Exchange interactions between dimers of chromium(III). A cluster approach

Exchange interactions in dimers, tetramers and octamers of chromium(III) are treated with an effective Hamiltonian of the Heisenberg-Dirac-vanVleck (HDvV) type. Energies and wave functions of the cluster states are computed. The results of interdimer interactions are: (i) energy splittings and (ii) a contamination of the cluster ground state with excited configurations. The results are used for a qualitative rationalisation of the observed low-temperature properties of Cs₃Cr₂Cl₉.     

Higher-order centrifugal distortion effects in ONCl

One hundred and nine new rotational transitions of ONCl in the ground vibrational state have been measured. A centrifugal distortion analysis of these transitions yields the values of six sextic and one octic centrifugal distortion constants. A comparison between experimental and calculated sextic distortion constants is reported.     

Syntheses, characterization and redox properties of oxo-centred triruthenium cluster dimers and trimers linked by ortho-metallated polypyridyl ligands

A series of mono-, di- and trimeric oxo-centred triruthenium cluster compounds with ortho-metallated polypyridyl ligands were prepared by reaction of oxo-centred triruthenium precursor compound [Ru3O(OAc)6(py)2(CH3OH)]+ (1) with bis(2,2'-bipyridin-5-yl)butadiyne (bpyC[triple bond, length as m-dash]C-C[triple bond, length as m-dash]Cbpy), bis(2,2'-bipridin-5-yl)ethyne (bpyC[triple bond, length as m-dash]Cbpy), 2,2'-bipyrazine (bpz), pyrazino[2,3-f]quinoxaline (pyq), or 4,7-phenanthroline (4,7-phen). As revealed by redox wave splitting, electronic interactions are operating between two Ru3O redox centres in most of the triruthenium cluster dimers and trimers. The cluster-cluster interactions are highly sensitive to the variations in bonding modes of the polypyridyl ligands. Ortho-metallation of the bridging ligand induces an enhanced electronic interaction between two triruthenium centres.     

Water effects on electron transfer in azurin dimers

Recent experimental and theoretical analyses indicate that water molecules between or near redox partners can significantly affect their electron-transfer (ET) properties. Here, we study the effects of intervening water molecules on the electron self-exchange reaction of azurin (Az) by using a newly developed ab-initio method to calculate transfer integrals between molecular sites. We show that the insertion of water molecules in the gap between the copper active sites of Az dimers slows down the exponential decay of the ET rates with the copper-to-copper distance. Depending on the distance between the redox sites, water can enhance or suppress the electron-transfer kinetics. We show that this behavior can be ascribed to the simultaneous action of two competing effects: the electrostatic interaction of water with the protein subsystem and its ability to mediate ET coupling pathways.     

A DFT study of substituent effects in corannulene dimers

Corannulene dimers made up of corannulene monomers with different curvature and substituents were studied using M06-2X, B97D and ωB97XD functionals and 6-31+G* basis set. Corannulene molecules were substituted with five alternating Br, Cl, CH(3), C(2)H or CN units. Geometric results showed that substituents gave rise to small changes in the curvature of corannulene bowls. So, there was not a clear relationship between the curvature of bowls and the changes on interaction energy generated by addition of substituents in the bowl. Electron withdrawing substituents gave rise to a more positive molecular electrostatic potential (MEP) of the bowl, which was able to get a strong interaction with the negative MEP at the surface of a fullerene. Substitution with CN caused the largest effect, giving rise to the most positive MEP and to a large interaction energy of -24.64 kcal mol(-1), at the ωB97XD/6-31+G* level. Dispersive effects must be taken into account to explain the catching ability of the different substituted corannulenes. For unsubstituted dimers, calculations with DFT-D methods employing ωB97XD and B97D functionals led to similar results to those previously reported at the SCS-MP2/cc-pVTZ level for corannulene dimers (A. Sygula and S. Saeb?, Int. J. Quant. Chem., 2009, 109, 65). In particular, the ωB97XD functional led to a difference of only 0.35 kcal mol(-1), regarding MP2 interaction energy for corannulene dimers. On the other hand, the M06-2X functional showed a general considerable underestimation of interaction energies. This functional worked quite well to study trends, but not to obtain absolute interaction energies.     

Kinetic temperature effects on 4He dimers in jets

The formation of dimers in a free jet cryogenic expansion of 4He gas has been studied by measuring mole fractions as a function of source temperature and pressure using diffraction from a nanostructured transmission grating. The data sets are limited to low source pressures for which dimers and trimers are the only appreciable cluster populations in the beam. The final cluster mole fractions are corrected for residual gas attenuation in the source chamber by an extrapolation over several residual gas pressures. A set of rate equations used to model the cluster formation in a free jet expansion has been extended to include the departure of the ambient translational temperature from the isentropic-equilibrium values as the density decreases with increasing distance. The effect of collisions in restoring the equilibrium temperature is treated with a relaxation time approximation. There are distinct distance ranges where the dimer and trimer mole fractions and the ambient temperature near their asymptotic values. The present modeling reproduces the apparent threshold observed at low source pressures for the survival of dimers in the asymptotic beam. Except for these low source pressures, there are only small changes relative to results based on the isentropic temperature.     

Theoretical study of electron correlation effects in transition metal dimers

Introduction of partially localized orbitals (PLOs) is shown to reduce the number of configurations needed to describe the bonding in transition metal clusters. Using this formalism, estimates are made of the molecular electron correlation energy that arises from including such terms as 3d → 3d′, 3p → 3p′ and 4s² → 4p² in the wavefunction. When this estimate of the additional correlation is added to the CAS SCF results of Walch, Bauschlicher, Roos and Nelin improved interaction potentials are obtained for the dimers V₂ and Cr₂.     

Cooperative effects in photon statistics of molecular dimers with spectral diffusion

The two-point fluorescence intensity correlation function g(2)t and the Mandel parameter Mt are calculated for a strongly pumped dimer of two-level molecules undergoing Gaussian-Markovian frequency fluctuations. The effects of detuning and saturation are examined. All fluctuation time scale regimes are explored using a continued fraction solution of the stochastic Liouville equation for the generating function. Bunching and antibunching are observed for slow and fast fluctuations, respectively. The short-time antibunching dip in g(2) and its variation with intermolecular coupling, the exciton annihilation rate, and laser detuning are studied.     

High pressure effects on benzoic acid dimers: Vibrational spectroscopy

To understand pressure effects on dimer structure stability, Raman and FTIR spectroscopies were used to examine changes in H-bonded dimers of benzoic acid (BA). Experiments were performed on single crystals compressed to 33 GPa in a diamond anvil cell (DAC). Several changes in Raman spectra were observed in the range 6–8 GPa indicating modification in the dimer structure suggesting the lowering of molecular symmetry. Pressure increase above 15 GPa induced strong luminescence and a gradual change of the crystal color from white to yellow/brownish. FTIR measurements on the sample released from 33 GPa indicated formation of a new compound. It is proposed that molecules of this compound are composed of the hydroxyl group associated with alcohol, carbonyl group associated with ketone, and the sp³ hydrocarbon groups. This study demonstrates that sufficient high pressure compression and subsequent decompression can lead to significant changes in the H-bonded dimer structure, including the breaking of bonds and formation of new chemical compound.     

Stabilization of antibodies by haptens

A new, sensitive and very simple spectrofluorimetric biparameter sensor is described for the determination of salicylamide and/or salicylic acid in pharmaceutical preparations. The method integrates the transitory retention and fluorescence detection of both compounds on Sephadex QAE A-25 resin packed into a conventional flow-through cell. A monochannel manifold with two alternative carriers is used. At pH 2.0 (first carrier) salicylic acid is selectively retained on the solid support and after developing the analytical signal it is desorbed. At pH 11.0 (second carrier) both salicylic acid and salicylamide are simultaneously and transitorily retained on the solid, the analytical signal now corresponding to both analytes. The monochromators were tuned at 260 (excitation) and 415 (emission) nm, respectively. The calibration graph for salicylamide is linear over the range 0.01 to 0.32 μg mL^–1 and for salicylic acid from 0.04 to 1.0 μg mL^–1 in the presence of each other. The relative standard deviation and the sampling frequency for the determination of salicylamide (0.20 μg mL^–1) and salicylic acid (0.50 μg mL^–1) were 1.1% and 35 h^–1, and 0.9% and 45 h^–1, respectively. Good results on application to individual determination or mixture resolution in pharmaceutical samples testify to the usefulness of the proposed sensor. Received: 20 April 1999 / Revised: 7 June 1999 / Accepted: 12 June 1999     

Porphyrin dimers bridged by a platinum-diacetylide unit

A series of butadiyne- and platinum diacetylide-bridged conjugated porphyrin dimers have been prepared; electrochemical and UV-vis absorption measurements on these compounds show that the butadiyne-bridge confers stronger porphyrin-porphyrin conjugation than the platinum diacetylide-bridge.     

Stabilization of a reactive polynuclear silver carbide cluster through the encapsulation within a supramolecular cage

We report herein the synthesis of a new metal cluster-encapsulated supramolecular capsule, [(C≡C)@Ag(5-6)@(Py6)(2)](CF(3)SO(3))(3-4) (3, Py6 = azacalix[6]pyridine), by use of a bowl-shaped macrocyclic ligand Py6. The multinuclear silver carbide cluster aggregate in 3 is encapsulated by two Py6 ligands through both metal-ligand coordination and cation-π interactions, spotlighting a new synthetic strategy for supramolecular capsules.     

Synthesis of new artemisinin-derived dimers by self-cross-metathesis reaction

[reaction: see text] New artemisinin-derived dimers, fluorinated or not, have been prepared by a self-cross metathesis reaction in the presence of first- or second-generation ruthenium catalysts without degradation of the endoperoxide bridge and with a good E/Z selectivity (up to 100:0).     

NH-tautomerism effects and isocyclic substitution in alkylporphyrins and their chemical dimers

A study has been made of the absorption spectra, circular dichroism in a magnetic field (MCD), and polarized fluorescence of individual NH-tautomers of free-base porphyrins with asymmetric substitution. The four-orbital model of Gouterman and the perimeter model of Michl have been used in demonstrating that, in spite of the basic differences in type of electronic spectra of the two NH-tautomers of each compound, their absorption spectra in the visible region are described by a system of two linear oscillators X and Y. The fluorescence polarization spectra of the tautomers in the region of band III are extremely sensitive to changes in the side substituents in the isocycle. From an analysis of the sign sequence of electronic bands in the MCD spectra, it has been established that these compounds may be classed as hard chromophores with identical type of orbital splitting for the HOMO (b₁ and b₂) and LUMO (c₂ and c₂) in the two tautomers of one and the same compound. A specific role has been found for the keto group in the isocycle, as manifested in inversion of the sign sequence of electronic bands in the MCD spectra of NH-tautomers with a cyclopentanone ring.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 3, pp. 295–305, May–June, 1989.     

Preparation of molybdenum target by centrifugal method

The molybdenum targets of 2–10 mg/cm² have been prepared on Al and Ti backings by centrifugal sedimentation. The thickness and purity of the produced targets were analysed with Rutherford back scattering measurements, scanning electron microscope and energy dispersive spectrometry. Targets were requested for pilot studies of an alternative accelerator way of ^99mTc production. Targets were tested with deuteron beam of 20 MeV energy and 10 nA intensity. They were irradiated for 10 min and showed no damage what proofed the targets suitability for irradiation with a weak beam of light projectiles.