Search

A molecular level understanding of the aqueous Mg corrosion mechanism will be essential in developing improved alloys for battery electrodes, automobile parts, and biomedical implants. The structure and reactivity of the hydroxylated surface is expected to be key to the overall mechanism because (i) it is predicted to be the metastable surface state (rather than the bare surface) under a range of conditions and (ii) it provides a reasonable model for the outer corrosion film/water interface. We investigate the structure, interactions, and reactivity at the hydroxylated Mg(0001)/water interface using a combination of static Density Functional Theory calculations and second-generation Car–Parrinello ab initio molecular dynamics. We carry out detailed structural analyses into, among other properties, near-surface water orientations, favored adsorption sites, and near-surface hydrogen bonding behavior. Despite the short timescale (tens of ps) of our molecular dynamics run, we observe a cathodic water splitting event; the rapid timescale for this reaction is explained in terms of near-surface water structuring lowering the reaction barrier. Furthermore, we observe oxidation of an Mg surface atom to effectively generate a univalent Mg species (Mg+). Results are discussed in the context of understanding the Mg corrosion mechanism: For example, our results provide an explanation for the catalytic nature of the Mg corrosion film toward water splitting and a feasible mechanism for the generation of the univalent Mg species often proposed as a key intermediate. [ABSTRACT FROM AUTHOR]

A molecular level understanding of metal/bulk water interface structure is key for a wide range of processes, including aqueous corrosion, which is our focus, but their buried nature makes experimental investigation difficult and we must mainly rely on simulations. We investigate the Mg(0001)/water interface using second generation Car–Parrinello molecular dynamics (MD) to gain structural information, combined with static density functional theory calculations to probe the atomic interactions and electronic structure (e.g., calculating the potential of zero charge). By performing detailed structural analyses of both metal–surface atoms and the near-surface water, we find that, among other insights: (i) water adsorption causes significant surface roughening (the planar distribution for top-layer Mg has two peaks separated by ≈ 0.6 A ̊ ), (ii) strongly adsorbed water covers only ≈ 1 4 of available surface sites, and (iii) adsorbed water avoids clustering on the surface. Static calculations are used to gain a deeper understanding of the structuring observed in MD. For example, we use an energy decomposition analysis combined with calculated atomic charges to show that adsorbate clustering is unfavorable due to Coulombic repulsion between adsorption site surface atoms. Results are discussed in the context of previous simulations carried out on other metal/water interfaces. The largest differences for the Mg(0001)/water system appear to be the high degree of surface distortion and the minimal difference between the metal work function and metal/water potential of zero charge (at least compared to other interfaces with similar metal–water interaction strengths). The structural information, in this paper, is important for understanding aqueous Mg corrosion, as the Mg(0001)/water interface is the starting point for key reactions. Furthermore, our focus on understanding the driving forces behind this structuring leads to important insights for general metal/water interfaces. [ABSTRACT FROM AUTHOR]

An ab initio study of the effects of implicit and explicit hosts on the excited state properties of pentacene and its nitrogen-based derivatives has been performed using ground state density functional theory (DFT), time-dependent DFT, and ΔSCF. We observe a significant solvatochromic redshift in the excitation energy of the lowest singlet state (S1) of pentacene from inclusion in a p-terphenyl host compared to vacuum; for an explicit host consisting of six nearest neighbour p-terphenyls, we obtain a redshift of 65 meV while a conductor-like polarisable continuum model (CPCM) yields a 78 meV redshift. Comparison is made between the excitonic properties of pentacene and four of its nitrogen-based analogs, 1,8-, 2,9-, 5,12-, and 6,13-diazapentacene with the latter found to be the most distinct due to local distortions in the ground state electronic structure. We observe that a CPCM is insufficient to fully understand the impact of the host due to the presence of a mild charge-transfer (CT) coupling between the chromophore and neighbouring p-terphenyls, a phenomenon which can only be captured using an explicit model. The strength of this CT interaction increases as the nitrogens are brought closer to the central acene ring of pentacene. [ABSTRACT FROM AUTHOR]