Electronic structure of linear thiophenolate-bridged heteronuclear complexes [LFeMFeL](n)(+) (M = Cr, Co, Fe; n = 1-3): a combination of kinetic exchange interaction and electron delocalization

The electronic properties of the isostructural series of heterotrinuclear thiophenolate-bridged complexes of the general formula [LFeMFeL](n)(+) with M = Cr, Co and Fe where L represents the trianionic form of the ligand 1,4,7-tris(4-tertbutyl-2-mercaptobenzyl)-1,4,7-triazacyclononane, synthesized and investigated by a number of experimental techniques in the previous work(1), are subjected now to a theoretical analysis. The low-lying electronic excitations in these compounds are described within a minimal model supported by experiment and quantum chemistry calculations. It was found indeed that various experimental data concerning the magnetism and electron delocalization in the lowest states of all seven compounds are completely reproduced within a model which includes the electron transfer between magnetic orbitals at different metal centers and the electron repulsion in these orbitals (the Hubbard model). Moreover, due to the trigonal symmetry of the complexes, only the electron transfer between nondegenerate orbital, a(1), originating from the t(2g) shell of each metal ion in a pseudo-octahedral coordination, is relevant for the lowest states. An essential feature resulting from quantum chemistry calculations, allowing to explain the unusual magnetic properties of these compounds, is the surprisingly large value and, especially, the negative sign of the electron transfer between terminal iron ions, beta'. According to their electronic properties the series of complexes can be divided as follows: (1). The complexes [LFeFeFeL](3+) and [LFeCrFeL](3+) show localized valences in the ground electronic configuration. The strong antiferromagnetic exchange interaction and the resulting spin 1/2 of the ground-state arise from large values of the transfer parameters. (2). In the complex [LFeCrFeL](+), due to a higher energy of the magnetic orbital on the central Cr ion than on the terminal Fe ones, the spin 3/2 and the single unpaired a(1) electron are almost localized at the chromium center in the ground state. (3). The complex [LFeCoFeL](3+) has one ground electronic configuration in which two unpaired electrons are localized at terminal iron ions. The ground-state spin S = 1 arises from a kinetic mechanism involving the electron transfer between terminal iron ions as one of the steps. Such a mechanism, leading to a strong ferromagnetic interaction between distant spins, apparently has not been discussed before. (4). The complex [LFeFeFeL](2+) is characterized by both spin and charge degrees of freedom in the ground manifold. The stabilization of the total spin zero or one of the itinerant electrons depends on beta', i.e., corresponds to the observed S = 1 for its negative sign. This behavior does not fit into the double exchange model. (5). In [LFeCrFeL](2+) the delocalization of two itinerant holes in a(1) orbitals takes place over the magnetic core of chromium ion. Although the origin of the ground-state spin S = 2 is the spin dependent delocalization, the spectrum of the low-lying electronic states is again not of a double exchange type. (6). Finally, the complex [LFeCoFeL](2+) has the ground configuration corresponding to the electron delocalization between terminal iron atoms. The estimated magnitude of the corresponding electron transfer is smaller than the relaxation energy of the nuclear distortions induced by the electron localization at one of the centers, leading to vibronic valence trapping observed in this compound.     

Model systems for hydrophobic interactions: Volumes and heat capacities ofn-alkoxyethanols in water

The densities and heat capacities of the first four members of the 2-n-alkoxyethanols were measured in water over the whole mole fraction range with a flow densimeter and a flow microcalorimeter. The methoxy and n-propoxy homologs were studied at 25°C, ethoxyethanol at 19, 25, and 40C, andn-butoxyethanol at 4, 10, 25, 40, and 55°C. While methoxyethanol behaves as a fairly typical polar nonelectrolyte in water,n-butoxyethanol shows trends in the concentration dependence which resemble micellization; some pseudo-microphase transition occurs at about 0.02 mole fraction, and this transition concentration decreases with increasing temperature. There is no simple relationship between this phenomenon and the existence of a lower critical solution temperature at 49°C since the sharpness of the thermodynamic changes is maximum at the lowest temperature and at 55°C the apparent molal quantities on both sides of the two-phase region appear to fall on the same continuous curve. In the region prior to the pseudo-microphase separation the apparent and partial molal heat capacities decrease regularly but beyond approximately 0.01 mole fraction increase sharply to a maximum, suggesting some type of pre-association. The apparent molal heat capacity of water in the binary solutions is larger than the molar heat capacity of water over the whole mole fraction range. The present data seem to be consistent with a clathrate model for hydrophobic hydration and interactions with these systems.     

Selective internal manipulation of a single molecule by scanning tunneling microscopy

We have studied the adsorption of the polyaromatic molecule 1,4"-paratriphenyldimethylacetone, which we have nicknamed Trima. The originality of this linear molecule is that it was designed and synthesized to have two functionalities. First, chemisorb itself to the surface by its two ends rather like a bridge. Second, the central part of the molecule could then be rotated by injecting electrons with the tip of the scanning tunneling microscope (STM). The length of the molecule corresponds exactly to the spacing between five dimers in a row on the Si(100)-2 x 1 surface. We found that the molecule adsorbs as expected on the clean silicon surface by using complementary STM and synchrotron radiation studies. Manipulation of individual molecules with the STM tip showed selective internal modifications that were highly voltage dependent. These manipulations were found to be compatible with an electronic excitation of the pi-pi* transition of the molecule.     

New linear high-valent tetranuclear manganese-oxo cluster relevant to the oxygen-evolving complex of photosystem II with oxo, hydroxo, and aqua coordinated to a single Mn(IV)

An unprecedented atom connectivity, MnIV(mu-O)MnIV(mu-O)2MnIV(mu-O)MnIV, is found in the complex [MnIV4O4(EtO-terpy)4(OH)2(OH2)2](ClO4)(6).8H2O (EtO-terpy=4'-ethoxyl-2,2':6',2' '-terpyridine), which has been characterized by X-ray crystallography, X-ray powder diffraction, EPR spectroscopy, and magnetic studies. This complex is the first example of a compound where a MnIV ion is coordinated by all three types of water-derived ligands: oxo, hydroxo, and aqua. Bond distances and angles for this complex are consistent with a MnIV4 oxidation state assignment. The di-mu-oxo- and mono-mu-oxo-bridged Mn-Mn distances are 2.80 and 3.51 A, respectively. The variable-temperature magnetic susceptibility data for this complex, in the range of 10-300 K, are consistent with an S=0 ground state and were fit using the spin Hamiltonian HHDvV=-J1S2S1-J2S1S1A-J1S1AS2A (S1=S1A=S2=S2A=3/2) with J1=-432 cm-1 and J2=-164 cm-1 (where J1 and J2 are exchange constants through the mono-mu-oxo and the di-mu-oxo bridges, respectively). The first excited spin state of this tetramer is a spin triplet state at 279 cm-1 above the diamagnetic ground state. The next spin states are the S=1 and S=2 levels at about 700 and 820 cm-1 above the S=0 ground state, respectively. These large energy gaps are consistent with the absence of an EPR signal for this complex, even at high temperature.     

Electrochemical behavior of n-and p-type GaxIn1−xP electrodes in sodium iodide liquid ammoniate at room temperature

The electrochemical behavior of p- and n-type Ga_xIn_1-xP electrodes in liquid ammoniate (NaI+3.3 NH₃) in the dark and under illumination has been investigated at room temperature. The flatband potentials of the semiconductors (SC) have been measured either by impedance determinations or by the photoperturbation method. Some specific physical parameters of the polycrystalline material have been deduced, such as E_gap, the electronic transition modes and the carrier densities. The SC behavior in water and in liquid ammoniate has been compared.     

Vinylallenes IX. - preparation de quelques tetraenes-1,2,4,6

The title hydrocarbons have been obtained by the reaction of Grignard reagents with derivatives of 4,6-diene-2-yne-1-ols in the presence of cuprous bromide.     

Formation of a long range ordered polysilsesquioxane O1.5Si-Ph-SiO1.5 by thermal polycondensation of 1,4-bis(trihydroxysilyl)benzene

A study of the polycondensation reaction of 1,4-bis(trihydroxysilyl)benzene 1H by thermal treatment and by a catalytic route is described. The resulting polysilsesquioxane materials O_1.5Si-Ph-SiO_1.5have been compared with those obtained by the classical sol-gel route from 1,4-bis(trimethoxysilyl)benzene 1M. From these results it emerges that the solids obtained by thermal treatment of the solid 1H were drastically different from those obtained in solution, either from 1H or 1M. The thermal polycondensation of 1H led to highly polycondensed non-porous solids having a high density. They were organized at the nanometric scale (X-ray) and at the micrometric scale (TEM images). In contrast, the solids obtained by catalytic polycondensation in solution from 1H or by classical sol-gel route from 1M presented similar properties (moderate level of condensation, high specific surface areas). However, they exhibited different TEM images and chemical reactivity. It has been shown that the long range order observed in the case of the thermal treatment of 1H was not the result of an ordinary thermodynamic rearrangement, since the same treatment applied to the solids obtained by polycondensation in solution of 1H and 1M did not allow such organization.