Di-sigma adsorbed ethene, bridge(30) adsorbed benzene (with four di-sigma-type and two pi-type interactions), and)71 and eta(2) adsorbed acetone on nanosized platinum clusters consisting of 19 to 38 Pt atoms were studied theoretically by density functional theory (DFT) calculations with general gradient approximation (GGA) utilizing plane wave and local basis sets. The cluster results were compared to plane wave calculations employed with periodic boundary conditions. It was found that the geometries obtained with different methods are very similar but the adsorption energy depends prominently on the cluster size. Adsorption was strongest on the 22- and 26-atom clusters and weakest on the 35- and 38-atom clusters. No correlation between the d-band center of the clusters and the adsorption energy was observed. The relative amount of adsorbed active and spectator species of an organic molecule may depend on the cluster size as exemplified by the eta(1) and eta(2) adsorption modes of acetone. It is also feasible that the reaction energies are size-dependent adhering to the Bronsted-Evans-Polanyi relationship. This indicates that the catalytic activity of Pt nanoparticles can be increased by carefully controlling their size.

Hydrogen-bonding interactions between alpha-hydroxyketones (i.e., (R)- and (S)-1-hydroxy-1-phenyl-2-propanones and (R)- and (S)-2-hydroxy-phenyl- 1-propanones) and protonated cinchonidine in Open(3) conformation relevant to enantioselective hydrogenation over Pt were studied computationally at the B3LYP/TZVP level. The density functional theory (DFT)-optimized structures were reoptimized on a flat Pt(111) surface with molecular mechanics using the condensed phase-optimized molecular potentials for atomistic simulation studies (COMPASS) force field. Two possible interaction modes-the so-called bifurcated and cyclic hydrogen bonded complexes-were studied. In the former, both oxygens of the reactant interact with the proton attached to the modifier's quinuclidine nitrogen; in the latter, one modifier's hydroxyl group also interacts with the substrate's oxygen. Bifurcated complexes were found to be 3-8 U mol(-1) more stable than the cyclic complexes by DFT calculations. By the force field calculations, only three cyclic complexes relevant to the reaction were found to be stable on the Pt(111) surface, and they were less stable than the bifurcated complexes. Thus, the relevance of the cyclic complexes between alpha-hydroxyketones and cinchonidine to enantiodifferentiation can be considered negligible. Furthermore, no bifurcated but single hydrogen bonded complexes were found to be stable on the Pt(111) surface for 1-hydroxy-1-phenyl-2-propanones with an sp(3)-hybridized carbon next to the phenyl ring. DFT calculations indicated that complexes leading to (R)-stereoisomers were more thermodynamically stable than the complexes leading to (S)-stereoisomers. In general, orbital analysis of the reacting C=O keto carbonyl orbitals indicated that formation of (R)-stereoisomers was kinetically preferred as well. However, DFT calculations of isolated complexes cannot qualitatively predict the enantiomeric excess, requiring that the steric restriction of the Pt surface be taken into account. By combining the results of DFT and force field calculations, a reasonable explanation for experimentally observed product distribution was obtained. (c) 2005 Elsevier Inc. All rights reserved.

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The proton affinities of seven different ketones, vicinal diketones, and α-keto esters (acetophenone, 2,2,2-trifluoroacetophenone, 2,3-butanedione, 1-phenyl-1,2-propanedione, methyl pyruvate, ethyl benzoylformate, and ketopantolactone) have been evaluated theoretically using the conventional ab initio HF and several post-HF methods (MP2, MP4, CCSD), density functional methods with the B3LYP hybrid functional, as well as some ab initio model chemistries [CBS-4M, G2(MP2), and G3(MP2)//B3LYP]. The chemical compounds studied are frequently used substrates in the asymmetric hydrogenation over chirally modified platinum catalysts where the protonation properties of the chiral modifier and the substrates are of great interest. In most cases, the proton affinities (PAs) evaluated with the CCSD/6-311+G(d,p)//B3LYP/TZVP and G2(MP2) methods are in good agreement with the existing experimental ones. However, the previously reported PA of 2,3-butanedione seems to be too high by 10–15kJmol−1. The B3LYP/TZVP//B3LYP/TZVP and MP2/6-311+G(d,p)//B3LYP/TZVP model chemistries predict proton affinities that are systematically higher and lower than the experimental PAs, respectively. If proton affinities are evaluated as the average of the PAs calculated with these two theoretical methods a very good agreement with the experimental results is obtained. The mean absolute deviation (MAD) from experiment of this combination method for the PAs of 13 test molecules is 4.0kJmol−1. For 9 molecules composed only of first-row atoms the MAD is 2.5kJmol−1. The B3LYP/TZVP//B3LYP/TZVP and MP2/6-311+G(d,p)//B3LYP/TZVP methods provide significant savings in computational time and disk space compared to the CCSD/6-311+G(d,p)//B3LYP/TZVP and G2(MP2) models. Therefore, it is suggested that if no experimental or highly accurate theoretical data is available (due to computational cost), the proton affinities of similar compounds as investigated in this paper, can be evaluated with the combination method. For the studied molecules, this method gives the following PAs (in kJmol−1): 788 (2,3-butanedione, exptl 802); 798 (2,2,2-trifluoroacetophenone, exptl 799); 811 (ketopantolactone); 813 (methyl pyruvate); 825 (1-phenyl-1,2-propanedione); 862 (acetophenone, exptl 861); 865 (ethyl benzoylformate).

In order to investigate enantiodifferentiation mechanism on a molecular level one-to-one complexes between protonated cinchonidine and methyl pyruvate, ketopantolactone as well as 1-phenylpropane-1,2-dione with and without an acetic acid molecule were investigated computationally with ab initio quantum chemical method at the Hartree–Fock level using 6-31G* basis set. The stabilities of the diastereomeric complexes and the electronic structure of the reactants were examined in order to correlate molecular level properties and enantioselectivity with the solvent effect (acetic acid versus toluene). A correlation between the keto carbonyl orbital energies and the complex stability was found. However, the enantiodiscrimination could not solely be explained in these terms. Acetic acid was found to have a large influence on the keto carbonyl orbital energies.

The enantioselective hydrogenation of vicinal diketones over cinchonidine-modified Pt resulted in enantiomeric excess of structurally similar (R)-enantiomers. Furthermore, the kinetic resolution was caused due to faster reaction of (S)-hydroxyketone further to diols, resulting in an increase of ee. The diastereoselectivities in diols were similar. The (R,S) or (S,R) diols were always the main products whereas considerably less (R,R) of (S,S) were formed. For the first time in 1-phenylpropane-1,2-dione (A) hydrogenation enantiomeric excesses of both C1double bondO1 and C2double bondO2 group have been reported. The ee(1-OH) and ee(2-OH) were 50 and 25%, respectively, at 50% conversion of A. Based on batch and continuous reactor experiments it could be concluded that the source of enantioselectivity is an increased formation rate of (R)enantiomer and decreased formation rate of (S)-enantiomer. Theoretical calculations revealed that in the substrate-modifier diastereomeric complex the reactant forms a nonplanar s-cis conformation and bonds to the protonated cinchonidine either via a bifurcated hydrogen bond or with two hydrogen bonds where the OH group is involved also. Optimized diastereomeric complexes were equal in energy. The calculated proton affinity of CD was high, 1000 kJ mol(-1), indicating that protonation is feasible under typical experimental conditions. (C) 2004 Elsevier Inc. All rights reserved.

Conformations of the two diastereomers of the natural lignan hydroxymatairesinol (HMR) from Norway Spruce (Picea abies), their interactions with alkali metal cations Na+ and K+, and the mechanism and some other aspects of the dehydrogenation reaction between HMR and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) were studied computationally with molecular mechanics, semiempirical methods (AM1 and PM3), ab initio quantum chemical method at the HF level and density functional theory with the B3LYP functional, using 6-31G(d) basis set with the two latter methods. The calculations involving water as a solvent were performed employing polarized continuum model. In the gas phase and in water solution the most stable conformers of the diastereomers were found to be of almost equal energy. The study on interactions between an alkali metal cation and HMR revealed that in the energetically favourable complexes the K+ cation may act as a bridge connecting two molecules to an agglomerate and thus promote nucleation. However, in water it may be energetically more favourable to solvate the metal cations rather than form the cation–HMR complexes. The oxidative reaction of HMR by DDQ through the hydride abstraction mechanism, yielding oxomatairesinol as the major product, is kinetically and thermodynamically more favoured than the reaction yielding 7′,8′-dehydro-7-hydroxymatairesinol. The observed differences in the outcome of the reactions, where the pure epimers of HMR interact with DDQ, can be explained in terms of the stereoelectronic effects.