Kinetic Rates and Linear Free Energy Relationships for Water Dissociation on Transition and Noble Metal Dimers

Photochemistry of Fe:H2O Adducts in Argon Matrixes: A Combined Experimental and Theoretical Study in the Mid-IR and UV-Visible Regions

The photochemistry of Fe:H₂O adducts is of interest in fields as diverse as catalysis and astrochemistry. Industrially, iron can be used as a catalyst to convert H₂O to H₂, whereas in the interstellar medium it may be an important component of dust grains, influencing the chemistry on their icy surfaces. This study consisted of the deposition and spectral characterization of binary systems of atomic iron with H₂O in cryogenic argon matrixes. In this way, we were able to obtain information about the interaction of the two species; we observed the formation of adducts of iron monomers and dimers with water molecules in the mid-IR and UV-visible spectral domains. Upon irradiation with a UV radiation source, the iron species were inserted into the water molecules to form HFeOH and HFe₂OH, leading in some cases to the formation of FeO possibly accompanied by the production of H₂. DFT and correlated multireference wave function calculations confirmed our attributions. This combination of IR and UV-visible spectroscopy with theoretical calculations allowed us to determine, for the first time, the spectral characteristics of iron adducts and their photoproducts in the UV-visible and in the OH stretching region of the mid-IR domain.

Establishing and Understanding Adsorption-Energy Scaling Relations with Negative Slopes

Adsorption-energy scaling relations are widely used for the design of catalytic materials. To date, only linear scaling relations are known in which the slopes are positive. Considering the adsorption energies of F, O, N, C, and B on transition metals, we show here that scaling relations with negative slopes also exist between certain adsorbates. The origin of such unconventional scaling relations is analyzed in terms of common descriptors such as d-band center, work function, number of outer electrons, electronic charge on the adsorbates, integrated crystal orbital overlap populations, and crystal orbital Hamilton populations. Conventional scaling relations are formed between adsorbates such as F, O, N, and C, which create ionic-like bonds with surfaces. Conversely, anomalous scaling relations are established between those and covalently bound adsorbates such as B. This widens the theory of adsorption-energy scaling relations and opens new avenues in physical chemistry and catalysis, for instance, in direct borohydride fuel cells.

Water Dissociation upon Adsorption onto Free Iron Clusters Is Size Dependent

Cationic iron clusters, produced through laser ablation and subsequently complexed with a water molecule Fen(+)-H2O (n = 6-15) are mass-selectively investigated via infrared multiple photon dissociation (IR-MPD) spectroscopy in the 300-1700 cm(-1) spectral range. The experimental data are complemented by density functional theory calculations at the OPBE/TZP level for the Fe13(+)-H2O system. The observed spectra can be explained by a mixture of clusters where for a majority water is adsorbed molecularly but for a small but significant fraction also dissociation of water molecules occurs. The bands observed at frequencies 300-700 cm(-1) exhibit regular, size-dependent frequency shifts, showing that (a) dissociation takes places on all cluster sizes and (b) the interaction of water with the cluster surface is not influenced much by the particular cluster structure. The intensity evolution of the absorption bands suggests that dissociation is increasingly probable for larger cluster sizes.

Dissociative addition of water to a main group 13 metal cluster: computational study of the reaction of gallium dimer with H2O

We have investigated the lowest triplet and singlet potential energy surfaces (PESs) for the reaction of Ga(2) dimer with water. Under thermal conditions, we predict formation of the triplet ground state addition complex Ga(2)···OH(2)((3)B(1)) involving Ga···O···Ga bridge interaction. At the coupled cluster CCSD(T)/AE (CCSD(T)/ECP) computational levels, Ga(2)···OH(2)((3)B(1)) is bound by 5.5 (5.7) kcal/mol with respect to the ground state reactants Ga(2)((3)Π(u)) + H(2)O. Identification of the addition complex is in agreement with the experimental evidence from matrix isolation infrared (IR) spectroscopy reported recently by Macrae and Downs. The located minimum energy crossing points (MECPs) between the triplet and singlet energy surfaces on the entrance channel of Ga(2) + H(2)O are not expected to be energetically accessible under the matrix conditions, consistent with the lack of occurrence of Ga(2) insertion into the O-H bond under such conditions. The computed energies and harmonic and anharmonic vibrational frequencies for the triplet and singlet Ga(2)(H)(OH) insertion isomers indicate the singlet double-bridged Ga(μ-H)(μ-OH)Ga isomer to be the most stable and support the experimental IR identification of this species. The energy barrier for elimination of H(2) from the second most stable singlet HGa(μ-OH)Ga insertion isomer found to be 13.9 (12.9) kcal/mol is also consistent with the available experimental data.