Molecular Engineering to Tune Functionality: The Case of Cl-Substituted [Fe(terpy)].

The properties of transition-metal complexes and their chemical dynamics can be effectively modified with ligand substitutions, and theory can be a great aid to such molecular engineering. In this paper, we first theoretically explored how substitution with a Cl atom at different positions of the terpyridine ligand affects the electronic structure of the [Fe(terpy)] complex. We found that besides the substitution at position 4', the next most promising candidate to cause substantial electronic effects is that where the side pyridine ring is substituted at position 5 (β).

Investigation of the Deactivation and Reactivation Mechanism of a Heterogeneous Palladium(II) Catalyst in the Cycloisomerization of Acetylenic Acids by XAS .

A well-studied heterogeneous palladium(II) catalyst used for the cycloisomerization of acetylenic acids is known to be susceptible to deactivation through reduction. To gain a deeper understanding of this deactivation process and to enable the design of a reactivation strategy, X-ray absorption spectroscopy (XAS) was used. With this technique, changes in the palladium oxidation state and coordination environment could be studied in close detail, which provided experimental evidence that the deactivation was primarily caused by triethylamine-promoted reduction of palladium(II) to metallic palladium nanoparticles.

Site-Selective Real-Time Observation of Bimolecular Electron Transfer in a Photocatalytic System Using L-Edge X-Ray Absorption Spectroscopy*.

Time-resolved X-ray absorption spectroscopy has been utilized to monitor the bimolecular electron transfer in a photocatalytic water splitting system. This has been possible by uniting the local probe and element specific character of X-ray transitions with insights from high-level ab initio calculations. The specific target has been a heteroleptic [Ir (ppy) (bpy)] photosensitizer, in combination with triethylamine as a sacrificial reductant and as a water reduction catalyst.

EXAFS Study on the Coordination Chemistry of the Solvated Copper(II) Ion in a Series of Oxygen Donor Solvents.

The structures of the solvated copper(II) ion in water and nine organic oxygen donor solvents with similar electron-pair donor ability, but with different space-demanding properties at coordination, have been studied by EXAFS. -Dimethylpropyleneurea and -tetramethylurea are sufficiently space demanding at coordination to make the axial positions not accessible, resulting in square-planar copper(II) solvate complexes with an intense green color. The mean Cu-O bond distances in these two solvate complexes are 1.

Vibrational wavepacket dynamics in Fe carbene photosensitizer determined with femtosecond X-ray emission and scattering.

The non-equilibrium dynamics of electrons and nuclei govern the function of photoactive materials. Disentangling these dynamics remains a critical goal for understanding photoactive materials. Here we investigate the photoinduced dynamics of the [Fe(bmip)] photosensitizer, where bmip = 2,6-bis(3-methyl-imidazole-1-ylidine)-pyridine, with simultaneous femtosecond-resolution Fe Kα and Kβ X-ray emission spectroscopy (XES) and X-ray solution scattering (XSS).

In Situ Structural Determination of a Homogeneous Ruthenium Racemization Catalyst and Its Activated Intermediates Using X-Ray Absorption Spectroscopy.

The activation process of a known Ru-catalyst, dicarbonyl(pentaphenylcyclopentadienyl)ruthenium chloride, has been studied in detail using time resolved in situ X-ray absorption spectroscopy. The data provide bond lengths of the species involved in the process as well as information about bond formation and bond breaking. On addition of potassium tert-butoxide, the catalyst is activated and an alkoxide complex is formed.

Highly Porous Amorphous Calcium Phosphate for Drug Delivery and Bio-Medical Applications.

Amorphous calcium phosphate (ACP) has shown significant effects on the biomineralization and promising applications in bio-medicine. However, the limited stability and porosity of ACP material restrict its practical applications. A storage stable highly porous ACP with Brunauer-Emmett-Teller surface area of over 400 m/g was synthesized by introducing phosphoric acid to a methanol suspension containing amorphous calcium carbonate nanoparticles.

Hot Branching Dynamics in a Light-Harvesting Iron Carbene Complex Revealed by Ultrafast X-ray Emission Spectroscopy.

Iron N-heterocyclic carbene (NHC) complexes have received a great deal of attention recently because of their growing potential as light sensitizers or photocatalysts. We present a sub-ps X-ray spectroscopy study of an Fe NHC complex that identifies and quantifies the states involved in the deactivation cascade after light absorption. Excited molecules relax back to the ground state along two pathways: After population of a hot MLCT state, from the initially excited MLCT state, 30 % of the molecules undergo ultrafast (150 fs) relaxation to the MC state, in competition with vibrational relaxation and cooling to the relaxed MLCT state.

Finding intersections between electronic excited state potential energy surfaces with simultaneous ultrafast X-ray scattering and spectroscopy.

Light-driven molecular reactions are dictated by the excited state potential energy landscape, depending critically on the location of conical intersections and intersystem crossing points between potential surfaces where non-adiabatic effects govern transition probabilities between distinct electronic states. While ultrafast studies have provided significant insight into electronic excited state reaction dynamics, experimental approaches for identifying and characterizing intersections and seams between electronic states remain highly system dependent. Here we show that for 3d transition metal systems simultaneously recorded X-ray diffuse scattering and X-ray emission spectroscopy at sub-70 femtosecond time-resolution provide a solid experimental foundation for determining the mechanistic details of excited state reactions.

Using Ultrafast X-ray Spectroscopy To Address Questions in Ligand-Field Theory: The Excited State Spin and Structure of [Fe(dcpp)].

We have employed a range of ultrafast X-ray spectroscopies in an effort to characterize the lowest energy excited state of [Fe(dcpp)] (where dcpp is 2,6-(dicarboxypyridyl)pyridine). This compound exhibits an unusually short excited-state lifetime for a low-spin Fe(II) polypyridyl complex of 270 ps in a room-temperature fluid solution, raising questions as to whether the ligand-field strength of dcpp had pushed this system beyond the T/T crossing point and stabilizing the latter as the lowest energy excited state. Kα and Kβ X-ray emission spectroscopies have been used to unambiguously determine the quintet spin multiplicity of the long-lived excited state, thereby establishing the T state as the lowest energy excited state of this compound.

Chemical equilibria of aqueous ammonium-carboxylate systems in aqueous bulk, close to and at the water-air interface.

Previous studies have shown that the water-air interface and a number of water molecule layers just below it, the surface region, have significantly different physico-chemical properties, such as lower relative permittivity and density, than bulk water. The properties in the surface region of water favor weakly hydrated species as neutral molecules, while ions requiring strong hydration and shielding of their charge are disfavored. In this study the equilibria NH(aq) + RCOO(aq) ⇌ NH(aq) + RCOOH(aq) are investigated for R = CH, n = 0-8, as open systems, where ammonia and small carboxylic acids in the gas phase above the water surface are removed from the system by a gentle controlled flow of nitrogen to mimic the transport of volatile compounds from water droplets into air.

Laboratory EXAFS determined structure of the stable complexes in the ternary Ni(ii)-EDTA-CN system.

Aqueous solutions of the ternary system Ni(ii)-EDTA-CN- are investigated with X-ray Absorption Spectroscopy (XAS) as a function of cyanide concentration with an enhanced laboratory von Hámos X-ray spectrometer. The near-edge structure of the spectra identifies unambiguously the formation of the pentacyanidonickel(ii) complex at excess CN- concentrations. An analysis of the extended energy range of the XAS spectra reveals the molecular structure of the distinct molecular components present and provides a detailed description of the barely detectable mixed ligand [NiEDTA(CN)]3- complex.

Amorphous Calcium Carbonate Constructed from Nanoparticle Aggregates with Unprecedented Surface Area and Mesoporosity.

Amorphous calcium carbonate (ACC), with the highest reported specific surface area of all current forms of calcium carbonate (over 350 m g), was synthesized using a surfactant-free, one-pot method. Electron microscopy, helium pycnometry, and nitrogen sorption analysis revealed that this highly mesoporous ACC, with a pore volume of ∼0.86 cm g and a pore-size distribution centered at 8-9 nm, is constructed from aggregated ACC nanoparticles with an estimated average diameter of 7.

Solvent control of charge transfer excited state relaxation pathways in [Fe(2,2'-bipyridine)(CN)].

The excited state dynamics of solvated [Fe(bpy)(CN)], where bpy = 2,2'-bipyridine, show significant sensitivity to the solvent Lewis acidity. Using a combination of optical absorption and X-ray emission transient spectroscopies, we have previously shown that the metal to ligand charge transfer (MLCT) excited state of [Fe(bpy)(CN)] has a 19 picosecond lifetime and no discernable contribution from metal centered (MC) states in weak Lewis acid solvents, such as dimethyl sulfoxide and acetonitrile. In the present work, we use the same combination of spectroscopic techniques to measure the MLCT excited state relaxation dynamics of [Fe(bpy)(CN)] in water, a strong Lewis acid solvent.

Simultaneous Speciation, Structure, and Equilibrium Constant Determination in the Ni-EDTA-CN Ternary System via High-Resolution Laboratory X-ray Absorption Fine Structure Spectroscopy and Theoretical Calculations.

Even quite simple chemical systems can involve many components and chemical states, and sometimes it can be very difficult to differentiate them by their hardly separable physical-chemical properties. The Ni-EDTA-CN (EDTA = ethylenediaminetetraacetic acid) ternary system is a good example for this problem where, in spite of its fairly simple components and numerous investigations, several molecular combinations can exist, all of them not having been identified unambiguously yet. In order to achieve a detailed understanding of the reaction steps and chemical equilibria, methods are required in which the structural transitions in the different reaction steps can be followed via element-selective complex spectral feature sets.

Laboratory von Hámos X-ray spectroscopy for routine sample characterization.

High energy resolution, hard X-ray spectroscopies are powerful element selective probes of the electronic and local structure of matter, with diverse applications in chemistry, physics, biology, and materials science. The routine application of these techniques is hindered by the complicated and slow access to synchrotron radiation facilities. Here we propose a new, economic, easily operated laboratory high resolution von Hámos type X-ray spectrometer, which offers rapid transmission experiments for X-ray absorption and is also capable of recording X-ray emission spectra.

Speciation and structure of tin(II) in hyper-alkaline aqueous solution.

The identity of the predominating tin(ii)-hydroxide complex formed in hyper-alkaline aqueous solutions (0.2 ≤CNaOH≤ 12 mol dm(-3)) is determined by potentiometric titrations, Raman, Mössbauer and XANES spectroscopy, supplemented by quantum chemical calculations. Thermodynamic studies using a H2/Pt electrode up to free hydroxide concentrations of 1 mol dm(-3) showed the presence of a single monomeric complex with a tin(II) : hydroxide ratio of 1 : 3.

Speciation and the structure of lead(II) in hyper-alkaline aqueous solution.

The identity of the predominating lead(ii) species in hyper-alkaline aqueous solution has been determined by Raman spectroscopy, and ab initio quantum chemical calculations and its structure has been determined by EXAFS. The observed and calculated Raman spectra for the [Pb(OH)3](-) complex are in agreement while they are different for two-coordinated complexes and complexes containing Pb[double bond, length as m-dash]O double bonds. Predicted bond lengths are also consistent with the presence of [Pb(OH)3](-) and exclude the formation of Pb[double bond, length as m-dash]O double bond(s).

Multinuclear complex formation between Ca(II) and gluconate ions in hyperalkaline solutions.

Alkaline solutions containing polyhydroxy carboxylates and Ca(II) are typical in cementitious radioactive waste repositories. Gluconate (Gluc(-)) is a structural and functional representative of these sugar carboxylates. In the current study, the structure and equilibria of complexes forming in such strongly alkaline solutions containing Ca(2+) and gluconate have been studied.