Ultraviolet irradiation can induce DNA damages. Plants suffer much more sunlight than animals. As a result, nature has selected efficient screening agents to protect plants. In adult Arabidopsis plants, sinapoyl malate (SM) is deposited in the leaf epidermis to prevent penetration of UV radiation, which is an ester derivative of sinapic acid (SA). However, the photoprotection mechanism of sinpoyl malate in aqueous solution is still unknown.
　　To understand the photoprotection mechanism, Prof. HAN Keli and his coworkers at Dalian Institute of Chemical Physics, CAS, use the femtosecond transient absorption spectroscopy and time-dependent density functional theory and find the ultrafast photoisomerization mechanism of SA and SM in neutral aqueous solution.
　　The Photoprotection Mechanism of Plant Sunscreen (Image by LUO Jian)
　　In the neutral aqueous solution, both SM and SA are deprotonated. Their excited states can relax to the ground states by ultrafast barrierless trans-cis photoisomerization, efficiently dissipating the ultraviolet energy into surroundings. However, the cis deprotonated SA has much lower ultraviolet absorption ability compared to its trans conformer. In contrast, the cis deprotonated SM still absorbs ultraviolet light strongly. Furthermore, the excited-state protonated SA can also decay to the ground state by ultrafast photoisomerization and its cis product still has strong ultraviolet absorption ability. Therefore, the protonated SA can be used as a new sunscreen molecule after proper chemical modification.
　　The related work was published on the Journal of Physical Chemistry Letters. This study was financially supported by National Basic Research Program of China and the National Natural Science Foundation of China. (Text and Image by LUO Jian)
　　Contact:
　　Dr. LU Xinyi
　　Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
　　457 Zhongshan Road, Dalian, 116023, China,
　　Tel: 86-411-84379201
　　E-mail: luxinyi@dicp.ac.cn 

　　Ultraviolet irradiation can induce DNA damages. Plants suffer much more sunlight than animals. As a result, nature has selected efficient screening agents to protect plants. In adult Arabidopsis plants, sinapoyl malate (SM) is deposited in the leaf epidermis to prevent penetration of UV radiation, which is an ester derivative of sinapic acid (SA). However, the photoprotection mechanism of sinpoyl malate in aqueous solution is still unknown.
　　To understand the photoprotection mechanism, Prof. HAN Keli and his coworkers at Dalian Institute of Chemical Physics, CAS, use the femtosecond transient absorption spectroscopy and time-dependent density functional theory and find the ultrafast photoisomerization mechanism of SA and SM in neutral aqueous solution.
　　The Photoprotection Mechanism of Plant Sunscreen (Image by LUO Jian)
　　In the neutral aqueous solution, both SM and SA are deprotonated. Their excited states can relax to the ground states by ultrafast barrierless trans-cis photoisomerization, efficiently dissipating the ultraviolet energy into surroundings. However, the cis deprotonated SA has much lower ultraviolet absorption ability compared to its trans conformer. In contrast, the cis deprotonated SM still absorbs ultraviolet light strongly. Furthermore, the excited-state protonated SA can also decay to the ground state by ultrafast photoisomerization and its cis product still has strong ultraviolet absorption ability. Therefore, the protonated SA can be used as a new sunscreen molecule after proper chemical modification.
　　The related work was published on the Journal of Physical Chemistry Letters. This study was financially supported by National Basic Research Program of China and the National Natural Science Foundation of China. (Text and Image by LUO Jian)
　　Contact:
　　Dr. LU Xinyi
　　Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
　　457 Zhongshan Road, Dalian, 116023, China,
　　Tel: 86-411-84379201
　　E-mail: luxinyi@dicp.ac.cn 

　　Reactions occurring at a carbon atom with a tetrahedral environment proceeding through the back-side attack Walden inversion mechanism are one of the most important and useful classes of reactions in chemistry. The bimolecular nucleophilic substitution (SN2) reaction is the most common reaction of that type. In a SN2 reaction, a nucleophile approaches a saturated carbon from one side, displaces a leaving group on the opposite side of the carbon atom, resulting in inversion of the carbon center. Decades of extensive studies revealed that a gas-phase SN2 reaction with a center barrier exhibits an inverse secondary kinetic isotope effect (KIE) on thermal rate constant at room temperature, i.e. kH/kD < 1 when isotopically substituted atom not directly involved in the reaction. In contrast, on the cross sections it shows a large normal secondary isotope effect with reaction threshold energy substantially higher than calculated barrier height. However, the origin of this intriguing difference on the isotope effects on the kinetic and cross sections as well as the high energy threshold remain unclear.
　　A research term led by prof. ZHANG Donghui from State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences has carried out an accurate quantum dynamics study of the H' + CH4 → CH3H' + H substitution reaction and its isotope analogies. The reaction is the simplest reaction proceeding through the back-side attack Walden inversion mechanism, with a D3h transition state and a static barrier height of 1.6 eV. It is very similar to the gas-phase SN2 reactions with central barriers, except that there exist pre- and post-reaction wells in SN2 reactions stemming from strong ion-dipole interaction between reagents/products. They found that the reaction threshold energy is considerably higher than the barrier height, and the reaction manifests different isotope effect. In term of cross section beyond reaction threshold energy, it has a large normal secondary isotope effect with the reactivity for the H' + CH4 → CH3H' + H reaction substantially larger than that for H' + CHD3 → CD3H' + H reaction. However, in term of thermal rate constant it has a reverse secondary KIE with the rate for H + CH4 slightly smaller than that for H + CHD3.
　　In order to explore the dynamics origin of these different isotope effects, they analyzed the change of the umbrella angle of the non-reacting methyl group as the reaction proceeds. According to the minimum energy path (MEP), the umbrella angle should change synchronously during the reaction with the incoming of H atom and elongation of the breaking CH bond to reach 90° at the static saddle point. However, it turned out that for the H' + CHD3 substitution reaction the umbrella motion of the non-reacting CD3 group is slow in responding to the attack of the incoming H atom during the reaction. The reaction does not proceed along MEP as shown by the black reaction path in the figure. Instead, it proceeds on the red path with the umbrella angle larger than 90° at the dynamical saddle point and the corresponding barrier height higher than the static barrier height. Because the umbrella motion depends on the masses of the methyl group, isotope replacement of D atom to H atom can enhance the reactivity substantially, resulting in a large normal isotope effect on cross sections. Furthermore, because the initial umbrella excitation of methyl group can accelerate the umbrella motion during reaction substantially, it can dramatically enhance the reactivity. As a result, the thermal rate constant for the reaction is exclusively determined by the contributions from the umbrella excited states. As the contributions from the umbrella excited states for the H + CD3H reaction beat those for the H + CH4 reaction, the reaction exhibits an inverse secondary KIE.
　　All the phenomena found from this study for the H+CH4 substitution reactions, including higher threshold energy, a reverse secondary KIE at room temperature, and a large normal isotope effect for reaction cross section, have been observed for gas-phase SN2 reactions with barriers. We anticipate that the mechanisms uncovered from this study play important roles in these gas-phase SN2 reactions. Therefore, this study only not represents the first accurate theoretical study of a Walden inversion reaction which provided unprecedented dynamical details and a clear physical picture of the dynamics, but also sheds light on the dynamics of gas-phase SN2 reactions.
　　H' + CH4 → CH3H' + H substitution reaction (Image by ZHANG Donghui)
　　The related work was published in Nature Communications entitled as “Dynamical barrier and isotope effects in the simplest substitution reaction via Walden inversion mechanism”. This study was financially supported by the Natural Science Foundation of China, Ministry of Science and Technology of China, and the Chinese Academy of Sciences. (Text and Image by ZHANG Donghui) (CN 约稿)
　　Contact:
　　Dr. LU Xinyi
　　Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
　　457 Zhongshan Road, Dalian, 116023, China,
　　Tel: 86-411-84379201
　　E-mail: luxinyi@dicp.ac.cn 
　　

　　Reactions occurring at a carbon atom with a tetrahedral environment proceeding through the back-side attack Walden inversion mechanism are one of the most important and useful classes of reactions in chemistry. The bimolecular nucleophilic substitution (SN2) reaction is the most common reaction of that type. In a SN2 reaction, a nucleophile approaches a saturated carbon from one side, displaces a leaving group on the opposite side of the carbon atom, resulting in inversion of the carbon center. Decades of extensive studies revealed that a gas-phase SN2 reaction with a center barrier exhibits an inverse secondary kinetic isotope effect (KIE) on thermal rate constant at room temperature, i.e. kH/kD < 1 when isotopically substituted atom not directly involved in the reaction. In contrast, on the cross sections it shows a large normal secondary isotope effect with reaction threshold energy substantially higher than calculated barrier height. However, the origin of this intriguing difference on the isotope effects on the kinetic and cross sections as well as the high energy threshold remain unclear.
　　A research term led by prof. ZHANG Donghui from State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences has carried out an accurate quantum dynamics study of the H' + CH4 → CH3H' + H substitution reaction and its isotope analogies. The reaction is the simplest reaction proceeding through the back-side attack Walden inversion mechanism, with a D3h transition state and a static barrier height of 1.6 eV. It is very similar to the gas-phase SN2 reactions with central barriers, except that there exist pre- and post-reaction wells in SN2 reactions stemming from strong ion-dipole interaction between reagents/products. They found that the reaction threshold energy is considerably higher than the barrier height, and the reaction manifests different isotope effect. In term of cross section beyond reaction threshold energy, it has a large normal secondary isotope effect with the reactivity for the H' + CH4 → CH3H' + H reaction substantially larger than that for H' + CHD3 → CD3H' + H reaction. However, in term of thermal rate constant it has a reverse secondary KIE with the rate for H + CH4 slightly smaller than that for H + CHD3.
　　In order to explore the dynamics origin of these different isotope effects, they analyzed the change of the umbrella angle of the non-reacting methyl group as the reaction proceeds. According to the minimum energy path (MEP), the umbrella angle should change synchronously during the reaction with the incoming of H atom and elongation of the breaking CH bond to reach 90° at the static saddle point. However, it turned out that for the H' + CHD3 substitution reaction the umbrella motion of the non-reacting CD3 group is slow in responding to the attack of the incoming H atom during the reaction. The reaction does not proceed along MEP as shown by the black reaction path in the figure. Instead, it proceeds on the red path with the umbrella angle larger than 90° at the dynamical saddle point and the corresponding barrier height higher than the static barrier height. Because the umbrella motion depends on the masses of the methyl group, isotope replacement of D atom to H atom can enhance the reactivity substantially, resulting in a large normal isotope effect on cross sections. Furthermore, because the initial umbrella excitation of methyl group can accelerate the umbrella motion during reaction substantially, it can dramatically enhance the reactivity. As a result, the thermal rate constant for the reaction is exclusively determined by the contributions from the umbrella excited states. As the contributions from the umbrella excited states for the H + CD3H reaction beat those for the H + CH4 reaction, the reaction exhibits an inverse secondary KIE.
　　All the phenomena found from this study for the H+CH4 substitution reactions, including higher threshold energy, a reverse secondary KIE at room temperature, and a large normal isotope effect for reaction cross section, have been observed for gas-phase SN2 reactions with barriers. We anticipate that the mechanisms uncovered from this study play important roles in these gas-phase SN2 reactions. Therefore, this study only not represents the first accurate theoretical study of a Walden inversion reaction which provided unprecedented dynamical details and a clear physical picture of the dynamics, but also sheds light on the dynamics of gas-phase SN2 reactions.
　　H' + CH4 → CH3H' + H substitution reaction (Image by ZHANG Donghui)
　　The related work was published in Nature Communications entitled as “Dynamical barrier and isotope effects in the simplest substitution reaction via Walden inversion mechanism”. This study was financially supported by the Natural Science Foundation of China, Ministry of Science and Technology of China, and the Chinese Academy of Sciences. (Text and Image by ZHANG Donghui) (CN 约稿)
　　Contact:
　　Dr. LU Xinyi
　　Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
　　457 Zhongshan Road, Dalian, 116023, China,
　　Tel: 86-411-84379201
　　E-mail: luxinyi@dicp.ac.cn 
　　

　　The research group led by Prof. BAO Xinhe and YANG Fan from Dalian Institute of Chemical Physics, Chinese Academy of Sciences discovered that oxide nanostructures (NSs) with a diameter below 3 nm could exhibit an oxidation resistance much more superior than larger NSs. By investigating the oxidation mechanism at the atomic level, the team proposed, for the first time, a "dynamic size effect", that determines the stability of supported nanoparticles. These findings were published in the latest issue of Nature Communications, entitled “Enhanced oxidation resistance of active nanostructures via dynamic size effect”. This study not only brings the atomic understanding of the dynamic remodeling mechanism of nanocatalyst under the atmosphere, but also provides a new interface control for the development of anti-corrosion and anti-oxidation nano-protective coating.
　　A major challenge limiting the practical applications of nanomaterials is that the activities of NSs increase with reduced size, often sacrificing their stability in the chemical environment. Under oxidative conditions, NSs with smaller sizes and higher defect densities are commonly expected to oxidize more easily, since high-concentration defects can facilitate oxidation by enhancing the reactivity with O2 and providing a fast channel for oxygen incorporation. Yet, several nanocrystalline materials were also reported previously to exhibit improved oxidation resistance with respect to bulk materials and have been applied as anti-corrosion coatings. The lack of general consensus on the oxidation resistance of oxide NSs has been attributed to the limited understanding on the underlying mechanism of oxidation. Particularly, the oxidation kinetics of NSs with diameters below 5 nm have rarely been studied.
　　Bao's team thus constructed FeO NSs with different sizes on Pt (111) and studied their oxidation kinetics using high resolution scanning tunneling microscopy (STM) and density-functional theory (DFT) calculations. Reducing the size of active FeO NSs was found to increase drastically their oxidation resistance and a maximum oxidation resistance is found for FeO NSs with dimensions below 3.2 nm. The team found the enhanced oxidation resistance originates from the size-dependent structural dynamics of FeO NSs in O2.
　　Specifically, the study shows that FeO NSs with a size below 3.2 nm could undergo a facile and complete reconstruction, when O2 dissociates at the coordinatively unsaturated ferrous centers at the edges of FeO NSs. Accompanying the reconstruction, the dissociated oxygen atoms are stabilized at the edges of FeO NSs and could not penetrate into the interface FeO and Pt, thereby inhibiting the further oxidation of FeO NSs. FeO NSs with dimensions above 3.2 nm are easier to be oxidized, because of their inability to complete the reconstruction, accompanied by the formation of surface dislocations. In other words, small FeO NSs are more susceptible to dynamic changes in the reaction, to achieve a relatively stable structure. The authors term this as the “dynamic size effect” and found it to govern the chemical properties of active NSs. To demonstrate the generality of dynamic size effect, the researchers also studied CoO NSs supported on Pt (111) or Au (111), and found similar oxidation-resistant behavior for NSs below 3 nm.
　　Enhanced oxidation resistance of active nanostructures via dynamic size effect (Image by YANG Fan)
　　The study was supported by the National Natural Science Foundation of China (21303195, 21473191 and 91545204), Ministry of Science and Technology of China (2013CB933100 and 2016YFA0202803), Strategic Priority Research Program of the Chinese Academy of Sciences (XDB17020200), CAS Center for Excellence in Nanoscience and Collaborative Innovation Center of Chemistry for Energy Materials. (Text and Image by YANG Fan)(CN 约稿)
　　Contact:
　　Dr. LU Xinyi
　　Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
　　457 Zhongshan Road, Dalian, 116023, China,
　　Tel: 86-411-84379201
　　E-mail: luxinyi@dicp.ac.cn 

　　The research group led by Prof. BAO Xinhe and YANG Fan from Dalian Institute of Chemical Physics, Chinese Academy of Sciences discovered that oxide nanostructures (NSs) with a diameter below 3 nm could exhibit an oxidation resistance much more superior than larger NSs. By investigating the oxidation mechanism at the atomic level, the team proposed, for the first time, a "dynamic size effect", that determines the stability of supported nanoparticles. These findings were published in the latest issue of Nature Communications, entitled “Enhanced oxidation resistance of active nanostructures via dynamic size effect”. This study not only brings the atomic understanding of the dynamic remodeling mechanism of nanocatalyst under the atmosphere, but also provides a new interface control for the development of anti-corrosion and anti-oxidation nano-protective coating.
　　A major challenge limiting the practical applications of nanomaterials is that the activities of NSs increase with reduced size, often sacrificing their stability in the chemical environment. Under oxidative conditions, NSs with smaller sizes and higher defect densities are commonly expected to oxidize more easily, since high-concentration defects can facilitate oxidation by enhancing the reactivity with O2 and providing a fast channel for oxygen incorporation. Yet, several nanocrystalline materials were also reported previously to exhibit improved oxidation resistance with respect to bulk materials and have been applied as anti-corrosion coatings. The lack of general consensus on the oxidation resistance of oxide NSs has been attributed to the limited understanding on the underlying mechanism of oxidation. Particularly, the oxidation kinetics of NSs with diameters below 5 nm have rarely been studied.
　　Bao's team thus constructed FeO NSs with different sizes on Pt (111) and studied their oxidation kinetics using high resolution scanning tunneling microscopy (STM) and density-functional theory (DFT) calculations. Reducing the size of active FeO NSs was found to increase drastically their oxidation resistance and a maximum oxidation resistance is found for FeO NSs with dimensions below 3.2 nm. The team found the enhanced oxidation resistance originates from the size-dependent structural dynamics of FeO NSs in O2.
　　Specifically, the study shows that FeO NSs with a size below 3.2 nm could undergo a facile and complete reconstruction, when O2 dissociates at the coordinatively unsaturated ferrous centers at the edges of FeO NSs. Accompanying the reconstruction, the dissociated oxygen atoms are stabilized at the edges of FeO NSs and could not penetrate into the interface FeO and Pt, thereby inhibiting the further oxidation of FeO NSs. FeO NSs with dimensions above 3.2 nm are easier to be oxidized, because of their inability to complete the reconstruction, accompanied by the formation of surface dislocations. In other words, small FeO NSs are more susceptible to dynamic changes in the reaction, to achieve a relatively stable structure. The authors term this as the “dynamic size effect” and found it to govern the chemical properties of active NSs. To demonstrate the generality of dynamic size effect, the researchers also studied CoO NSs supported on Pt (111) or Au (111), and found similar oxidation-resistant behavior for NSs below 3 nm.
　　Enhanced oxidation resistance of active nanostructures via dynamic size effect (Image by YANG Fan)
　　The study was supported by the National Natural Science Foundation of China (21303195, 21473191 and 91545204), Ministry of Science and Technology of China (2013CB933100 and 2016YFA0202803), Strategic Priority Research Program of the Chinese Academy of Sciences (XDB17020200), CAS Center for Excellence in Nanoscience and Collaborative Innovation Center of Chemistry for Energy Materials. (Text and Image by YANG Fan)(CN 约稿)
　　Contact:
　　Dr. LU Xinyi
　　Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
　　457 Zhongshan Road, Dalian, 116023, China,
　　Tel: 86-411-84379201
　　E-mail: luxinyi@dicp.ac.cn