In the first part of this review, the characteristics of Au–H bonds in gold hydrides are reviewed including the data of recently prepared stable organometallic complexes with gold(I) and gold(III) centers. In the second part, the reports are summarized where authors have tried to provide evidence for hydrogen bonds to gold of the type Au⋯H–X. Such interactions have been proposed for gold atoms in the Au(−I), Au(0), Au(I), and Au(III) oxidation states as hydrogen bonding acceptors and H–X units with X = O, N, C as donors, based on both experimental and quantum chemistry studies. To complement these findings, the literature was screened for examples with similar molecular geometries, for which such bonding has not yet been considered. In the discussion of the results, the recently issued IUPAC definitions of hydrogen bonding and the currently accepted description of agostic interactions have been used as guidelines to rank the Au⋯H–X interactions in this broad range of weak chemical bonding. From the available data it appears that all the intra- and intermolecular Au⋯H–X contacts are associated with very low binding energies and non-specific directionality. To date, the energetics have not been estimated, because there are no thermochemical and very limited IR/Raman and temperature-dependent NMR data that can be used as reliable references. Where conspicuous structural or spectroscopic effects have been observed, explanations other than hydrogen bonding Au⋯H–X can also be advanced in most cases. Although numerous examples of short Au⋯H–X contacts exist in the literature, it seems, at this stage, that these probably make only very minor contributions to the energy of a given system and have only a marginal influence on molecular conformations which so far have most often attracted researchers to this topic. Further, more dedicated investigations will be necessary before well founded conclusions can be drawn.

The gold–hydrogen bond, Au–H, and the hydrogen bond to gold, Au⋯H–X

A joint QM/MM and ab initio study on the decomposition of urea in the gas phase and in aqueous solution is reported. Numerous possible mechanisms of intramolecular decomposition and hydrolysis have been explored; intramolecular NH3 elimination assisted by a water molecule is found to have the lowest activation energy. The solvent effects were elucidated using the TIP4P explicit water model with free energy perturbation calculations in conjunction with QM/MM Monte Carlo simulations. The explicit representation of the solvent was found to be essential for detailed resolution of the mechanism, identification of the rate-determining step, and evaluation of the barrier. The assisting water molecule acts as a hydrogen shuttle for the first step of the elimination reaction. The forming zwitterionic intermediate, H3NCONH, participates in 8−9 hydrogen bonds with water molecules. Its decomposition is found to be the rate-limiting step, and the overall free energy of activation for the decomposition of urea in water...

Why Urea Eliminates Ammonia Rather Than Hydrolyzes in Aqueous Solution

We report the results of theoretical calculations on interaction of the nucleotide bases of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) with a (ZnO)12 cluster, carried out within density-functional theory framework. In all cases, (ZnO)12 prefers to bind with a ring nitrogen atom having a lone electron pair relative to the other possible binding sites of the bases. The degree of hybridization between Zn-d and N-p orbitals determines the relative interaction strength at the N-site of individual nucleobases with (ZnO)12 in contrast to the cases of interaction of metallic clusters and carbon nanostructures with nucleobases where either electrostatic or van der Waals interactions dominates the bonding characteristics of the conjugate complexes. The predicted site-preference of (ZnO)12 toward the nucleobases appears to be similar to that of the metal clusters, which indicates that the metal clusters retain their site-preference even in their oxidized state.

First-Principles Study of Nanoparticle–Biomolecular Interactions: Anchoring of a (ZnO)12 Cluster on Nucleobases

The nucleobase-gold complexes were studied with anion photoelectron spectroscopy and density functional calculations. The vertical detachment energies of uracil-Au−, thymine-Au−, cytosine-Au−, adenine-Au−, and guanine-Au− were estimated to be 3.37 ± 0.08 eV, 3.40 ± 0.08 eV, 3.23 ± 0.08 eV, 3.28 ± 0.08 eV, and 3.43 ± 0.08 eV, respectively, based on their photoelectron spectra. The combination of photoelectron spectroscopy experiments and density functional calculations reveals the presence of two or more isomers for these nucleobase-gold complexes. The major isomers detected in the experiments probably are formed by Au anion with the canonical tautomers of the nucleobases. The gold anion essentially interacts with the nucleobases through N-H···Au hydrogen bonds.

Hydrogen bonds in the nucleobase-gold complexes: photoelectron spectroscopy and density functional calculations.

Density functional approximation (B3LYP) calculations have been used in order to analyze the interaction of gold anionic clusters (Au(n) with n = 2-4) with uracil ([uracil-Au(n)](-1)), adenine ([adenine-Au(n)](-1)), and the adenine-uracil base pair ([uracil-adenine-Au(n)](-1)). It was found that there is only one stable isomer for [uracil-Au(2)](-1) and [uracil-Au(3)](-1), whereas there are two or more stable structures for [uracil-Au(4)](-1), [adenine-Au(n)](-1), and [uracil-adenine-Au(n)](-1). For all these systems, nonconventional hydrogen bonds are formed. Strengthening the bond between anionic gold clusters and the nitrogen base pair weakens the conventional hydrogen bond of the adenine-uracil base pair. This destabilization is not very strong (2-4 kcal/mol) and is at the limits of calculations, but anionic gold clusters bound in between the uracil-adenine base pair indicate that gold anionic clusters may dissociate it. These results will be useful for further experimental studies and may be important for future applications.

Do Anionic Gold Clusters Modify Conventional Hydrogen Bonds? The Interaction of Anionic Aun (n = 2−4) with the Adenine−Uracil Base Pair

Binding of gold nanoclusters with size-expanded DNA bases, xA, xC, xG, and xT, is studied using quantum chemical methods. Geometries of the neutral xA-Au6, xC-Au6, xG-Au6, and xT-Au6 complexes were fully optimized using the B3LYP density functional method (DFT). The gold clusters around xA and xT adopt triangular geometries, whereas irregular structures are obtained in the case of gold clusters complexed around xC and xG. The lengths of the bonds between atoms in the x-bases increase on gold complexation. The aromatic character of the x-bases also increases on gold complexation except for the five-member rings. A significant charge transfer from the x-base to gold atoms is seen in these complexes. Second-order interactions are observed in addition to direct covalent bonds between gold atoms and x-bases.

Binding of Gold Nanoclusters with Size-Expanded DNA Bases: A Computational Study of Structural and Electronic Properties

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Binding of Gold Nanoclusters with Size-Expanded DNA Bases: A Computational Study of Structural and Electronic Properties