Bimetallic silver-gold clusters offer an excellent opportunity to study changes in metallic versus ''ionic'' properties involving charge transfer as a function of the size and the composition, particularly when compared to pure silver and gold clusters. We have determined structures, ionization potentials, and vertical detachment energies for neutral and charged bimetallic Ag m Au n ͓3р(mϩn)р5͔ clusters. Calculated VDE values compare well with available experimental data. In the stable structures of these clusters Au atoms assume positions which favor the charge transfer from Ag atoms. Heteronuclear bonding is usually preferred to homonuclear bonding in clusters with equal numbers of hetero atoms. In fact, stable structures of neutral Ag 2 Au 2 , Ag 3 Au 3 , and Ag 4 Au 4 clusters are characterized by the maximum number of hetero bonds and peripheral positions of Au atoms. Bimetallic tetramer as well as hexamer are planar and have common structural properties with corresponding one-component systems, while Ag 4 Au 4 and Ag 8 have 3D forms in contrast to Au 8 which assumes planar structure. At the density functional level of theory we have shown that this is due to participation of d electrons in bonding of pure Au n clusters while s electrons dominate bonding in pure Ag m as well as in bimetallic clusters. In fact, Au n clusters remain planar for larger sizes than Ag m and Ag n Au n clusters. Segregation between two components in bimetallic systems is not favorable, as shown in the example of Ag 5 Au 5 cluster. We have found that the structures of bimetallic clusters with 20 atoms Ag 10 Au 10 and Ag 12 Au 8 are characterized by negatively charged Au subunits embedded in Ag environment. In the latter case, the shape of Au 8 is related to a pentagonal bipyramid capped by one atom and contains three exposed negatively charged Au atoms. They might be suitable for activating reactions relevant to catalysis. According to our findings the charge transfer in bimetallic clusters is responsible for formation of negatively charged gold subunits which are expected to be reactive, a situation similar to that of gold clusters supported on metal oxides.

Structures and energetics of complexes between the guanine−cytosine Watson−Crick DNA base pair and pentahydrated Mg2+, Ca2+, Sr2+, Ba2+, Zn2+, Cd2+, and Hg2+ metal cations were studied. Comparison has been made with the data for the unsolvated cations. The complexes were fully optimized within the Hartree−Fock approximation applying the 6-31G* basis set of atomic orbitals, while relativistic pseudopotentials were used for the cations except magnesium. The energetics have been studied with the inclusion of electron correlation using the full second-order Møller−Plesset perturbation theory. The cation with its hydration sphere has been considered as one subsystem in the calculations of interaction energy. Thus, the complete system for a calculation would include the hydrated cation−guanine−cytosine trimer. The interaction between hydrated cation and guanine is significantly reduced compared to the guanine−unsolvated cation interaction. Though the stabilizing three-body contribution has been reduced by almost 50% by hydration, it still remains significant. The stability of the guanine−cytosine Watson−Crick base pair is enhanced by ca. 20−30% due to the coordination of the hydrated cation. All the transition metal and Mg2+ cations are tightly bound to the N7 atom of guanine, constituting an octahedral coordination sphere. The Ca2+, Sr2+, and Ba2+ cations are coordinated simultaneously to the N7 and O6 atoms of guanine and the base−cation distance increases with the row number in this series. However, the energy difference between the N7 and N7−O6 types of coordination is rather small. The calculations show a different balance between the transition metal and alkaline earth cations with respect to the cation−base and cation−water interactions. Zn2+ compared to Mg2+ is bound more tightly to the base, and the hydration shell around Zn2+ is more flexible. The replacement of Mg2+ by Zn2+ can be viewed, to some extent, as a shift from the interaction between nucleobase and a hydrated cation toward hydration of a metalated base. This is likely to contribute to the different biological role of Zn2+ and Mg2+.

The interaction of guanine and adenine with ions of groups Ia, Ib, IIa, and IIb were studied at the Hartree−Fock and second-order Møller−Plesset levels employing all-electron (AE) and pseudopotential treatmetns. Christiansen's average relativistic effective pseudopotentials (AREP) were used for all the ions with the exception of Li+. AE and AREP treatments were tested for the Na+, K+, Mg2+, and Ca2+ complexes; very good agreement between the results suggests that pseudopotentials can be used with confidence also for other cations. Intermolecular X−N7 distances for complexes containing adenine are shorter than those for complexes containing guanine. The stabilization energies for guanine···X complexes are larger than those of adenine···X complexes. Relativistic effects are most pronounced for Au+ and Hg2+ ions.

Interaction of Watson−Crick adenine−thymine (AT) and guanine−cytosine (GC) base pairs with various metal (M) cations (Mg2+, ..., Hg2+) were studied by nonempirical ab initio methods with inclusion of correlation energy. Cations were allowed to interact with the N7 nitrogen of adenine and the N7 and O6 atoms of guanine. All of the cations were described by Christiansen's average relativistic effective potentials using the DZ+P basis set, while the 6-31G** basis set was used for the elements of base pairs. Disruption of the adenine−thymine as well as guanine−cytosine pairs in the presence of all studied cations is energetically more demanding than that for isolated base pairs; the addition stabilization of the base pair is about 100% for complexes with dication. The interaction is highly nonadditive. The three-body term is for the MGC complex considerably larger than that for MAT; the intercomplex charge transfer is also much larger for the former complex.

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Hydration of selected platinum complexes [PtCl42−, Pt(NH3)42+, and cis- and trans-platin–PtCl2(NH3)2] have been studied. Up to two solvent molecules have been considered to replace the ligands. In order to be able to draw conclusions about pH changes in the course of the hydration process, both H2O and OH− species were considered in the solvating process. The modified Gaussian 3 theory was adapted for the pseudopotential treatment of platinum complexes. Since a heavy element was present in the complexes, an additional stabilization due to the spin–orbit coupling and core-polarization potentials have been evaluated above the scheme of a G3 treatment. This spin–orbit coupling stabilization amounts to 2–5 kcal/mol but does not qualitatively change the hydration preferences. In accord with the experiment, neutral Pt(NH3)2(OH)2 was found to be the most stable complex for hydration of both cis- and trans-platin.

Enantiomeric excess of chiral compounds is a key parameter that determines their activity or therapeutic action. The current paradigm for rapid measurement of enantiomeric excess using NMR is based on the formation of diastereomeric complexes between the chiral analyte and a chiral resolving agent, leading to (at least) two species with no symmetry relationship. Here we report an effective method of enantiomeric excess determination using a symmetrical achiral molecule as the resolving agent, which is based on the complexation with analyte (in the fast exchange regime) without the formation of diastereomers. The use of N,N′-disubstituted oxoporphyrinogen as the resolving agent makes this novel method extremely versatile, and appropriate for various chiral analytes including carboxylic acids, esters, alcohols and protected amino acids using the same achiral molecule. The model of sensing mechanism exhibits a fundamental linear response between enantiomeric excess and the observed magnitude of induced chemical shift non-equivalence in the 1H NMR spectra.

The mechanism of substitution water exchange reactions in square planar trans-Pt[(NH(3))(2)T(H(2)O)](n+) complexes is studied (T = H(2)O, NH(3), OH(-), F(-), Cl(-), Br(-), H(2)S, CH(3)S(-), SCN(-), CN(-), PH(3), CO, CH(3)(-), H(-), C(2)H(4)). The trans effect is explained in terms of sigma-donation and pi-back-donation whose relative strengths are quantified by the changes of electron occupations of 5d platinum atomic orbitals. The sigma-donation strength is linearly correlated with the Pt-H(2)O (leaving ligand) bond length (trans influence). The kinetic trans effect strength correlates proportionally with the sigma-donation ability of the trans-ligand except the ligands with strong pi-back-donation ability that stabilizes transition state structure. The sigma-donation ability of the ligand is dependent on the sigma-donation strength of the ligand in the trans position. Therefore the trans effect caused by sigma-donation can be understood as a competition between the trans-ligands for the opportunity to donate electron density to the central Pt(II) atom. The influence of the trans effect on the reaction mechanism is also shown. For ligands with a very strong sigma-donation (e.g. CH(3)(-) and H(-)), the substitution proceeds by a dissociative interchange (I(d)) mechanism. Ligands with strong pi-back donation ability (e.g. C(2)H(4)) stabilize the pentacoordinated intermediate and the substitution proceeds by a two step associative mechanism. For ligands with weak sigma-donation and pi-back-donation abilities, the highest activation barriers have to be overcome and substitutions can be described by an associative interchange (I(a)) mechanism. The results are supported by the energy decomposition and the natural orbital analysis.