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The composition and thermodynamic stability of the (110) surface of Sn 1 - x Ti x O 2 rutile solid solutions was investigated as a function of Ti-distribution and content up to the formation of a full TiO 2 surface monolayer. The bulk and (110) surface properties of Sn 1 - x Ti x O 2 were compared to that of the pure SnO 2 and TiO 2 crystal. A large supercell of 720 atoms and a localized basis set based on the Gaussian and plane wave scheme allowed the investigation of very low Ti-content and symmetry. For the bulk, optimization of the crystal structure confirmed that up to a Ti-content of 3.3 at.%, the lattice parameters ( a , c ) of SnO 2 do not change. Increasing further the Ti-content decreased both lattice parameters down to those of TiO 2 . The surface energy of these solid solutions did not change for Ti-substitution in the bulk of up to 20 at.%. In contrast, substitution in the surface layer rapidly decreased the surface energy from 0.99 to 0.74 J/m 2 with increasing Ti-content from 0 to 20 at.%. As a result, systems with Ti atoms distributed in the surface (surface enrichment) had always lower energies and thus were thermodynamically more favorable than those with Ti homogeneously distributed in the bulk. This was attributed to the lower energy necessary to break the Ti O bonds than Sn O bonds in the surface layer. In fact, distributing the Ti atoms homogeneously or segregated in the (110) surface led to the same surface energy indicating that restructuring of the surface bond lengths has minimal impact on thermodynamic stability of these rutile systems. As a result, a first theoretical prediction of the composition of Sn 1 - x Ti x O 2 solid solutions is proposed.

A magnetic Pt/SiO2/Fe3O4 catalyst consisting of chirally modified platinum supported on silica coated magnetite nanoparticles was prepared using an easy synthetic route and successfully applied for the enantioselective hydrogenation of various activated ketones. The magnetic catalyst modified with cinchonidine showed a catalytic performance (activity, enantioselectivity) in the asymmetric hydrogenation of various activated ketones in toluene comparable to the best known Pt/alumina catalyst used for these reactions. The novel catalyst can be easily separated from the reaction solution by applying an external magnetic field and recycled several times with almost complete retention of activity and enantioselectivity.

The directing effect of the CF 3 group on Pt/alumina modified by cinchonidine in the enantioselective hydrogenation of activated ketones was studied experimentally and theoretically using aliphatic α , α , α -trifluoromethyl ketones as model compounds. α , α , α -Trifluoromethyl ketones with varying steric hindrances of the groups on the two sides of the keto group (methyl-trifluoromethyl ketone 1a , adamantyl-trifluoromethyl ketone 2a , and tert -butyl-trifluoromethyl ketone 3a ) were examined. The catalytic results show that in weakly polar solvents, in which the interaction of the solvent with the substrate, modifier, and Pt is less important, the three alcohols 1b , 2b , and 3b have the same absolute configuration ( R ). This indicates that enantioselectivity is guided by the trifluoromethyl substitution rather than by the relative bulkiness of the substituents at the two sides of the carbonyl group. The fluorinated substrate has a preferential interaction on the side of the α , α , α -trifluoromethyl group, as corroborated by the calculation of hydrogen–bonding interactions. When this interaction mode is calculated for the system alkaloid–trifluoroacetone, an energy difference between the Pro( R ) and the Pro( S ) configurations is found. Our findings suggest that electronic effects give a bias toward the formation of the ( R )-enantiomer.

The conformations of cinchonidine (CD) adsorbed on a Pt(111) surface were studied using first-principles methods. Eight conformationally different adsorption states due to different degrees of rotation around the τ 1 and τ 2 degrees of freedom were identified and their possible role in the formation of chiral surface sites relevant to enantioselective hydrogenation investigated in light of the currently existing experimental evidence. Comparison of the conformational behavior of CD in solution and on platinum has revealed the effect of the metal surface on the internal mobility of the alkaloid. Although the study corroborates the outstanding role of the adsorbed Open(3) conformer suggested previously, the rich conformational flexibility observed on the platinum surface points to the possibility that other conformers of CD also may be involved in enantiodifferentiation. Closed conformations of CD are found to play an important role in the conformational equilibria on the surface due to their stability and are identified as precursors of the less stable, but presumably more active, open conformers. Although the open and closed conformers are closely related to the correspondent ones found in solution, surface species that are also adsorbed via quinuclidine moiety are characteristic of the metal–modifier interaction and should serve as precursors to catalytically active conformations.

The adsorption mode of cinchonidine on platinum is discussed in the light of new computational studies in which the alkaloid is adsorbed on a large metal cluster. Previous computations focused on the role of 1(S)-(4-quinolinyl)ethanol as anchoring group, but in the present study the role of the quinuclidine ring is analyzed, leading to a consistent view of the roles of both moieties of the alkaloid in the adsorption process. The tertiary nitrogen of the quinuclidine moiety can participate in the anchoring of the alkaloid and can be protonated by surface hydrogen. The conformational flexibility of the quinuclidine moiety was investigated by attenuated total reflection infrared experiments under nearly in situ conditions, by comparing the adsorption behavior of cinchonidine and O -phenyl-cinchonidine on platinum. Close agreement was found between experimental observations and theoretical calculations. A mechanism is proposed whereby the tertiary nitrogen can promote charge polarization of hydrogen and its transfer to the substrate.

The adsorption of several ketones interesting for the enantioselective hydrogenation on cinchona-modified platinum has been modeled using relativistically corrected density functional theory. Two metal clusters, containing 19 and 31 Pt atoms, respectively, have been used to model a Pt(111) surface. The two adsorption modes η1 and η2 have been described, and their importance for the mechanism of hydrogenation has been pointed out. The effect of an ester group in α position and of α-fluorination of a ketone on its adsorption has been studied, and an explanation for the reactivity enhancement due to the ketone substitution has been proposed.

The relation between the electronic structure of α-substituted ketones and their reactivity in the racemic and enantioselective platinum-catalyzed hydrogenation has been investigated using a combined theoretical and experimental approach. A correlation between the keto carbonyl orbital energy and the hydrogenation rate has been found, which rationalizes the effect of the substituent on the rate of hydrogenation. The uncovered relationship between the keto carbonyl orbital energy and the hydrogenation rate provides a rational explanation for the often observed rate acceleration that occurs when cinchona-modified platinum is used as a enantioselective hydrogenation catalyst. The previously suggested model for enantiodiscrimination based on the different stability of the diastereomeric complexes formed between the reactant and the cinchona modifier is discussed in the light of the new kinetic findings.

The previously proposed model for reactant–modifier interaction in the enantioselective hydrogenation of activated carbonyl compounds over platinum chirally modified by cinchona alkaloids has been extended to platinum modified by synthetic pyrrolidinyl–naphthyl–ethanol modifiers. As in the case of cinchonidine, the most used modifier, the model predicts enantiomeric excess in nearly quantitative agreement with experiment. Excellent agreement is achieved despite the fact that structural assumptions had to be made and the platinum surface was not explicitly taken into account. The one-to-one interaction between modifier and reactant was calculated at the ab initio level. A comparison of the results for different modifiers leads to the conclusion that steric repulsion caused by the anchoring group plays an important role in the enantiodifferentiating interaction. The favoured formation of the (R)-product is traced to the fact that the pro-(S) complex leading to the (S)-product upon hydrogenation is more destabilised due to repulsive interaction than the pro-(R) complex. The model calculations are a useful tool for designing effective modifiers and for gaining insight into the mechanism of enantiodifferentiation.