Search results for "ORBIT"
showing 10 items of 1104 documents
Long-term X-ray variability of the microquasar system LS 5039/RX J1826.2−1450
2003
We report on the results of the spectral and timing analysis of a BeppoSAX observation of the microquasar system LS 5039/RX J1826.2-1450. The source was found in a low-flux state with Fx(1-10 keV)= 4.7 x 10^{-12} erg cm^{-2} s^{-1}, which represents almost one order of magnitude lower than a previous RXTE observation 2.5 years before. The 0.1--10 keV spectrum is described by an absorbed power-law continuum with photon-number spectral index Gamma=1.8+-0.2 and hydrogen column density of NH=1.0^{+0.4}_{-0.3} x 10^{22} cm^{-2}. According to the orbital parameters of the system the BeppoSAX observation covers the time of an X-ray eclipse should one occur. However, the 1.6-10 keV light curve does…
Molecular orbital study of conformational isomers and rotational barriers of methyl substituted hydroquinone cation radicals
1998
Abstract The torsional potential energy curve of the hydroxyl group of hydroquinone and tetramethyl-hydroquinone cation radicals were explored with various ab initio methods. The minimum and the torsional transition state geometries and energies were computed by using high accuracy density functional methods yielding the rotation barrier height and the energy difference between the cis- and trans-isomers. The obtained minimum energy geometry for the hydroquinone cation radical indicates that the CO bond has shortened when compared to the neutral species. We attribute this to the increased double-bond character of this bond. The energy minima were located for methyl-hydroquinone, 2,3-dimeth…
An ab initio study of the unimolecular decomposition mechanism of formamidine. 4-31G Characterization of potential energy hypersurface
1991
Ab initio MO calculations have been carried out for the unimolecular decomposition of formamidine. The Hartree–Fock method in LCAO approximation with the 4-31G basis set was used. The 4-31G potential hypersurface has been further studied. The stationary points (R, TS, and P) were localized. A reaction analysis by correlation of bond-order indices and localized molecular orbitals demonstrated that the decomposition is an asynchronous process. The TS can be described as four-membered ring.
A pseudo-Jahn–Teller model of the photochromic effect in sodium nitroprusside
2003
Abstract A new model for the photochromic effect in sodium nitroprusside Na2[Fe(CN)5(NO)]·2H2O based on the concept of the pseudo-Jahn–Teller effect is proposed. The model takes into account the electron transfer from the Fe2+ ion to the π* orbitals of the NO-ligand as well as the vibronic mixing of three electronic states of the Fe NO fragment through the non-symmetric and full-symmetric modes. The problem is solved within the adiabatic approximation. Under certain conditions the lower sheet of the adiabatic potential is shown to possess three minima with the increasing energies that correspond to the N-bound, sideways bound and O-bound NO group. The barriers between the minima are estimat…
Further orthometalated dinuclear palladium(iii) compounds with bridging N,S-donor ligands
2013
New dinuclear palladium(III) compounds of general formula Pd2[(C6H4)PPh2]2[N-S]2Cl2, N-S being 2-mercaptopyridinate, 3a; 2-mercapto-6-methylpyridinate, 3b; 2-quinolinethiolate, 3c; 2-mercaptopyrimidinate, 3d; 1-methyl-1H-imidazole-2-thiolate, 3e; 1-methyl-1H-benzimidazole-2-thiolate, 3f; 2-mercaptobenzothiazolate, 3g and 5-mercapto-1-methyltetrazolate, 3h have been obtained by oxidation with PhICl2 of the corresponding palladium(II) counterparts. The stability of the new compounds has been studied by (31)P NMR spectroscopy from 200 to 298 K. Compounds 3f-h were relatively stable until room temperature and they have been synthesized and characterized by (31)P, (1)H and (13)C NMR spectroscopy…
Schiff-Base Podates – X-ray, NMR and Ab Initio Molecular-Orbital Studies of the Cadmium(II) Complexes of Linear and Three-Armed Podands in Solution a…
1998
Cadmium(II) complexes of two Schiff bases, 1,3-di(pyridine-2-carboxaldimino)propane (C15H16N4, L1)[1] and tris[4-(2′-pyridyl)-3-aza-3-butenyl]amine (C24H27N7, L2)[2] are described. An efficient route utilising molecular sieves for the synthesis of Schiff bases is presented. The ligands L1 and L2 can be described as linear and three-armed podands, respectively, L1 being conformationally flexible and L2 preorganised. Cadmium perchlorate in methanol with L1 yields a crystalline complex [Cd(L1)2](ClO4)2 (1), the structure of which was determined by X-ray structure analysis. The complex 1 has an unusual nonsymmetrical 8-coordinated helical structure and crystallizes in an acentric space group (C…
Spin Density Distribution in Transition Metal Complexes: Some Thoughts and Hints
1998
Abstract The spin density distribution in transition metal complexes is discussed in qualitative terms, taking into account the coexistence of spin delocalization and spin polarization mechanisms, with the help of numerical results for several complexes obtained from density functional calculations. The covalent character of the metal-ligand bonds as well as the σ- or π-characteristics of the partially filled d orbitals must be taken into account to qualitatively predict the sign of the spin density at a particular atom within a ligand. The same patterns can be applied to binuclear complexes and can be helpful in determining the ferro- or antiferromagnetic character of the exchange coupling…
Control of Exchange Interactions in Trinuclear Complexes Based on Orthogonal Magnetic Orbitals
2009
The reaction of copper(II) acetate with the tetradentate Schiff base like ligand H4L {(E,E)-[{diethyl 2,2′-[4,5-dihydroxy-1,2-phenylenebis(iminomethylidyne)]bis(3-oxobutanoate)}] leads to the formation of the square planar N2O2 coordinated complex [H2CuL]. Reaction of two equivalents of this complex with copper(II) acetylacetonato or vanadyl(IV) acetylacetonato yields the trinuclear complexes [V(O)Cu2L2][N(nBu)4]2·2MeOH (1) and [Cu3L2][N(nBu)4]2·2DMF (2). Both complexes were characterised by using magnetic measurements and X-ray crystallography. Special attention was given to the spin-exchange coupling through the bridging phenylene ring. The principle of strict orthogonality of the magneti…
X‐ray Absorption Spectra of Cu II and Cu III Complexes of N , N ′ ‐1,2‐Phenylenebis(2‐mercapto‐2‐methylpropionamide)
2005
The X-ray absorption spectra of Cu I I and Cult complexes of N,N'-1,2-phenylenebis(2-mercapto-2-methylpropionamide) were recorded and analysed in solid phase. The EXAFS spectra of the two samples were refined with full multiple scattering path. Geometry optimisations on the Cu I I and Cu I I I complexes were performed by the B3LYP density functional method, with the 6-31G(d,p) basis set, considering different spin multiplicities. The singlet state of the Cu I I I complex was shown to be more stable than the triplet state, and a good agreement between the calculated and the corresponding experimental structure was found. Further single-point calculations on the optimised geometry were carrie…
1,2,3,4,5‐[1′,8′] Anthra‐8,9;10,11‐dibenzo[13]annulene and 1,8‐Distyrylanthracene as Models for Phenylenevinylene Species. Syntheses, Structures, and…
1987
The synthesis of the title compound 4, formally a [13] perimeter, and of its acyclic model system, 1,8-distyrylanthracene (6) is described. The crystal structure of 4 reveals the configuration of the double bonds and the conformation of the macrocycle. Related findings come from the 1H-NMR spectroscopic characterization of 4 and 6 in solution. The spin density distribution of the radical anions 4−· and 6−· is interpretet in terms of the singly occupied molecular orbital and of the prevailing geometry. In contrast to 4, compund 6 proves to be an efficient electron acceptor since it can even be reduced to a tetraanion salt. The complete redox sequence is characterized by NMR and ESR spectrosc…