DFT Study of the BH4− Hydrolysis on Au(111) Surface

The mechanism of the catalytic hydrolysis of BH₄⁻ on Au(111) as studied by DFT is reported. The results are compared to the analogous process on Ag(111) that was recently reported. It is found that the borohydride species are adsorbed stronger on the Au⁰-NP surface than on the Ag⁰-NP surface. The electron affinity of the Au is larger than that of Ag. The results indicate that only two steps of hydrolysis are happening on the Au(111) surface and the reaction mechanism differs significantly from that on the Ag(111) surface. These remarkable results were experimentally verified. Upon hydrolysis, only three hydrogens of BH₄⁻ are transferred to the Au surface, not all four, and H₂ generation is enhanced in the presence of surface H atoms. Thus, it is proposed that the BH₄⁻ hydrolysis and reduction mechanisms catalyzed by M⁰-NPs depend considerably on the nature of the metal.     

Hydride Transfer to Gold: Yes or No? Exploring the Unexpected Versatility of Au⋅⋅⋅H−M Bonding in Heterobimetallic Dihydrides

The potential for coordination and H-transfer from Cp₂MH₂ (M=Zr, W) to gold(I) and gold(III) complexes was explored in a combined experimental and computational study. [(L)Au]⁺ cations react with Cp₂WH₂ giving [(L)Au(κ²-H₂WCp₂)]⁺ (L=IPr ( 1 ), cyclic (alkyl)(amino)carbene ( 2 ), PPh₃ ( 3 ) and Dalphos-Me ( 4 ) [IPr=1,3-bis(diisopropylphenyl)imidazolylidene; Dalphos-Me=di(1-adamantyl)-2-(dimethylamino)phenyl-phosphine], while [Au(DMAP)₂]⁺ (DMAP=p-dimethylaminopyridine) affords the C₂-symmetric [Au(κ-H₂WCp₂)₂]⁺ ( 5 ). The Dalphos complex 4 can be protonated to give the bicationic adduct 4 H, showing Au^I⋅⋅⋅H⁺−N hydrogen bonding. The gold(III) Lewis acid [(C^N−CH)Au(C₆F₅)(OEt₂)]⁺ binds Cp₂WH₂ to give an Au-H-W σ-complex. By contrast, the pincer species [(C^N^C)Au]⁺ adds Cp₂WH₂ by a purely dative W→Au bond, without Au⋅⋅⋅H interaction. The biphenylyl-based chelate [(C^C)Au]⁺ forms [(C^C)Au(μ-H)₂WCp₂]⁺, with two 2-electron-3-centre W−H⋅⋅⋅Au interactions and practically no Au−W donor acceptor contribution. In all these complexes, strong but polarized W−H bonds are maintained, without H-transfer to gold. On the other hand, the reactions of Cp₂ZrH₂ with gold complexes led in all cases to rapid H-transfer and formation of gold hydrides. Relativistic DFT calculations were used to rationalize the striking reactivity and bonding differences in these heterobimetallic hydride complexes along with an analysis of their characteristic NMR parameters and UV/Vis absorption properties.     

Parameterization of peptide 13C carbonyl chemical shielding anisotropy in molecular dynamics simulations.

NMR chemical shielding anisotropy (CSA) relaxation is an important tool in the study of dynamical processes in proteins and nucleic acids in solution. Herein, we investigate how dynamical variations in local geometry affect the chemical shielding anisotropy relaxation of the carbonyl carbon nucleus, using the following protocol: 1) Using density functional theory, the carbonyl (13)C' CSA is computed for 103 conformations of the model peptide group N-methylacetamide (NMA). 2) The variations in computed (13)C' CSA parameters are fitted against quadratic hypersurfaces containing cross terms between the variables. 3) The predictive quality of the CSA hypersurfaces is validated by comparing the predicted and de novo calculated (13)C' CSAs for 20 molecular dynamics snapshots. 4) The CSA fluctuations and their autocorrelation and cross correlation functions due to bond-length and bond-angle distortions are predicted for a chemistry Harvard molecular mechanics (CHARMM) molecular dynamics trajectory of Ca(2+)-saturated calmodulin and GB3 from the hypersurfaces, as well as for a molecular dynamics (MD) simulation of an NMA trimer using a quantum mechanically correct forcefield. We find that the fluctuations can be represented by a 0.93 scaling factor of the CSA tensor for both R(1) and R(2) relaxations for residues in helix, coil, and sheet alike. This result is important, as it establishes that (13)C' relaxation is a valid tool for measurement of interesting dynamical events in proteins.     

Density Functional Investigations of Electronic Structure and Dehydrogenation Reactions of Al‐ and Si‐Substituted Magnesium Hydride

The effect on the hydrogen storage attributes of magnesium hydride (MgH₂) of the substitution of Mg by varying fractions of Al and Si is investigated by an ab initio plane‐wave pseuodopotential method based on density functional theory. Three supercells, namely, 2×2×2, 3×1×1 and 5×1×1 are used for generating configurations with varying amounts (fractions x=0.0625, 0.1, and 0.167) of impurities. The analyses of band structure and density of states (DOS) show that, when a Mg atom is replaced by Al, the band gap vanishes as the extra electron occupies the conduction band minimum. In the case of Si‐substitution, additional states are generated within the band gap of pure MgH₂—significantly reducing the gap in the process. The reduced band gaps cause the Mg? H bond to become more susceptible to dissociation. For all the fractions, the calculated reaction energies for the stepwise removal of H₂ molecules from Al‐ and Si‐substituted MgH₂ are much lower than for H₂ removal from pure MgH₂. The reduced stability is also reflected in the comparatively smaller heats of formation (ΔH_f) of the substituted MgH₂ systems. Si causes greater destabilization of MgH₂ than Al for each x. For fractions x=0.167 of Al, x=0.1, 0.167 of Si (FCC) and x=0.0625, 0.1 of Si (diamond), ΔH_f is much less than that of MgH₂ substituted by a fraction x=0.2 of Ti (Y. Song, Z. X. Guo, R. Yang, Mat. Sc. & Eng. A 2004 , 365, 73). Hence, we suggest the use of Al or Si instead of Ti as an agent for decreasing the dehydrogenation reaction and energy, consequently, the dehydrogenation temperature of MgH₂, thereby improving its potential as a hydrogen storage material.     

Solvent‐Dependent Structure of Iridium Dihydride Complexes: Different Geometries at Low and High Dielectricity of the Medium

The hydride iridium pincer complex [(PCyP)IrH₂] (PCyP=cis‐1,3‐bis[(di‐tert‐butylphosphino)methyl]cyclohexane, 1 ) reveals remarkably solvent‐dependent hydride chemical shifts, isotope chemical shifts, J_HD and T₁(min), with r_HH increasing upon moving to more polar medium. The only known example of such behaviour (complex [(POCOP)IrH₂], POCOP=2,6‐(tBu₂PO)₂C₆H₃) was explained by the coordination of a polar solvent molecule to the iridium (J. Am. Chem. Soc. 2006 , 128, 17114). Based on the existence of an agostic bond between α‐C?H and iridium in 1 in all solvents, we argue that the coordination of solvent can be rejected. DFT calculations revealed that the structures of 1 and [(POCOP)IrH₂] depend on the dielectric permittivity of the medium and these compounds adopt trigonal‐bipyramidal geometries in non‐polar media and square‐pyramidal geometries in polar media.