The scalable and sustainable production of hydrogen fuel through water splitting demands efficient and robust Earth-abundant catalysts for the hydrogen evolution reaction (HER). Building on promising metal compounds with high HER catalytic activity, such as pyrite structure cobalt disulphide (CoS2), and substituting non-metal elements to tune the hydrogen adsorption free energy could lead to further improvements in catalytic activity. Here we present a combined theoretical and experimental study to establish ternary pyrite-type cobalt phosphosulphide (CoPS) as a high-performance Earth-abundant catalyst for electrochemical and photoelectrochemical hydrogen production. Nanostructured CoPS electrodes achieved a geometrical catalytic current density of 10 mA cm(-2) at overpotentials as low as 48 mV, with outstanding long-term operational stability. Integrated photocathodes of CoPS on n(+)-p-p(+) silicon micropyramids achieved photocurrents up to 35 mA cm(-2) at 0 V versus the reversible hydrogen electrode (RHE), onset photovoltages as high as 450 mV versus RHE, and the most efficient solar-driven hydrogen generation from Earth-abundant systems.

X-ray and neutron diffractions, vibrational spectroscopy, and x-ray Raman scattering and absorption experiments on water are often interpreted in terms of hydrogen bonding. To this end a number of geometric definitions of hydrogen bonding in water have been developed. While all definitions of hydrogen bonding are to some extent arbitrary, those involving one distance and one angle for a given water dimer are unnecessarily so. In this paper the authors develop a systematic procedure based on two-dimensional potentials of mean force for defining cutoffs for a given pair of distance and angular coordinates. They also develop an electronic structure-based definition of hydrogen bonding in liquid water, related to the electronic occupancy of the antibonding OH orbitals. This definition turns out to be reasonably compatible with one of the distance-angle geometric definitions. These two definitions lead to an estimate of the number of hydrogen bonds per molecule in liquid simple point charge/extended (SPC/E) water of between 3.2 and 3.4. They also used these and other hydrogen-bond definitions to examine the dynamics of local hydrogen-bond number fluctuations, finding an approximate long-time decay constant for SPC/E water of between 0.8 and 0.9 ps, which corresponds to the time scale for local structural relaxation.

We present improvements on our previous approaches for calculating vibrational spectroscopy observables for the OH stretch region of dilute HOD in liquid D2O. These revised approaches are implemented to calculate IR and isotropic Raman spectra, using the SPC/E simulation model, and the results are in good agreement with experiment. We also calculate observables associated with three-pulse IR echoes: the peak shift and 2D-IR spectrum. The agreement with experiment for the former is improved over our previous calculations, but discrepancies between theory and experiment still exist. Using our proposed definition for hydrogen bonding in liquid water, we decompose the distribution of frequencies in the OH stretch region in terms of subensembles of HOD molecules with different local hydrogen-bonding environments. Such a decomposition allows us to make the connection with experiments and calculations on water clusters and more generally to understand the extent of the relationship between transition frequency and local structure in the liquid.vibrational spectroscopy ͉ water W ater is ubiquitous in science and nature (1), so it is natural that a tremendous amount of effort has been expended trying to describe and understand the structure and dynamics of its liquid state. Vibrational spectroscopy, both IR and Raman, provides an excellent probe of the local structure in water, because a local mode's vibrational frequency is exquisitely sensitive to the local mode's molecular environment. Actually, the cleanest information about local structure in water comes from the vibrational spectroscopy not of neat water, but rather of dilute HOD in either H 2 O or D 2 O, because in these cases, respectively, the OD or OH local-mode stretch is almost completely decoupled from the other stretches in the liquid, thus functioning well as a local chromophore. IR and Raman spectra on these systems have been measured by many (2-9).Valuable information about local dynamics in liquid water can also be obtained from vibrational spectroscopy experiments, in this case of the subpicosecond time-domain variety. On this time scale a local mode's vibrational frequency is continually changing because of molecular dynamics. The resulting dynamic frequency fluctuations, also known as spectral diffusion, can be measured by transient vibrational hole-burning and three-pulse echoes (5,6,(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22). In particular, these experiments provide information about both the short-time (stretching) and longtime (making and breaking) aspects of intermolecular hydrogen bonds (23-35).We and others have developed methods for the theoretical calculation of steady-state and ultrafast vibrational spectroscopy observables (7,(23)(24)(25)(26)(27)(36)(37)(38)(39)(40). In our approach, the single vibrational mode of interest, for example, the OH stretch of HOD (when it is immersed in D 2 O), is treated quantum mechanically, whereas all other degrees of freedom (the bath) are treated classically. Thus we are making the adiabatic app...

In the context of vibrational spectroscopy in liquids, non-Condon effects refer to the dependence of the vibrational transition dipole moment of a particular molecule on the rotational and translational coordinates of all the molecules in the liquid. For strongly hydrogen-bonded systems, such as liquid water, non-Condon effects are large. That is, the bond dipole derivative of an OH stretch depends strongly on its hydrogen-bonding environment. Previous calculations of nonlinear vibrational spectroscopy in liquids have not included these non-Condon effects. We find that for water, inclusion of these effects is important for an accurate calculation of, for example, homodyned and heterodyned three-pulse echoes. Such echo experiments have been "inverted" to obtain the OH stretch frequency time-correlation function, but by necessity the Condon and other approximations are made in this inversion procedure. Our conclusion is that for water, primarily because of strong non-Condon effects, this inversion may not lead to the correct frequency time-correlation function. Nevertheless, one can still make comparison between theory and experiment by calculating the experimental echo observables themselves.

The vast chemical and structural diversity of metal-organic frameworks (MOFs) opens up the exciting possibility of "crystal engineering" MOFs tailored for particular catalytic or separation applications. Yet the process of reaction discovery, optimization, and scale-up of MOF synthesis remains extremely challenging, presenting significant obstacles to the synthetic realization of many otherwise promising MOF structures. Recently, significant new insights into the fundamental processes governing MOF nucleation and growth, as well as the relationship between reaction parameters and synthetic outcome, have been derived using powerful in situ, time-resolved and/or mechanistic studies of MOF crystallization. This Review provides a summary and associated critical analysis of the results of these and other related "direct" studies of MOF nucleation and growth, with a particular emphasis on the recent advances in instrument technologies that have enabled such studies and on the major hypotheses, theories, and models that have been used to explain MOF formation. We conclude with a summary of the major insights that have been gained from the work summarized in this Review, outlining our own perspective on potential fruitful new directions for investigation.

Kwac and Cho [J. Chem. Phys. 119, 2247 (2003)] have recently developed a combined electronic structure/molecular dynamics approach to vibrational spectroscopy in liquids. The method involves fitting ab initio vibrational frequencies for a solute in a cluster of solvent molecules to a linear combination of the electrostatic potentials on the solute atoms due to the charges on the solvent molecules. These authors applied their method to the N-methylacetamide-D/D(2)O system. We (S. A. Corcelli, C. P. Lawrence, and J. L. Skinner, [J. Chem. Phys. 120, 8107 (2004)]) have recently explored a closely related method, where instead of the electrostatic potential, the solute vibrational frequencies are fit to the components of the electric fields on the solute atoms due to the solvent molecules. We applied our method to the HOD/D(2)O and HOD/H(2)O systems. In order to make a direct comparison of these two approaches, in this paper we apply their method to the water system, and our method to the N-methylacetamide system. For the water system we find that the electric field method is superior to the potential approach, as judged by comparison with experiments for the absorption line shape. For the N-methylacetamide system the two methods are comparable.

We present a general methodology for generating accurate and transferable ab initio force fields, employing the framework of symmetry adapted perturbation theory (SAPT). The resulting force fields are "physically-motivated" in that they contain separate, explicit terms to account for the various fundamental intermolecular interactions, such as exchange, electrostatics, induction, and dispersion, with each term parametrized to a corresponding term in the SAPT energy decomposition. Crucially, the resulting force fields are largely compatible with existing, standard simulation packages, requiring only minimal modifications. We present several novel parametrization techniques that yield robust, physically meaningful atomic parameters that are transferable between molecular environments. We demonstrate the accuracy and generality of our method by validating against experimental second virial coefficients for a variety of small molecules. We then show that the resulting atomic parameters can be combined using physically motivated ansatzes to accurately predict arbitrary heteromolecular interaction energies, with example applications including prediction of gas adsorption in functionalized metal-organic framework materials.

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