An energy decomposition analysis (EDA) method is proposed to isolate physically relevant components of the total intermolecular interaction energies such as the contribution from interacting frozen monomer densities, the energy lowering due to polarization of the densities, and the further energy lowering due to charge-transfer effects. This method is conceptually similar to existing EDA methods such as Morokuma analysis but includes several important new features. The first is a fully self-consistent treatment of the energy lowering due to polarization, which is evaluated by a self-consistent field calculation in which the molecular orbital coefficients are constrained to be block-diagonal (absolutely localized) in the interacting molecules to prohibit charge transfer. The second new feature is the ability to separate forward and back-donation in the charge-transfer energy term using a perturbative approximation starting from the optimized block-diagonal reference. The newly proposed EDA method is used to understand the fundamental aspects of intermolecular interactions such as the degree of covalency in the hydrogen bonding in water and the contributions of forward and back-donation in synergic bonding in metal complexes. Additionally, it is demonstrated that this method can be used to identify the factors controlling the interaction of the molecular hydrogen with open metal centers in potential hydrogen storage materials and the interaction of methane with rhenium complexes.
Metal-alkane binding energies have been calculated for [CpRe(CO) 2](alkane) and [(CO)2M(C5H4)C'C(C5H4)M(CO)2](alkane), where M ؍ Re or Mn. Calculated binding energies were found to increase with the number of metal-alkane interaction sites. In all cases examined, the manganese-alkane binding energies were predicted to be significantly lower than those for the analogous rhenium-alkane complexes. The metal (Mn or Re)-alkane interaction was predicted to be primarily one of charge transfer, both from the alkane to the metal complex (70 -80% of total charge transfer) and from the metal complex to the alkane (20 -30% of the total charge transfer).binding energy ͉ COH activation ͉ DFT calculations ͉ manganese C arbon-hydrogen (COH) bond activation reactions are important from a fundamental point of view, as well as in more practical terms to medicine, academia, and industry, as they are being used for the conversion of common, inexpensive alkanes into reactive (and physiologically active) molecules (1, 2). Many transition metal complexes are now known that can be used to activate COH bonds in alkanes (3). With low-valent metal complexes, the first bond-breaking step involves oxidative addition of an alkane to an unsaturated metal center (Fig. 1). The goal of subsequent steps is to convert the alkyl group R into an alcohol ROH or other potentially useful functionalized organic molecule.Many researchers today are focusing on the development of industrially practical organometallic oxidation catalysts (5-8), but there remain fundamental aspects of the COH activation process that are not fully understood. For instance, it is widely accepted that COH activation reactions proceed via alkane -complex intermediates (3) (Fig. 1, compound 3), and such species have recently been detected and studied in lowtemperature NMR experiments (9-11). Although complexes with intramolecular COH/metal (so-called agostic) interactions have been isolated and crystallographically characterized (3), in systems where strong evidence has been provided for intermolecular metal-alkane coordination in solution (9-11), isolation and solid state structural characterization have not yet been accomplished.† † Intermolecular metal-alkane complexes are therefore one of the most important targets in the study of COH activation; an understanding of the mechanisms of COH activation, which will allow researchers to better control and manipulate the outcome of the reactions, is crucial for the advancement of this area of chemistry.Quantum chemical methods have become useful and practical tools in study of TM complexes (14-17). Particularly, advancements in density functional theory (DFT) (16) and the use of effective core potentials (14, 15) have made qualitatively accurate predictions of the structures and chemistry of TM complexes possible at a reasonable computational cost (17). The goals of the present work are twofold: to make computational predictions of the relative stabilities of a selection of alkane -complexes, which will aid synthetic chemists in th...
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