡ = −50 ± 6 J/(K mol). 1H NMR spectra of Ni(admh)2 in CHClF2 down to −170 °C suggests that only one isomer (probably trans) exists in solution.

Photochemistry of Bacteriorhodopsin with Various Oligomeric Statuses in Controlled Membrane Mimicking Environments: A Spectroscopic Study from Femtoseconds to Milliseconds.

Preparing transmembrane protein in controllable lipid bilayers is essential for unravelling the coupling of the environments and its dynamic functions. Monomerized bacteriorhodopsin (mbR) embedded in covalently circularized nanodiscs was prepared with dimyristoylphosphatidylglycerol (DMPG) lipid and circular membrane scaffold proteins of two different sizes, cE3D1 and cΔ H5, respectively. The retinal photoisomerization kinetics and thermodynamic photocycle were examined by femtosecond and nanosecond transient absorption, respectively, covering the time scale from femtoseconds to hundreds of milliseconds. The kinetics of the retinal isomerization and proton migration from the protonated Schiff base to Asp-85 were not significantly different for monomeric bR solubilized in Triton X-100 or embedded in circularized nanodiscs. This can be ascribed to the local tertiary structures at the retinal pocket vicinity being similar among monomeric bR in various membrane mimicking environments. However, the aforementioned processes are intrinsically different for trimeric bR in purple membrane (PM) and delipidated PM. The reprotonation of the deprotonated Schiff base from Asp-96 in association with the decay of intermediate M, which involved wide-ranged structural alteration, manifested a difference in terms of the oligomeric statuses, as well as a slight dependence on the size of the nanodisc. In summary, bR oligomeric statuses, rather than the environmental factors, such as membrane mimicking systems and nanodisc size, play a significant role in bR photocycle associated with short-range processes, such as the retinal isomerization and deprotonation of protonated Schiff base at the retinal pocket. On the other hand, the environmental factors, such as the types of membrane mimicking systems and the size of nanodiscs, affect those dynamic processes involving wider structural alterations during the photocycle.

Raman spectroscopy of glycolic acid complexes with N2

High overtone excitation induced conformational isomerization of glycolic acid – nitrogen complex in an argon matrix was investigated by Raman spectroscopy. The interaction between glycolic acid and nitrogen change the green light (532 nm) induced isomerization processes compared to non-complexed glycolic acid. The 180° rotation around of the CC bond and stabilization of carboxyl OCOH dihedral angle to trans position were the main conformational changes observed in the complex of the lowest energy conformer and nitrogen. Interestingly, only one stable light-induced conformational product was observed, which isomerises back to the lowest energy conformer upon prolonged irradiation at 532 nm. The vibrational Raman spectrum of glycolic acid – nitrogen complex and the analysis of the isomerization kinetics are presented.

Towards high-level theoretical studies of large biodiesel molecules: an ONIOM/RRKM/Master-equation approach to the isomerization and dissociation kinetics of methyl decanoate radicals

The isomerization and dissociation reactions of methyl decanoate (MD) radicals were theoretically investigated by using high-level theoretical calculations based on a two-layer ONIOM method, employing the QCISD(T)/CBS method for the high layer and the M06-2X/6-311++G(d,p) method for the low layer. Temperature- and pressure-dependent rate coefficients for the involved reactions were computed by using the transition state theory and the Rice-Ramsperger-Kassel-Marcus/Master-equation method. The structure-reactivity relationships were explored for the complicated multiple-well interconnected system of ten isomeric MD radicals. Comparative studies of methyl butanoate (MB) and MD were also performed systematically. Results show that the isomerization reactions are appreciably responsible for the population distribution of MD radicals at low and intermediate temperatures, while the β-scission reactions are dominant at higher temperatures. Although the rate constants of MB specific to methyl esters are close to those of MD in certain temperature ranges, MB is unable to simulate most of the dissociation reactions due to its short aliphatic chain. Significant differences of rate constants for isomerization reactions were observed between the calculated results and the literature data, which were estimated by analogy to alkane systems, but the rate constants of β-scissions show generally good agreement between theory and experiment. The current work extends kinetic data for isomerization and dissociation reactions of MD radicals, and it serves as a reference for the studies of detailed combustion chemistry of practical biodiesels.

Understanding the isomerization kinetics in the gas phase of a triazole-3-thione derivative: A theoretical approach

The isomerization reactions of the 4-amino-5-methyl-2,4-dihydro- -3H-1,2,4-triazole-3-thione were studied using the B3LYP and M06-2x, as well as the CBS-QB3 theoretical methods. The measured energy profiles were complemented with kinetic rate constants using the transition state theory (TST). Based on the isomers geometries optimized using the CBS-QB3 method, a natural bond orbital (NBO) analysis shows that the stabilization energies of non-bonding lone-pair orbitals [LP(e)S7] to the ?*N2?C3 antibonding orbital increase from isomers 1 to 2. Moreover, the LP(e)S7 ? ?*N2?C3 delocalizations could fairly explain the increase in the occupancies of LP(e)S7 orbitals for isomers 1 and 2 (2 > 1). The studied stabilization energy increases the stability of the ground state structure, and could fairly explain the kinetics of the isomerization reactions 1 and 2 (k2 > k1). NBO results also suggest that the kinetics of these processes are controlled by the LP ??* resonance energies.

Photo-responsive foam control base on nonionic azobenzene surfactant as stabilizer

A nonionic photo-responsive surfactant (NPS) was synthesized for obtaining photo control of foamability, exposed in ultraviolet (UV) light or visible (Vis) light irradiation. The foamability depended on the surface activity of NPS, which possessed low critical micelle concentration (CMC) and high adsorption capacity compared with the similar type of nonionic surfactants. The azobenzene component in NPS as the light-responsive group, showed high sensitive and reversibility, i.e., the trans → cis isomerization occurred only for 1 s and the time to reach reversible isomerization was 85 s. And the fatigue resistance of isomerization was also outstanding, which remained stable for 14 cycles of alternate irradiation. Due to the lack of free volume and increase of barrier for isomerization, the irradiation time and first-order kinetics of isomerization decreased with the increase of NPS concentration. The foaming ratio and foam stability promoted with the NPS concentration increase, however, the stereospecific blockade amongst cis isomers was stronger than that of trans isomers, resulting in the steep decrease of foam half-life. The sustainable foam stability control of NPS applied into foam dyeing to solve the problem of residual foam.

Catalytic performance of micro-mesoporous materials as the supports for Pt catalysts in n-heptane isomerization

The n-heptane isomerization was comparatively investigated over various catalysts, and significantly higher n-heptane conversion and isomerization selectivity were achieved with Pt/SBA-15 catalyst. The kinetics of n-heptane isomerization has been examined as a function of n-heptane and hydrogen pressures at various temperatures. Two kinetics model based on power law and Langmuir-Hinshelwood mechanism were developed for the reaction and compared with the obtained experimental data. The apparent kinetics parameters were estimated using multiple regression analysis. The reaction order is positive for n-heptane over most catalysts, suggesting strong coverage and negative order with respect to H2, indicating the inhibitory effect of H2.

Theory and Practice of the Preparation of Adamantylarenes

The adamantylation of diphenyl oxide and phenol with 1-chloro(bromo)adamantanes in a liquid phase has been studied. The reaction has been performed under kinetic control and under the conditions of reaching chemical equilibrium using the following catalysts: Amberlyst 36 Dry, in situ HBr, and AlCl3. The kinetics of isomerization of (1-adamantyl)- and (2-adamantyl)diphenyl oxides has been studied at 333 and 417 K in the presence of AlCl3 and Amberlyst 36 Dry, respectively. The equilibrium of the positional and structural “bridge–bridgehead” isomerization has been studied in the range of 333–523 K. It has been found that high selectivity of the adamantylation of diphenyl oxide or phenol can be achieved in the case of the use of sulfonated cation-exchange resins as a catalyst. In the presence of aluminum halides or HBr, the reaction mixture has been represented by all the possible isomers and adamantane.

Fast dark cis-trans isomerization of azopyridine derivatives in comparison to their azobenzene analogues: Experimental and computational study

Thermal cis-trans isomerization of azobenzene and azopyridine derivatives was studied. The impact of the structural features of azo derivatives on its isomerization kinetics being important for molecular switching materials was investigated experimentally and theoretically based on the density-functional theory (DFT). Eleven azo-dyes in which the phenyl rings were substituted in the para-position to NN– linkage were prepared. Additionally, their thermal properties were evaluated by thermogravimetric analysis and differential scanning calorimetry. DFT calculations were also used for optimization ground and transition state geometries, the density of states, electronic structures of studied azo-dyes. The kinetics of the dark-return cis to trans-form of azo-compounds was investigated after UV-irradiation. It was found that attaching hydroxyalkoxy-substituents to the azo-dyes structure retarding the photoisomerization process in comparison to the hydroxyl group. Moreover, dark-return cis to trans-isomer was ca. twice times faster for azopyridine derivatives than for their azobenzene analogues. The calculated rate constants were in qualitative agreement with the experimental values.

Building Strain with Large Macrocycles and Using It To Tune the Thermal Half-Lives of Hydrazone Photochromes.

Strain has been used as a tool to modulate the reactivity (e.g., mechanochemistry) and thermal isomerization kinetics of photochromic compounds. Macrocyclization is used to build-up strain in such systems, and in general the reactivity and rates increase with the decrease in macrocycle size. To ascertain the effect of strain on recently reported bistable hydrazone photoswitches, we incorporated them into macrocycles having varying aliphatic linker lengths (C3-C7), and studied their switching behavior, and effect of macrocycle size on the thermal isomerization rate. Surprisingly, while the systems with C3-C5 linkers behave as expected (i.e., the rate is faster with smaller linkers), the isomerization rate in the systems with larger aliphatic linkers (C6-C8) is enhanced up to 4 orders of magnitude. NMR spectroscopy, X-ray crystallography and DFT calculations were used to elucidate this unexpected behavior, which on the basis of our analyses results from the buildup of Pitzer (torsional), Prelog (transannular) and Baeyer (large angle) strain in the longer linkers.

Isomerization kinetics of flexible molecules in the gas phase: Atomistic versus coarse-grained sampling.

Isomerization kinetics of molecules in the gas phase naturally falls on the microcanonical ensemble of statistical mechanics, which for small systems might significantly differ from the more traditional canonical ensemble. In this work, we explore the examples of cis-trans isomerization in butane and bibenzyl and to what extent the fully atomistic rate constants in isolated molecules can be reproduced by coarse-graining the system into a lower dimensional potential of mean force (PMF) along a reaction coordinate of interest, the orthogonal degrees of freedom acting as a canonical bath in a Langevin description. Time independent microcanonical rate constants can be properly defined from appropriate state residence time correlation functions; however, the resulting rate constants acquire some time dependence upon canonical averaging of initial conditions. Stationary rate constants are recovered once the molecule is placed into a real condensed environment pertaining to the canonical ensemble. The effective one-dimensional kinetics along the PMF, based on appropriately chosen inertia and damping parameters, quantitatively reproduces the atomistic rate constants at short times but deviates systematically over long times owing to the neglect of some couplings between the system and the bath that are all intrinsically present in the atomistic treatment. In bibenzyl, where stronger temperature effects are noted than in butane, the effective Langevin dynamics along the PMF still performs well at short times, indicating the potential interest of this extremely simplified approach for sampling high-dimensional energy surfaces and evaluating reaction rate constants.

OCS isomerization and dissociation kinetics from statistical models

In this work computational modeling of the isomerization reactions of the linear OCS, CSO, COS, and the cyclic Δ-OCS structures, on the ground state Potential Energy Surface (PES), is performed using Rice–Ramsperger–Kassel–Marcus (RRKM), Semiclassical Transition State Theory (SCTST), and the Master Equation (ME). Adiabatic dissociations of the sulfur atom from the OCS, $$\Delta $$ -OCS and COS structures are also studied with the Variational Transition State Theory (VTST). Intramolecular Vibrational Energy Redistributions (IVRs), calculated within the adiabatic approximation framework, appear to be faster than the corresponding reactions, justifying the use of statistical methods to study the kinetics of the reactions. From the microcanonical rate coefficients, we establish that the lifetimes of isomers $$\Delta $$ -OCS and COS are insufficient to complete one vibrational period of the corresponding molecules. The energy-specific branching ratios lead us to conclude that only 0.08 and 0.07% of molecules reacting from the most stable isomer OCS produce CSO and COS, respectively, at 180 kcal/mol (when all the reaction channels are open). This can be attributed to the dominance of the dissociation over the isomerizations. From the SCTST treatment, we find that coupling and tunneling are very important in this system, as deviations from the Arrhenius-type behavior in the reaction kinetics are considerable, particularly at low temperatures.