A new computational scheme integrating ab initio and molecular mechanics descriptions in different parts of the same molecule is presented. In contrast with previous approaches, this method is especially designed to allow the introduction of molecular mechanics corrections in full geometry optimizations concerning problems usually studied through ab initio calculations on model systems. The scheme proposed in this article intends to solve some of the systematic error associated with modeling through the use of molecular mechanics corrections. This method, which does not require any new parameter, evaluates explicitly the energy derivatives with respect to geometrical parameters and therefore has a straightforward application to geometry optimization. Examples of its performance on two simple cases are provided: the equilibrium geometry of cyclopropene and the energy barriers on S , 2 reactions of alkyl chloride systems. Results are in satisfactory agreement with those of full ab initio calculations in both cases. 0 1995 by John Wiley & Sons, Inc.

We present the ioChem-BD platform ( www.iochem-bd.org ) as a multiheaded tool aimed to manage large volumes of quantum chemistry results from a diverse group of already common simulation packages. The platform has an extensible structure. The key modules managing the main tasks are to (i) upload of output files from common computational chemistry packages, (ii) extract meaningful data from the results, and (iii) generate output summaries in user-friendly formats. A heavy use of the Chemical Mark-up Language (CML) is made in the intermediate files used by ioChem-BD. From them and using XSL techniques, we manipulate and transform such chemical data sets to fulfill researchers' needs in the form of HTML5 reports, supporting information, and other research media.

A new family of tetra-anionic tetradentate amidate ligands, N1,N1'-(1,2-phenylene)bis(N2-methyloxalamide) (H4L1), and its derivatives containing electron-donating groups at the aromatic ring have been prepared and characterized, together with their corresponding anionic Cu(II) complexes, [(LY)Cu](2-). At pH 11.5, the latter undergoes a reversible metal-based III/II oxidation process at 0.56 V and a ligand-based pH-dependent electron-transfer process at 1.25 V, associated with a large electrocatalytic water oxidation wave (overpotential of 700 mV). Foot-of-the-wave analysis gives a catalytic rate constant of 3.6 s(-1) at pH 11.5 and 12 s(-1) at pH 12.5. As the electron-donating capacity at the aromatic ring increases, the overpotential is drastically reduced down to a record low of 170 mV. In addition, DFT calculations allow us to propose a complete catalytic cycle that uncovers an unprecedented pathway in which crucial O-O bond formation occurs in a two-step, one-electron process where the peroxo intermediate generated has no formal M-O bond but is strongly hydrogen bonded to the auxiliary ligand.

I. Introduction 601 A. Hydride Is the Smallest Ligand and the Only One To Make a Pure Single Bond to a Metal 601 B. H Is Easy To Locate through Quantum Calculations but Hard To Locate through Experimental Techniques 602 C. Hydride Complexes Are Full of Surprises 603 D. H 2 Is the Ideal Model for a σ Bond Coordinating to a Metal 603 E. H 2 Is the Ideal Model for Activation of a Single Bond 603 F. Scope and Limitations of the Review 603 II. Small Gas-Phase Systems 603 III. H as an Ideal Ligand 605 A. General Approach for Bonding in Transition Metal Complexes 605 B. The d 0 ML 6 Case 606 C. The d 6 ML 5 Case 606 D. Other Systems 607 IV. "Computational Crystallography" of Hydride Complexes, a Cost-Effective Method for High-Quality Structural Determination 607 A. Good Success with Simplified Models 607 B. Improving the Model 610 V. The Dihydrogen Saga 611 A. Dihydrogen as a Ligand 611 B. Dihydrogen versus Dihydride 613 C. Theoretical Tools for Analysis of the H‚‚‚H Interaction 614 VI. Interactions with M−H and with M−H 2 . Hydrogen Bonds Again! 615 VII. Hydrogen Exchange Processes 617 A. Pairwise Exchange 617 B. Polytopal Rearrangements 619 C. Site Exchange with H Atoms outside the Coordination Sphere 619 D. Dihydrogen Rotation 620 E. Exchange Processes in M(H)(H 2 ) Complexes 622 VIII. Quantum Exchange Couplings in Polyhydrides 623 A. Physical Origin of Quantum Exchange Couplings 623 B. Simulation of Quantum Exchange Couplings 624 IX. Stretched Dihydrogen Complexes 625 A. The Electronic Point of View 626 B. The Dynamic Point of View 627 X. Breaking the H−H Bond by Transition Metal Complexes 627 A. Oxidative Addition 628 B. σ Bond Metathesis 629 XI. Methodological Peculiarities in the Study of Polyhydride Systems 629 XII. Conclusions and Perspectives 631 XIII. Acknowledgment 631 XIV. References 631 10.

Under the usual conditions, the Pd-catalyzed arylation does not involve an electrophilic aromatic substitution reaction. On the basis of DFT calculations, we propose a mechanism for the Pd-catalyzed arylation that involves a proton abstraction by a carbonate or related ligand and that provides a satisfactory explanation for the experimental data.

A computational study with the Becke3LYP DFT functional is carried out on the cross-coupling reaction of vinyl bromide H 2 CdCHBr and vinylboronic acid H 2 CdCHB(OH) 2 catalyzed by palladium diphosphine [Pd(PH 3 ) 2 ] in the presence of an excess of base OH -. The full catalytic cycle is computed, starting from the separated reactants and the catalyst and finishing with the cross-coupled product and the regeneration of the catalyst. The different stages in the cycle (oxidative addition, isomerization, transmetalation, reductive elimination) are characterized through calculation of the corresponding intermediates and transition states. Different alternative mechanisms are considered, depending on the number of phosphine ligands at palladium, and on the cis or trans isomery around the metal center. The results indicate the existence of a number of competitive pathways of reasonably low energy.

The use of phenolic compounds as organocatalysts is discussed in the context of the atom-efficient cycloaddition of carbon dioxide to epoxides, forming useful cyclic organic carbonate products. The presence and cooperative nature of adjacent phenolic groups in the catalyst structure results in significantly enhanced catalytic efficiencies, allowing these CO(2) fixation reactions to operate efficiently under virtually ambient conditions. The cooperative effect has also been studied by computational methods. Furthermore, when the cycloaddition reactions are carried out on a larger scale and under solvent-free conditions, further enhancements in activity are observed, combined with the advantageous requirement of reduced loadings of the binary organocatalyst system. The reported system is among one of the mildest and most effective metal-free catalysts for this conversion and contributes to a much more sustainable development of organic carbonate production; this feature has not been the main focus of previous contributions in this area.

The regioselectivity observed in the intramolecular palladium-catalyzed arylation of substituted bromobenzyldiarylmethanes as well as theoretical results demonstrate that the Pd-catalyzed arylation proceeds by a mechanism involving a proton abstraction by the carbonate, or a related basic ligand. The reaction is facilitated by electron-withdrawing substituents on the aromatic ring, which is inconsistent with an electrophilic aromatic-substitution mechanism. The more important directing effect is exerted by electron-withdrawing substituents ortho to the reacting site.