Hydrogen Transfer Mechanism Research Articles

Examination of the Antioxidant Activity of Psoralidin: Computational Mechanistic Study and Impact on the ROS Level in Human Keratinocytes.

Psoralidin (Pso) is a coumestan-type compound found in Psoralea corylifolia L. that exhibits a broad spectrum of pharmacological properties. The current work aimed to study, for the first time, the antioxidant capacities of Pso under physiological circumstances. Tandem experimental and computational approaches were used to fully understand the interaction of Pso with ROS (reactive oxygen species) at the molecular level as well as its impact on the basal ROS level in cells. Pso has been found to be a potent radical scavenger in physiological polar media, acting via the single electron-transfer mechanism rather than the hydrogen-transfer mechanism. In contrast, Pso is a moderate radical scavenger in lipid media, and its reaction is determined by hydrogen transfer from the 7OH group. The in vitro assays revealed that Pso moderately reduces the basal ROS level in human keratinocytes at non-toxic concentrations, which is in agreement with the computational study. These findings indicate that Pso is a promising antioxidant, but in its natural form it has no significant effects on basal cell conditions.

A novel insight into the bimolecular concerted interaction in lignin pyrolysis: Tautomer-assisted hydrogen transfer mechanism

Bimolecular concerted interaction mechanism plays a vital role in lignin pyrolysis. According to the recently proposed hydroxyl-assisted hydrogen transfer (AHT) mechanism, the primary pyrolytic products can assist the hydrogen transfer process of linkage decomposition with their hydroxy groups acting as the mediator. Herein, the concerted interaction between two hydrogen transfer processes was further studied for the β-O-4 linked lignin. A novel concerted interaction mechanism named tautomer-AHT was found and carefully confirmed by combining theoretical calculations, electronic analyses, pyrolysis and isotope experiments, with 2-phenoxy-1-phenylethanol (α-OH-PPE) as the β-O-4 lignin model compound. During this interaction, the tautomer can make the best of its saturated and unsaturated groups to carry out hydrogen-donating and hydrogen-receiving processes of the β-O-4 breakage, respectively, accompanied by the hydrogen transfer of the mediator itself. As a result, the β-O-4 breakage can be promoted and the unstable tautomer turns into its corresponding stable isomer, leading to the formation of stable phenolics and ketones. In the early pyrolysis stage, the tautomer-AHT reaction triggered by unstable tautomers is basically superior to the hydroxyl-AHT reaction due to the lower energy barrier. With the consumption of unstable tautomers and the accumulation of stable hydroxy products, hydroxyl-AHT interaction becomes predominant in the latter pyrolysis stage. The bimolecular tautomer-AHT mechanism builds a special relationship between hydrogen transfer reactions, and lays a theoretical foundation for exploring complex interactions in lignin pyrolysis.

Theoretical study on hydrogen transfer in the dissociation of dimethyl disulfide radical cations.

Hydrogen transfer (HT) is of crucial importance in biochemistry and atmospheric chemistry. Here, HT processes involved in the dissociation reaction of dimethyl disulfide radical cations (DMDS˙+, CH3SSCH3˙+) are investigated using quantum chemical calculations. Four HTs from the C to S atom and one HT from the S to S atom are observed and the most probable paths are proposed in the dissociation channel from DMDS˙+ to CHnS+ (n = 2-4). The mechanisms of all these five HTs are described as hydrogen atom transfer (HAT) and four of them are accompanied by electron transfer (ET). Considering the catalytic effect of water molecules existing in organisms and the atmosphere, five HT processes in the dissociation of the [DMDS + H2O]˙+ complex are further explored, which show lower free energy barriers. With the participation of water molecules acting as a base, two HTs from the C to the S atom, which have the largest decrease in energy barriers, are characterized as concerted proton-coupled electron transfer (cPCET). These results can be extended to understand the mechanism of the HT process during the dissociation of disulfide and help provide a strategy to design a rare cPCET mechanism for the activation of the C-H bond.

Hydroxyl radical-initiated decomposition of metazachlor herbicide in the gaseous and aqueous phases: Mechanism, kinetics, and toxicity evaluation

The oxidation of widely-used herbicide metazachlor (MTZ) by hydroxyl radical (HO•) in the gas and the aqueous phases was investigated in terms of mechanistic and kinetic behaviors using the M06–2X/6–311++G (3df, 3pd)//M06–2X/6-31 + G (d,p) level of theory over the temperature range 250–400 K. The formal hydrogen transfer, HO•-addition, and single electron transfer mechanisms were considered. The overall rate constants in the gas phase range from 8.40 × 1010 to 8.31 × 109 M−1 s−1 at the temperature from 250 to 400 K, respectively, while the ones in the aqueous phase are close to diffusion-controlled rates, with diffusion-corrected rate constants being 1.31 × 109 to 1.27 × 109 M−1 s−1. The formal hydrogen transfer mechanism is the most dominant in the gas phase, whereas the HO•-addition is the most favorable in the aqueous phase. The H-abstraction at two methyl groups and the HO•-addition to C11 and C12 atoms (pyrazole ring), C16 and C18 atoms (benzyl ring) are significant. The short lifetime in the environment, equal to only 4.16 h, requires more attention to this herbicide compound, whereas its lifetime in the aqueous condition varies sharply from half second to several thousand days depending on the HO• concentration. The ecotoxicity estimation of MTZ and its principal transformation products to aquatic organisms suggests that they are harmful or toxic substances. Moreover, the MTZ is a developmental toxicant and mutagenicity-positive, while its decomposed products are developmental toxicants with no mutagenic toxicity. Their bioaccumulation in aquatic organisms is negligible.

Computational analysis of peroxyl radical scavenging capacity of coumestrol: insights into kinetics and reaction mechanisms

AbstractCoumestrol (COM) is a phytoestrogen from the coumestan family that occurs naturally in various foods such as clover, soybeans, alfalfa sprouts, and legumes. Besides its antiestrogenic activity, COM exerts various biological activities, including anticancer, antioxidant, and α‐glucosidase inhibitory. In terms of antioxidant activity, COM has shown an excellent profile according to experimental studies; however, its mechanism of action has not been successfully determined thus far. In this contribution, we evaluated the kinetics and mechanism of the peroxyl radical (ROO•) scavenging activity of COM using quantum chemistry methods. It has been shown that the environment and the nature of the ROO• radical play a determining role in the antioxidant activity of COM. In lipid media, COM exhibits moderate ROO• scavenging activity via the formal hydrogen transfer (FHT) mechanism regardless of the nature of the ROO• radical. However, in polar environments, its activity becomes more important as the reactivity of the ROO• radical increases. Under these conditions, the single electron transfer (SET) mechanism becomes dominant or even exclusive.

The mechanism and rate constant of hydrogen transfer from solvent radicals to coal-based model compounds in direct coal liquefaction

In direct coal liquefaction, in addition to stabilizing coal radicals, the ability of solvent-derived cyclohexadienyl radicals to hydrolyze bridge bonds in coal is a part of their hydrogen-donating activity. In this work, the density functional theory calculations and transition state theory were employed quantitatively to analyze the rate constants of hydrogen transfer from solvent-derived cyclohexadienyl radicals to coal model compounds. Results indicated that polycyclic 9,10-dihydrophenanthrene, 4,5-dihydropyrene and 4,5,9,10-tetrahydropyrene radicals have higher hydrogenolysis ability, therefore they can promote the breakage of the Caryl–Calkyl bonds more effectively. There is no obvious correlation between the rate constants of hydrogen transfer from H-donors radicals to ipso-C of different coal model compounds and the dissociation enthalpies of Caryl–Calkyl bond. However, it can be found that model compounds with higher bond dissociation energies are more susceptible to hydrogenolysis by solvent-derived cyclohexadienyl radicals. Among the coal model compounds containing the same functional group, the coal model compounds with substituent groups directly attached to aromatic rings are more easily hydrolyzed by solvent-derived cyclohexadienyl radicals.

7-O-Galloyltricetifavan: a promising natural radical scavenger.

7-O-Galloyltricetifavan (7OGT), a natural flavonoid, is isolated from the leaves of Pithecellobium clypearia. The compound exhibits a variety of biological activities. This study details the evaluation of the HOO• antiradical activity of 7OGT by quantum chemistry calculations. The HOO• trapping activity of 7OGT in the gas phase (reference state) was discovered to follow the formal hydrogen transfer mechanism with a rate constant of k = 4.58 × 108 M−1 s−1. In physiological environments, 7OGT is predicted to be an excellent HOO• radical scavenger with koverall = 2.65 × 108 and 1.40 × 104 M−1 s−1 in water and pentyl ethanoate solvents, respectively. The HOO• antiradical activity of 7OGT in water at physiological pH is approximately 2000 times that of Trolox and substantially higher than that of other well-known natural antioxidants such as trans-resveratrol or ascorbic acid. Thus, 7OGT is an excellent natural antioxidant in polar environments.

The radical scavenger capacity and mechanism of prenylated coumestan-type compounds: a DFT analysis

The antiradical capacity and mechanisms of two representative coumestan-type compounds, namely isosojagol (Iso) and phaseoul (Pha), were examined using quantum chemistry calculations and computational kinetics methods. From a thermodynamic point of view, the 18CH groups of the prenyl substituent have been found to be the most suitable sites for radical attacks via the formal hydrogen transfer (FHT) mechanism. However, the kinetic study revealed that the reaction at these CH groups is slow and does not contribute to the overall reactivity of these compounds, which the phenolic groups mainly define. The kinetic study also revealed that the studied compounds are good free radical scavengers with overall rate coefficients as high as recognized antioxidants such as carnosic acid, artepillin C, thymol, and rosefuran.

Sulfonic groups functionalized Zr-metal organic framework for highly catalytic transfer hydrogenation of furfural to furfuryl alcohol

The highly selective catalytic transfer hydrogenation (CTH) of furfural (FF) to furfuryl alcohol (FOL) is a significant route of biomass valorization. Herein, a series microporous Zr-metal organic framework (Zr-MOF) functionalized by sulfonic groups are prepared. Based on the comprehensive structural characterizations by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2 physisorption, Thermogravimetric (TG) and Fourier transformed infrared spectroscopy (FTIR), we find that sulfonic acid (–SO3H) functional groups are tethered on the UIO-66 without affecting the structure of the framework. Systematic characterizations (NH3-TPD, CO2-TPD, and in-situ FTIR) demonstrate that modifying of sulfonic groups on UIO-66 results in the formation of stronger Lewis acidic-basic and Brønsted acidis sites. The cooperative role of the versatile Lewis acidic-basic and Brønsted acidic sites in 60% mol fraction of sulfonic acid-containing UIO-66 (UIO-S0.6) retain high surface area and exhibit excellent catalytic performance of 94.7% FOL yield and 16.9 h−1 turnover number (TOF) under mild conditions. Kinetic experiments reveal that the activation energy of the CTH of furfural (FF) over UIO-S0.6 catalyst is as low as 50.8 kJ mol−1. Besides, the hydrogen transfer mechanism is investigated through isotope labeling experiments, exhibiting that the β-H in isopropanol is transferred to the α-C of FF by forming six-membered intermediates on the Lewis acidic-basic and Brønsted acidic sites of the UIO-S0.6, which is the rate-determining step in the formation of FOL.

Mechanistic and Kinetic Studies of the Radical Scavenging Activity of 5-O-Methylnorbergenin: Theoretical and Experimental Insights.

5-O-Methylnorbergenin (5-OMB), a natural compound isolated from Rourea harmandiana, is a compound with potential antioxidant activity based on its chemical structure; however, this activity has not been investigated thus far. In this study, the antioxidant activity of 5-OMB was evaluated by experimental and computational methods. 5-OMB exhibited high activity in DPPH (IC50 = 7.25 ± 0.94 μM) and ABTS•+ (IC50 = 4.23 ± 0.12 μM) assays, higher than the reference compound Trolox. The computational results consistently show that 5-OMB is an excellent HOO• radical scavenger (koverall = 8.14 × 108 M-1 s-1) in water at physiological pH, however it only exerts weak activity in lipid medium (koverall = 3.02 × 102 M-1 s-1). The reaction follows the formal hydrogen transfer mechanism in nonpolar solvents, whereas both the sequential proton loss electron transfer and the formal hydrogen transfer pathways contribute to the activity in aqueous solution. There is a good agreement between experimental and computational data, suggesting that 5-OMB is a promising natural radical scavenger in aqueous physiological environment.

Oxoberberine: a promising natural antioxidant in physiological environments.

Oxoberberine (OB, 2,10-dihydroxy-3,9-dimethoxy-8-oxo-protoberberine, artathomsonine), which was isolated from Artabotrys thomsonii, was shown to exhibit potent antioxidant activity in vitro, however that is the only reported evidence of the radical scavenging activity of this compound thus far. In the present study, thermodynamic and kinetic calculations were used to determine the free radical scavenging activity of OB against a range of biologically important species, under physiological conditions. In the first part the activity is calculated against the HOO˙ radical that is both biologically important and a reference radical for comparison. It was found that OB has high antiradical capacity against HOO˙ in both lipid medium and water at physiological pH with koverall = 1.33 × 105 and 1.73 × 106 M−1 s−1, respectively. The formal hydrogen transfer mechanism defined the activity in nonpolar environments, whereas in the aqueous solution the single electron transfer competes with the hydrogen transfer pathway. The results showed that, in lipid medium, the HOO˙ trapping capability of OB is better than typical antioxidants such as Trolox, BHT, resveratrol and ascorbic acid. Similarly, the activity of OB in water at pH 7.4 is roughly 19 and 7 times faster than those of Trolox and BHT, respectively, but slightly lower than the activities of resveratrol or ascorbic acid. In the second part, it was found that OB also exhibits high activity against other typical free radicals such as CH3O˙, CH3OO˙, CCl3OO˙, NO2, SO4˙−, DPPH and ABTS˙+ with kf ranging from 2.03 × 105 to 5.74 × 107 M−1 s−1. Hence, it is concluded that OB is a promising radical scavenger in the physiological environment.

The hydroperoxyl antiradical activity of natural hydroxycinnamic acid derivatives in physiological environments: the effects of pH values on rate constants.

Hydroxycinnamic acid derivatives (HCA) are a type of phenolic acid that occurs naturally. HCA are widely known for their anti-inflammatory, anti-cancer, and especially antioxidant capabilities; however, a comprehensive study of the mechanism and kinetics of the antiradical activity of these compounds has not been performed. Here, we report a study on the mechanisms and kinetics of hydroperoxyl radical scavenging activity of HCA by density functional theory (DFT) calculations. The ability of HCA to scavenge hydroperoxyl radicals in physiological environments was studied. The results showed that HCA had moderate and weak HOO˙ antiradical activity in pentyl ethanoate solvent, with the overall rate constant koverall = 8.60 × 101 − 3.40 × 104 M−1 s−1. The formal hydrogen transfer mechanism of phenyl hydroxyl groups defined this action. However, in water at physiological pH, 2-coumaric acid (1), 4-coumaric acid (2), caffeic acid (3), ferulic acid (4), sinapic acid (5) and 4-hydroxyphenylpyruvic acid (7) exhibit a significant HOO˙ antiradical activity with ktotal = 105 − 108 M−1 s−1 by the electron transfer reaction of the phenolate anions. Following a rise in pH levels in most of the studied substances, the overall rate constant varied. The acid 5 exhibited the highest HOO˙ radical scavenging activity (log(koverall) = 4.6–5.1) at pH < 5; however, at pH = 5.4–8.8, the highest HOO˙ radical scavenging activity were observed for 3 with log(koverall) = 5.2–5.7. At pH > 6.2, acids 2, 3, 4, and 5 presented the largest radical scavenging activity. By contrast, acid 3-coumaric acid (8) had the lowest antiradical activity at most pH values. Thus, the hydroperoxyl radical scavenging activity in pentyl ethanoate follows the order 3 > 5 > 1 ∼ 2 ∼ 4 ∼ 6 (homovanillic acid) ∼ 7 > 8, whereas it follows the order 3 > 2 ∼ 4 ∼ 5 > 6 ∼ 7 > 1 > 8 in water at pH = 7.40. The activity of 1, 2, 3, 4, 5, 6, and 7 are faster than those of the reference Trolox, suggesting that these HCA could be useful natural antioxidants in the aqueous physiological environment.

Theoretical insights into the antiradical activity and copper-catalysed oxidative damage of mexidol in the physiological environment.

Mexidol (MD, 2-ethyl-6-methyl-3-hydroxypyridine) is a registered therapeutic agent for the treatment of anxiety disorders. The chemical structure suggests that MD may also act as an antioxidant. In this study, the hydroperoxyl radical scavenging activity of MD was studied to establish baseline antioxidant activity, followed by an investigation of the effect of MD on the copper-catalysed oxidative damage in biological systems, using computational methods. It was found that MD exhibits moderate radical scavenging activity against HOO• in water and pentyl ethanoate solvents following the single electron transfer and formal hydrogen transfer mechanisms, respectively. MD can chelate Cu(II), forming complexes that are much harder to reduce than free Cu(II): MD chelation completely quenches the Cu(II) reduction by ascorbic acid and suppresses the rate of reduction reaction by that are the main reductants of Cu(II) in biological environments. Therefore, MD exerts its anti-HO• activity primarily as an OIL-1 inhibitor.

Design and Antioxidant Properties of Bifunctional 2H-Imidazole-Derived Phenolic Compounds-A New Family of Effective Inhibitors for Oxidative Stress-Associated Destructive Processes.

The synthesis of inhibitors for oxidative stress-associated destructive processes based on 2H-imidazole-derived phenolic compounds affording the bifunctional 2H-imidazole-derived phenolic compounds in good-to-excellent yields was reported. In particular, a series of bifunctional organic molecules of the 5-aryl-2H-imidazole family of various architectures bearing both electron-donating and electron-withdrawing substituents in the aryl fragment along with the different arrangements of the hydroxy groups in the polyphenol moiety, namely derivatives of phloroglucinol, pyrogallol, hydroxyquinol, including previously unknown water-soluble molecules, were studied. The structural and antioxidant properties of these bifunctional 5-aryl-2H-imidazoles were comprehensively studied. The redox transformations of the synthesized compounds were carried out. The integrated approach based on single and mixed mechanisms of antioxidant action, namely the AOC, ARC, Folin, and DPPH assays, were applied to estimate antioxidant activities. The relationship “structure-antioxidant properties” was established for each of the antioxidant action mechanisms. The conjugation effect was shown to result in a decrease in the mobility of the hydrogen atom, thus complicating the process of electron transfer in nearly all cases. On the contrary, the conjugation in imidazolyl substituted phloroglucinols was found to enhance their activity through the hydrogen transfer mechanism. Imidazole-derived polyphenolic compounds bearing the most electron-withdrawing functionality, namely the nitro group, were established to possess the higher values for both antioxidant and antiradical capacities. It was demonstrated that in the case of phloroglucinol derivatives, the conjugation effect resulted in a significant increase in the antiradical capacity (ARC) for a whole family of the considered 2H-imidazole-derived phenolic compounds in comparison with the corresponding unsubstituted phenols. Particularly, conjugation of the polyphenolic subunit with 2,2-dimethyl-5-(4-nitrophenyl)-2H-imidazol-4-yl fragment was shown to increase ARC from 2.26 to 5.16 (104 mol-eq/L). This means that the considered family of compounds is capable of exhibiting an antioxidant activity via transferring a hydrogen atom, exceeding the activity of known natural polyphenolic compounds.

Antioxidant Activity of Natural Samwirin A: Theoretical and Experimental Insights.

Samwirin A (SW), a natural compound isolated from Sambucus williamsii or Rourea harmandiana, is known to exhibit potent antiosteoporosis activity and promote cell proliferation in rat osteoblast-like UMR 106 cells. Antiosteoporosis activity suggests that the compound must also exhibit antioxidant activity but this has not been studied thus far. In the present study, the antioxidant activity of SW was examined by experimental and computational studies. It was found that SW exhibits good hydroperoxyl scavenging activity, particularly in water at physiological pH (koverall = 1.01 × 107 M–1 s–1). The single-electron transfer mechanism defines the HOO• + SW reaction in water, while the activity in the lipid medium is moderate and it follows the formal hydrogen transfer mechanism. The rate constant of the HOO• scavenging reaction in the aqueous solution is about 78 times higher than the reference compound Trolox. The computational results are in line with experimental data underscoring that SW is a promising radical scavenger in aqueous media at physiological pH.

Synthesis, antioxidant and cytoprotective activity evaluation of C-3 substituted indole derivatives

A series of fifteen indole derivatives substituted at the C-3 position were synthesized and characterized. The antioxidant activity of all derivatives was investigated by three in vitro antioxidant assays, and the derivative with pyrrolidinedithiocarbamate moiety was the most active as a radical scavenger and Fe3+-Fe2+ reducer. It can be stated that possible hydrogen and electron transfer mechanism is suggested for the quenching of the free radical. Moreover, the indolyl radical stabilization and the presence of unsubstituted indole nitrogen atom are mandatory for the observed antioxidant activity, which strongly depends on the type of the substituent directly connected to the methylene group at the C-3 position. Human red blood cells (RBC) have been used as a cell model to study derivatives interaction with the cell membrane. Haemolytic activity and RBC shape transformation were observed for certain derivatives in a concentration-dependent manner. However, most of the derivatives at sublytic concentration showed high cytoprotective activity against oxidative haemolysis induced by 2,2′-azobis(2-methylpropionamidine) dihydrochloride (AAPH). The cytoprotective properties of derivatives can be explained mostly due to their interactions with the RBC membrane components. Taking together, theoretical estimations and experimental data confirm the beneficial interactions between the selected C-3 substituted indole derivatives and the RBC membrane under oxidative stress conditions. These results encourage us to further structural optimization of C-3 substituted indole derivatives as potent antioxidant compounds.

Exploring the nature of hydrogen of Rutile TiO2(110) at 78 K

The hydrogen on the surface or in the subsurface of rutile TiO2 play an important role in the surface chemical reactions and understanding the catalytic reactions due to its excess electrons. In this work, we have systematically explored the nature of hydrogen atom by atomic force microscopy (AFM)/scanning tunneling microscopy (STM) and investigated the behaviors and mechanism of hydrogen atom desorption and transfer between the surface and subsurface by Kelvin probe force spectroscopy (KPFS). The atomic contrast of surface hydrogen (Hsurf) and subsurface hydrogen (Hsub) are clearly identified by dynamic and static STM. The Hsub-Hsub interactions are systematically investigated, demonstrating the unusual no-repulsive effect of species with the same polarity. Importantly, the reversible hydrogen atom transfer between the sample surface and subsurface was successfully manipulated by KPFS performance, and mechanism of hydrogen transfer proceeds was proposed that force field together with electric field between the tip-sample space reduce the height and width of barrier resulting in the hydrogen atom transfer. The present study is expected to provide some new insights into the nature and manipulation of hydrogen atom and is useful to design the new catalyst of metal oxide.

The hydroperoxyl radical scavenging activity of sulfuretin: insights from theory.

Sulfuretin (SFR), which is isolated from Rhus verniciflua, Toxicodendron vernicifluum, Dahlia, Bidens tripartite and Dipterx lacunifera, is one of the most important natural flavonoids. This compound is known to have numerous biological activities; among these, the antioxidant activity has not been thoroughly studied yet. In this study, the hydroperoxyl scavenging activity of SFR was examined by using density functional theory calculations. SFR is predicted to be an excellent HOO• scavenger in water at pH = 7.40 with koverall = 4.75 × 107 M−1 s−1, principally due to an increase in the activity of the anionic form following the single-electron transfer mechanism. Consistently, the activity of the neutral form is more prominent in the non-polar environment with koverall = 1.79 × 104 M−1 s−1 following the formal hydrogen transfer mechanism. Thus, it is predicted that SFR exhibits better HOO• antiradical activity than typical antioxidants such as resveratrol, ascorbic acid or Trolox in the lipid medium. The hydroperoxyl radical scavenging of SFR in the aqueous solution is approximately 530 times faster than that of Trolox and similar to ascorbic acid or resveratrol. This suggests that SFR is a promising radical scavenger in physiological environments.

Separation of salicylic acid and two phenols by polyvinyl chloride/N,N-bis (1-methylheptyl) acetamide polymer inclusion membrane

The separation behaviour and mass transfer mechanism of salicylic acid (SA) and two phenols by a polymer inclusion membrane (PN-PIM) based on polyvinyl chloride (PVC) as a base polymer and N,N-bis (1-methylheptanyl) acetamide (N503) as a carrier were studied. The correlation between the effects of carrier content and membrane thickness on the morphology, structure and mass transfer stability of PN-PIM was discussed. PN-PIM presented a good mass transfer capability for SA and p-nitrophenol (PNP) through a hydrogen transfer mechanism in weakly acidic environments and with low-concentration NaOH as a stripping phase. The mass transfer rate was in accordance with the first-order kinetic equation. At pH 6, the phenol and PNP coexisting with SA can be selectively removed. The mass transfer of PNP in PN-PIM was consistent with the diffusion mechanism, whereas that of SA exhibited the characteristics of a fixed-site hopping mechanism. When the carrier content was 68.8 %, the permeability coefficient (P) of PNP and SA reached at 9.02 and 8.74 μm/s, respectively. The mass transfer activation energies of PNP and SA totalled 13.23 and 19.08 kJ/mol, respectively. Moreover, intramembrane diffusion was the main controlling step of the mass transfer of PNP and SA in PN-PIM. Given the dual effects of PN-PIM membrane thickness on mass transfer stability and mass transfer rate, the suitable thickness range of PN-PIM was 160–180 μm.

Enantioselective reduction of prochiral ketones promoted by amino amide ruthenium complexes: A DFT study

The origin of enantioselectivity in the reaction of chiral Ru amino amide complexes in the asymmetric transfer hydrogenation of acetophenone was investigated using DFT calculations. For the most stable active catalysts, the full free energy profiles for the reaction were calculated according to the concerted hydrogen transfer mechanism. We succeeded in reproducing the experimentally observed enantioselectivity for the studied Ru amino amide complexes. Our results indicate that the high enantioselectivity can be explained by a stabilizing CH-π interaction existing between the phenyl group of acetophenone and the aromatic substituent of the catalyst, which plays a significant role in selectivity. Finally, our results show that important insights can be obtained with such a theoretical approach not simply to explain the origin of the reaction asymmetry but also to predict the enantiomeric excess. This can help experimentalists to design new enantioselective catalysts.