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Common one-electron reduced density matrix (1-RDM) functionals that depend on Coulomb and exchange-only integrals tend to underestimate dynamic correlation, preventing reduced density matrix functional theory (RDMFT) from achieving comparable accuracy to density functional theory in main-group thermochemistry and thermochemical kinetics. The recently developed ωP22 functional introduces a semi-local density functional to screen the erroneous short-range portion of 1-RDM functionals without double-counting correlation, potentially providing a better treatment of dynamic correlation around equilibrium geometries. Herein, we systematically evaluate the performance of this functional model, which consists of two parameters, on main-group thermochemistry, thermochemical kinetics, nonbonded interactions, and more. Tests on atomization energies, vibrational frequencies, and reaction barriers reveal that the ωP22 functional model can reliably predict properties at equilibrium and slightly away from equilibrium geometries. In particular, it outperforms commonly used density functionals in the prediction of reaction barriers, nonbonded interactions, and singlet diradicals, thus enhancing the predictive power of RDMFT for routine calculations of thermochemistry and thermochemical kinetics around equilibrium geometries. Further development is needed in the future to refine short- and long-range approximations in the functional model in order to achieve an excellent description of properties both near and far from equilibrium geometries. [ABSTRACT FROM AUTHOR]

In this study, the conformational preferences and photochemistry of acrylic acid (AA, CH2=CHCOOH) monomer isolated in cryogenic argon and krypton matrices were interpreted, based on results of quantum chemical calculations. Natural bond orbital analysis allowed to shed light on the main electronic effects determining the relative stability of the conformers of the molecule in the ground electronic state. The conformational isomerization taking place upon UV-irradiation of the matrix-isolated compound (λ ∼ 243 nm) was explained, based on theoretical complete active space self-consistent field/complete active space with second order perturbation theory (CASSCF/CASPT2) and time-dependent density functional theory (TD-DFT) results, allowing to rationalize the nearly equal populations of the two lowest energy conformers of the molecule observed in the photostationary state. Besides, details of the infrared spectra of the compound were reinterpreted based on the calculated spectra for the two most stable conformers of the molecule. In particular, the assignments for the out-of-plane A″ symmetry vibrations were revised. [ABSTRACT FROM AUTHOR]

We report transient resonance Raman spectra and density functional theory calculations for the photoproducts produced after ultraviolet excitation of CHBr[sub 2]Cl and CCl[sub 3]Br in cyclohexane solution. Comparison of the computed vibrational frequencies to the experimental Raman frequencies revealed that the iso-CHBrCl-Br and iso-CHClBr-Br species are mainly responsible for the transient resonance Raman spectrum observed following ultraviolet excitation of CHBr[sub 2]Cl. Similar comparisons for CCl[sub 3]Br showed the iso-CCl[sub 2]Cl-Br species is mainly responsible for the transient resonance Raman spectrum observed following ultraviolet excitation of CCl[sub 3]Br. Additional density functional theory computations were done to examine the chemical reactions of iso-CH[sub 2]Br-Cl and iso-CH[sub 2]Cl-Br with ethylene to give cyclopropane and Br-Cl product. We briefly discuss the possibility for release of reactive halogens into the atmosphere via the photochemical and chemical reactions of iso-polyhalomethane molecules formed after ultraviolet excitation of polyhalomethanes in condensed phase environments. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have taken (dA), (dT), and (dA)·(dT) as model systems to study concerted effects of base pairing and stacking on excited-state nature of DNA oligonucleotides using density functional theory (DFT) and time dependent DFT methods. The spectroscopic states are determined to be of a partial → charge-transfer nature in the oligonucleotides. The → charge-transfer transitions produce dark states, which are hidden in the energy region of the steady-state absorption spectra. This is different from the previous assignment that the → charge-transfer transition is responsible for a shoulder at the red side of the first strong absorption band. The → charge-transfer states were predicted to have relatively high energies in the oligonucleotides. The present calculations predict that the → charge-transfer states are not involved in the spectra and excited-state dynamics of the oligonucleotides. In addition, the influence of base pairing and stacking on the nature of the nπ* and ππ* states are discussed in detail. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011 [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: 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]

The potential energy surfaces of dissociations and isomerizations for butyrophenone and its α-substituted derivative in low-lying electronic states have been studied with use of the CASSCF and DFT methods. A three-surface (S[SUB1], T[SUB1] and T[SUB2]) intersection was found in the present work and this plays an important role in the S[SUB1] → T[SUB1] process for butyrophenone and related aromatic carbonyl compounds. Examination of these results provides some new insights into the mechanistic photochemistry of aromatic ketones. The most probable mechanism leading to formation of a cyclopropane moiety was obtained on the basis of the calculated potential energy surfaces and their intersections. The selection of a suitable α-substituent was found to be a key step for cyclopropanation to occur by photoexcitation of butyrophenone derivatives. [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]