Open-system theory of photodetection

The transfer of the statistical properties of the electromagnetic field to the detector signal is reinvesti?ated in terms of the Dicke model with coupling of the photosensitive atoms to a reservoir. The interrelated dynamics of the atomic moments and the field moments as well as the photocounting rate for any initial field state are given in terms of not only the field-atom, but also the atom-reservoir, coupling constants.     

Thiodiacetate-manganese chemistry with N ligands: unique control of the supramolecular arrangement over the metal coordination mode

Compounds based on the Mn-tda unit (tda=S(CH(2)COO)(2)(-2) ) and N co-ligands have been analyzed in terms of structural, spectroscopic, magnetic properties and DFT calculations. The precursors [Mn(tda)(H(2)O)](n) (1) and [Mn(tda)(H(2)O)(3)]·H(2)O (2) have been characterized by powder and X-ray diffraction, respectively. Their derivatives with bipyridyl-type ligands have formulas [Mn(tda)(bipy)](n) (3), [{Mn(N-N)}(2)(μ-H(2)O)(μ-tda)(2)](n) (N-N=4,4'-Me(2)bipy (4), 5,5'-Me(2)bipy, (5)) and [Mn(tda){(MeO)(2)bipy}·2H(2)O](n) (6). Depending on the presence/position of substituents at bipy, the supramolecular arrangement can affect the metal coordination type. While all the complexes consist of 1D coordination polymers, only 3 has a copper-acetate core with local trigonal prismatic metal coordination. The presence of substituents in 4-6, together with water co-ligands, reduces the supramolecular interactions and typical octahedral Mn(II) ions are observed. The unicity of 3 is also supported by magnetic studies and by DFT calculations, which confirm that the unusual Mn coordination is a consequence of extended noncovalent interactions (π-π stacking) between bipy ligands. Moreover, 3 is an example of broken paradigm for supramolecular chemistry. In fact, the desired stereochemical properties are achieved by using rigid metal building blocks, whereas in 3 the accumulation of weak noncovalent interactions controls the metal geometry. Other N co-ligands have also been reacted with 1 to give the compounds [Mn(tda)(phen)](2)·6H(2)O (7) (phen=1,10-phenanthroline), [Mn(tda)(terpy)](n) (8) (terpy=2,2':6,2'-terpyridine), [Mn(tda)(pyterpy)](n) (9) (pyterpy=4'-(4-pyridyl)-2,2':6,2'-terpyridine), [Mn(tda)(tpt)(H(2)O)]·2H(2)O (10) and [Mn(tda)(tpt)(H(2)O)](2)·2H(2)O (11) (tpt=2,4,6-tris(2-pyridyl)-1,3,5-triazine). Their identified mono-, bi- or polynuclear structures clearly indicate that hydrogen bonding is variously competitive with π-π stacking.     

O2 and vacancy diffusion on rutile(110): pathways and electronic properties.

The binding structures and diffusion pathways of molecular oxygen on a defective TiO2(110) surface are studied by means of a recently developed first-principles string molecular dynamics approach. A variety of molecular and dissociated O2 adsorption states are identified and the kinetics of their interconversion is analyzed. These results, as well as calculations of the electronic properties and of scanning tunneling microscopy (STM) images, are used to discuss recent experimental observations of the interactions between surface oxygen vacancies and the adsorbed oxygen molecule.     

Symmetrization of methylmercury(II) and phenylmercury(II) salts induced by the tripodal ligand N(CH2CH2PPh2)3

Both ionic [HgR(DMSO)][CF₃SO₃] (R = Me or Ph) and covalent HgMeI organomercury(II) compounds react with the tripodal ligand N(CH₂CH₂PPh₂)₃ (np₃) to yield as ultimate products Hg(II) complexes, the new five-coordinated [Hg(OSO₂CF₃)(np₃)]⁺ or the known tetrahedral [HgI(np₃)]⁺ and symmetric diorganomercurials respectively. Monitoring of the reactions by ¹H, ³¹P and ¹³C NMR spectroscopy has shown that the mechanistic pathways depend on the nature of the reagents.     

Different complexation properties of some hydroxy keto heterocycles toward beryllium(II) in aqueous solutions: experimental and theoretical studies

Four heterocycles containing hydroxy and keto functionalities have been tested as chelating agents of beryllium(II). These are in the order (i) 3-hydroxy-2-methyl-4H-pyran-4-one (maltol, Hma), (ii) 5-hydroxy-2-(hydroxymethyl)-4H-pyran-4-one (kojic acid, Hka), (iii) 3-hydroxy-1,2-dimethyl-4-pyridinone (Hdpp), (iv) 1-(3-hydroxy-2-furanyl)ethanone (isomaltol, Hima). Although the skeletons of the first three species, with one nitrogen or oxygen heteroatom at the six-membered ring, are almost superimposable, straightforward synthesis and crystallization is achieved only for the 1:2 adduct Be(dpp)(2), 1. Also the complex Be(ima)(2), 2, precipitates in high yield but the ima(-) ligand has a different skeletal structure. X-ray determinations of 1 and 2 showed that the Be(2+) ion is pseudotetrahedrally coordinated by two chelating ligands with slightly asymmetric Be-O(alkoxo) and Be-O(keto) bonds. The complex Be(ma)(2) precipitates in low yields together with large amounts of unreacted Hma while, under the same conditions, no trace of the analogous species Be(ka)(2) has been observed. This paper presents the results of potentiometric and NMR studies in the aqueous solutions as well as of DFT structural optimizations for all of the free acids, their associated bases, and the adducts of the type [BeL(H(2)O)(2)](+) and BeL(2) in the gas phase. It is consistently found that the basicity of the ligands and the stability of their complexes decrease in the order dpp(-) > ma(-) > ka(-) > ima(-). In solution, all of the anionic ligands form adducts of the type [BeL(H(2)O)(2)](+) at low pH values, whereas higher concentrations of the free anion are required to form 1:2 adducts. The pH, the basicity, and the stability constants of the complexes as well as the formation of competing beryllium hydroxide species are strictly correlated factors for the obtainment of the latter type of adduct. The DFT calculations account nicely for the different donor powers of the various chelates in terms of electronic redistribution and associated energetics.     

Mechanism of H2 production by the [FeFe]H subcluster of di-iron hydrogenases: implications for abiotic catalysts

To explore the possibility that the active center of the di-iron hydrogenases, the [FeFe] H subcluster, can serve by itself as an efficient hydrogen-producing catalyst, we perform comprehensive calculations of the catalytic properties of the subcluster in vacuo using first principles density functional theory. For completeness, we examine all nine possible geometrical isomers of the Fe(II)Fe(I) active-ready state and report in detail on the relevant ones that lead to the production of H 2. These calculations, carried out at the generalized gradient approximation level, indicate that the most efficient catalytic site in the isolated [FeFe] H subcluster is the Fe d center distal (d) to the [4Fe-4S] H cluster; the other iron center site, the proximal Fe p, also considered in this study, has much higher energy barriers. The pathways with the most favorable kinetics (lowest energy barrier to reaction) proceed along configurations with a CO ligand in a bridging position. The most favorable of these CO-bridging pathways start from isomers where the distal CN (-) ligand is in up position, the vacancy V in down position, and the remaining distal CO is either cis or trans with respect to the proximal CO. These isomers, not observed in the available enzyme X-ray structures, are only marginally less stable than the most stable nonbridging Fe d-CO-terminal isomer. Our calculations indicate that this CO-bridging CN-up isomer has a small barrier to production of H 2 that is compatible with the observed rate for the enzyme. These results suggest that catalysis of H 2 production could proceed on this stereochemically modified [FeFe] H subcluster alone, thus offering a promising target for functional bioinspired catalyst design.     

Small Au and Pt clusters at the anatase TiO2(101) surface: behavior at terraces, steps, and surface oxygen vacancies

The adsorption properties of Au and Pt metal nanoclusters on TiO2 anatase (101) were calculated using density functional theory. Structures and energetics of adsorbed Au and Pt monomers, dimers, and trimers at clean anatase TiO2(101) terraces and two major step edges, as well as O-vacancies, were systematically determined. The theoretical predictions were tested by vapor-depositing small coverages of Au and Pt on anatase (101) and investigating the resulting clusters with Scanning Tunneling Microscopy. On the clean surface, Au shows a strong tendency to form large clusters that nucleate on step edges. A preference for adsorption at type D-(112) steps is observed, which is probably a result of kinetic effects. For Pt, clusters as small as monomers are observed on the terraces, in agreement with the predicted large binding energy of 2.2 eV. Step edges play a less important role than in the case of Au. Oxygen vacancies, produced by electron irradiation, dramatically influence the growth of Au, while the nucleation behavior of Pt was found to be less affected.