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Graphical abstract Highlights • In Os1 and Os2, the MLCT state character changes in the excited-state relaxation dynamics. • Inter-ligand electron transfer plays a major role in the excited-state relaxation dynamics. • Ligand properties have significant influence on the excited-state relaxation dynamics. Abstract Photoinduced excited-state relaxation dynamics of organometallic compounds, due to their potential applications in solar cells, organic light-emitting diodes, photocatalysis, etc., have been recently explored by a wealth of ultrafast experimental techniques. However, the corresponding nonadiabatic dynamics simulations are rarely reported. In this work, the early-time excited-state relaxation dynamics of two Os(II) compounds i.e. Os(bpy) 3 and Os(bpy) 2 (dpp) (Os1 and Os2) have been investigated by our recently implemented TD-DFT based generalized surface-hopping dynamics simulation method in combination with the quantum mechanics/molecular mechanics (QM/MM) approach. The metal-to-ligand charge transfer (MLCT) excited singlet states from the Os atom to the bpy and dpp ligands are first populated in the Franck-Condon region. These initially populated excited singlet states will be converted through a series of ultrafast internal conversion and intersystem crossing processes to the lowest triplet state as a result of significant nonadiabatic and spin-orbit couplings among singlet and triplet electronic excited states. The intersystem crossing rates are estimated to be 72 and 53 fs for Os1 and Os2, respectively, which are consistent with experimental measured 100 fs and less than 50 fs. The time-dependent transition density analysis reveals that the MLCT excited states are dominant in the entire relaxation dynamics for either Os1 or Os2; however, the MLCT character changes in these processes. In Os1, the initially populated MLCT state is mainly from the Os atom to one bpy ligand, which is changed to the other two MLCT states. In Os2, the MLCT state that is mainly from the Os atom to the two bpy ligands is converted to the MLCT state from the Os to the dpp ligand. Finally, we have found that inter-ligand electron transfer plays a major role in the excited-state relaxation dynamics of Os1 and Os2; while, the hole transfer is minor. Our results also show that the ligand properties have significant influence on the excited-state relaxation dynamics of organometallic compounds. The insights gained in the present study provide atomistic insights for early-time excited-state relaxation dynamics of Osmium(II) compounds. [ABSTRACT FROM AUTHOR]

Abstract: The glyoxalase system catalyzes the conversion of toxic methylglyoxal to nontoxic d-lactic acid using glutathione (GSH) as a coenzyme. Glyoxalase II (GlxII) is a binuclear Zn enzyme that catalyzes the second step of this conversion, namely the hydrolysis of S-d-lactoylglutathione, which is the product of the Glyoxalase I (GlxI) reaction. In this paper we use density functional theory method to investigate the reaction mechanism of GlxII. A model of the active site is constructed on the basis of the X-ray crystal structure of the native enzyme. Stationary points along the reaction pathway are optimized and the potential energy surface for the reaction is calculated. The calculations give strong support to the previously proposed mechanism. It is found that the bridging hydroxide is capable of performing nucleophilic attack at the substrate carbonyl to form a tetrahedral intermediate. This step is followed by a proton transfer from the bridging oxygen to Asp58 and finally C–S bond cleavage. The roles of the two zinc ions in the reaction mechanism are analyzed. Zn2 is found to stabilize the charge of tetrahedral intermediate thereby lowering the barrier for the nucleophilic attack, while Zn1 stabilizes the charge of the thiolate product, thereby facilitating the C–S bond cleavage. Finally, the energies involved in the product release and active-site regeneration are estimated and a new possible mechanism is suggested. [Copyright &y& Elsevier]

Abstract: The mechanistic photodissociations of several series of aliphatic and aryl halides have been theoretically investigated using multireference ab initio calculations. The photodissociations of halomethanes were also investigated by spin–orbit ab initio calculations. Based on the calculated results of complete active space self-consistent field (CASSCF), CASSCF with second-order perturbation (CASPT2), multistate CASPT2 (MS-CASPT2), or MS-CASPT2 with spin–orbit interaction through complete active space state (MS-CASPT2/CASSI-SO), the experimentally observed photodissociation channels were clearly assigned, the photochemical and photophysical processes were quantitatively described. The photodissociation mechanisms of the targeted molecules were deeply discussed by high-quality potential energy curves (PECs) and critical points of the potential energy surfaces (PESs). [Copyright &y& Elsevier]

Abstract: Structures of divalent osmium and ruthenium complexes of [M(bpy)3]2+, [M(bpy)2P2]2+ and [M(dpb)2P2]2+ (M=Ru, Os, bpy=2,2′-bipyridine, dpb=4,4′-diphenyl-2,2′-bipyridine, P2=cis-1,2-bis(diphenylphosphino)ethylene) in the ground and the lowest triplet states were fully optimized with density functional theory. The features of the absorption bands and phosphorescent emission from the lowest triplet state for the complexes have been described by time-dependent density functional theory. The [Os(bpy)3]2+ complex in methanol solution exhibits two strong absorption bands at ∼495 and ∼445nm, which are well reproduced by the present calculations. There is one absorption peak observed at 440-460nm for the divalent Ru polypyridyl complexes in methanol solution, which is close to the TDDFT calculated values of 415–440nm for the complex of [Ru(bpy)3]2+. All evidences from the optimized structures, the calculated atomic charges and spin densities for the triplet complexes support the conclusion that phosphorescent emission in the divalent Os and Ru complexes originate from the triplet metal-to-ligand-charge-transfer state. This has been discussed in detail. [Copyright &y& Elsevier]

Abstract: In the present Letter, density functional theory has been used to optimize geometries of Zn2(η5-Cp*)2 (Cp*=C5Me5). The CCSD(T) calculated dissociation energies of 41.9 and 58.5kcal/mol for the metal–metal and metal–ligand bonds predict that Zn2(η5-Cp*)2 is a stable complex at room temperature. Ionic bond dominates the interaction between metal atom and ligand and a unique unit of was found to exist in the complex. The Zn–Zn bond is a σ single bond, which is further strengthened by a weak d–d interaction. [Copyright &y& Elsevier]

Taking thioacetaldehyde as an example, the purpose of this work is to investigate the reactivity difference between carbonyl compounds and thiocarbonyl compounds. Five possible unimolecular reaction pathways for ground-state thioacetaldehyde have been investigated in this work with MP2 and B3LYP methods in conjugation with cc-pVDZ basis set. The present calculations predict that the unimolecular reactions for ground-state thioacetaldehyde are quite different from those for ground-state acetaldehyde. In addition, it is found that this is mainly due to the larger size of sulfur atom than oxygen and the less stability of compounds containing sulfur with respect to the corresponding carbonyl compounds. [Copyright &y& Elsevier]

Seven possible unimolecular reaction pathways of thioformyl cyanide in its ground state have been investigated at the HF/cc-pVDZ, B3LYP/cc-pVDZ, MP2/cc-pVDZ, and MP2/6-31G** levels of theory. The theoretical analysis indicates that thioformyl cyanide is rather stable at room temperature in the case of unimolecular reactions, which is consistent with the experimental findings. [Copyright &y& Elsevier]

In the present Letter, potential energy surfaces for the 1,5-H shift and α-bond cleavages in the S0, T1 and S1 states have been investigated using the CASSCF, DFT and MP2 methods with the cc-pVDZ basis set, which provides some new insights into the mechanistic photochemistry of butanal. The 1,5-H shift proceeds mainly along the S1 pathway, while the α-C&z.sbnd;C bond cleavage occurs as a result of intersystem crossing from S1 to T1. However, the α-C&z.sbnd;H bond fission takes place with less probability. [Copyright &y& Elsevier]

Chemical passivation is a predominant approach to inhibit intrinsic defects responsible for electron–hole recombination in perovskite solar cells. Using time-domain density functional theory combined with nonadiabatic molecular dynamics, we demonstrate defect passivation by separation of electrons and holes in metal halide perovskites (MHPs), and show that bidentate ligands exhibit the best performance. Defects in traditional semiconductors create deep midgap states, and passivation eliminates these states. In contrast, common defects produce no deep midgap states in MHPs. Instead, defects localize electrons and holes around defect sites, enhancing electron–hole interaction. Defect passivation in MHPs acts to separate charges, decreasing electron–hole and charge–phonon interactions, and increasing charge lifetimes. Bidentate ligands work best, because they can passivate both unsaturated chemical bonds created due to vacancy defects. Bidentate ligands with spatially separated binding sites are preferred, since they provide better match to the sparse inorganic lattice of MHPs. Similar to the Lewis base ligands, water also acts as a ligand and extends charge lifetimes, with the oxygen atom donating its lone electron pair to the defect site. However, water accelerates chemical degradation of MHPs. The binding energy of most ligands is larger than that of water, and therefore, the ligands displace water, increase MHP stability and prolong carrier lifetimes. The established physical mechanism of defect passivation, and the specific principles guiding the choice of passivating molecules advance our understanding of the exceptional properties of MHPs and suggest routes for further improvement of MHP performance in solar energy and optoelectronic applications. Passivating defects with bidentate ligands is better than monodentate ligands in perovskite. ga1 • Binding of water and Lewis bases to I vacancies on CH3NH3PbI3 surface separates and extends lifetime of electrons and holes. • Monodentate water, thiophene and pyridine passivate one under-coordinated Pb, while bidentate 2-MP and D4TBP passivate both. • Charge recombination is driven by slow Pb-I vibrations. Quantum coherence is extremely short, favoring long-lived charges. • The state-of-the-art simulation combines non-adiabatic molecular dynamics and time-domain density functional theory. [ABSTRACT FROM AUTHOR]

[4 + 4] and [4 + 2] photocycloadditions have not computationally attracted the same level of attention as [2 + 2] photocycloadditions. Herein we have employed density-functional theory (DFT), complete active-space self-consistent field (CASSCF), and its multi-state second-order perturbation theory (MS-CASPT2) methods to systematically explore excited-state intramolecular proton transfer (ESIPT) induced [4 + 4] and [4 + 2] photocycloaddition mechanisms of an oxazoline. On the basis of the results, we have found that the ESIPTs in the S 1 state are not as efficient as those in the T 1 state concerning experimentally used irradiation wavelength. Instead, the T 1 ESIPTs generate two sets of endo and exo precursors of [4 + 4] and [4 + 2] photocycloadditions. In these reactions, the C atom of the furan group and the N atom of the aminophenyl group are first bonded leading to a diradical intermediate. This reaction step has a small barrier and is thermodynamically favorable. However, the remaining C C bond formation is blocked in the T 1 state due to its huge reaction barrier. Therefore, the system hops to the S 0 state via T 1 → S 0 intersystem crossings at T 1 /S 0 crossing points. These nonadiabatic processes are efficient because of structural and energetic similarities of T 1 /S 0 crossing points with their nearby T 1 intermediates. In the S 0 state, the C C bond formation becomes a barrierless process. Finally, we have discussed possible origins for high regioselectivity and enantioselectivity of these photocycloaddition reactions. [ABSTRACT FROM AUTHOR]

Abstract: Astacin, acting as the prototype of astacin family and of the metzincin superfamily, is a mononuclear zinc enzyme that catalyzes the hydrolysis of polypeptides and proteins. In the present article, the reaction mechanism of astacin has been investigated using density functional theory (DFT) method. Using a model of the active site constructed on the basis of the X-ray crystal structure of astacin, the potential energy surface for catalytic reaction has been mapped and the transition states and intermediates along the reaction pathway are characterized. The calculations give general support to the previously proposed mechanism by experiments, which mainly involve nucleophilic attack on carbonyl group of substrate and C―N bond cleavage, and reveal more detailed mechanistic features. The Glu93 functions as a crucial general base to activate the zinc-bound water and the resulting zinc-bound hydroxide acts as the real nucleophile. It is demonstrated that there exists a “reactant region”, where the interconversion between zinc-bound water and zinc-bound hydroxide occurs rapidly. The rate-limiting step is predicted to be the nucleophilic attack, which leads to an anionic gem-diolate tetrahedral intermediate. During the catalysis, zinc ion provides main catalytic power by stabilizing the developing charge of tetrahedral intermediate, while the Tyr149 residue also partially contributes to the catalysis by stabilizing the anionic intermediate. In addition, it is shown that the Cys64 plays roles in assisting in binding and orientating the substrate via electrostatic interaction. [Copyright &y& Elsevier]

Abstract: The photodissociations of acetophenone (C6H5COCH3) have been investigated by density functional theory (DFT) approach. The experimentally observed three photodissociation channels were clarified from the theoretical calculations on the related reactants, transition states (TSs), and products. Two of the three channels, C6H5COCH3 →C6H5CO+CH3 and C6H5COCH3 →C6H5 +CH3CO, were assigned to Norrish I reactions on the potential energy surfaces (PESs) of the lowest triplet state (T1). And, the first one is more favorable for lower barrier. The subsequent decompositions, C6H5CO→C6H5 +CO and CH3CO→CH3 +CO, were also studied by the similar calculations as above. The third photodissociation channel, C6H5COCH3 →C6H5CH3 +CO, has been documented on the PESs of the ground state (S0). The third one played a minor role in the photodissociations of C6H5COCH3 for much higher barrier than the first two. [Copyright &y& Elsevier]

The complete active space self-consistent field method has been used to optimize planar minimum energy structures of the lowest five electronic states for benzaldehyde, acetophenone and the related aromatic ketones. The adiabatic excitation energies from the ground state to the 3nπ*(T1), 1nπ*(S1), 3ππ*(T2) and 1ππ*(S2) states were determined, which are in a good agreement with the experimental band origins where available. The present calculations predict that the adiabatic excitation energy to each excited electronic state investigated is nearly a constant for a wide range of aromatic carbonyl compounds. [Copyright &y& Elsevier]

Abstract: The photodissociation of dichloromethane (CH2Cl2) has been investigated by spin–orbit ab initio calculations. The potential energy curves were calculated by multi-state second order multiconfigurational perturbation theory in conjunction with spin–orbit interaction through complete active space state interaction (MS-CASPT2/CASSI-SO). The calculated results clearly assigned the experimentally observed photodissociation channels from the first and second singlet excited states leading to CH2Cl + Cl(2P3/2) and CH2Cl + Cl∗(2P1/2). The reason of higher Cl∗/Cl ratios in vibrationally mediated photodissociation than in vibrationless one was discussed. [Copyright &y& Elsevier]