Search results for "Orbital"
showing 10 items of 679 documents
Millenial-scale paleoenviromental changes in the central Mediterranean during the Last Interglacial: comparison with European and North Atlantic reco…
2010
Abstract The environment of the central Mediterranean Sea is investigated on the basis of high-resolution planktonic foraminifera and calcareous nannofossil data (mean sampling resolutions of about 80 and 160 yr, respectively). MIS 5 is characterized by warm, oligotrophic and stratified waters, while coccolithophore communities developed a vertical zonation that, in today's oceans, is typical of the low-latitude gyres. The temperate-subtropical configuration of planktonic foraminifera and calcareous nannofossil assemblages is repeatedly transformed into a temperate-subpolar one during the suborbital cooling episodes C25-C18. A comparison with European pollen sequences and North Atlantic cor…
Coordination polymers undergoing spin crossover and reversible ligand exchange in the solid
2006
Here we report the synthesis and characterisation of a polymer made up of a system of parallel 2-D grids of Fe(II) ions linked by [Au(CN)2]– bridges and its transformation into a new system of three interpenetrated 3-D coordination open frameworks with the NbO topology. Reversibility of this crystal-to-crystal transformation is evidenced by X-ray crystallographic data and from their spin crossover properties. Real Cabezos, Jose Antonio, Jose.A.Real@uv.es
Structural incorporation of nitrogen into zeolites, and alpos: ab initio molecular orbital calculations on stability and basicity
1998
Ab initio molecular orbital calculations (HF/6-31G*) are used to obtain thermodynamical information on the possibility to form NH-bridging or NH2-terminal groups in amorphous and crystalline materials containing Si–O–Si and Al–O–P structures, such as for instance, zeolites and aluminophosphates. We have employed dimeric model clusters Si–O–Si and Al–O–P which contain NH2-terminal groups or an oxygen atom substituted by a NH bridging unit. The Bronsted basicity and the softness–hardness of these structures have been determined using the proton affinity and the HOMO energy, respectively. The obtained results indicate that for both Si–O–Si and Al–O–P linkages and from a thermodynamic standpoin…
Valence Topological Charge-Transfer Indices for Dipole Moments
2003
Valence topological charge-transfer (CT) indices are applied to the calculation of dipole moments. The dipole moments calculated by algebraic and vector semisums of the CT indices are defined. The combination of the CT indices allows the estimation of the dipole moments. The model is generalized for molecules with heteroatoms. The ability of the indices for the description of the molecular charge distribution is established by comparing them with the dipole moment of the valence-isoelectronic series of benzene and styrene. Two CT indices, μ v e c (vector semisum of vertex-pair dipole moments) and μ V v e c (valence μ v e c ) are proposed. μ v e c and μ V v e c are important for the predicti…
Chelation of a proton by oxidized diphosphines
2012
Abstract The chelation of a proton by oxidized diphosphines is studied for the first time both experimentally and theoretically. As a proof of concept the rare case where two different H-bond systems exist in one compound, H[7,8-(OP i Pr 2 ) 2 -7,8- nido -C 2 B 9 H 10 ] is reported. Based on NBO, QTAIM and ELF calculations, the P–O⋯H + ⋯O–P interactions were characterized as strong hydrogen bonds.
Intramolecular Communication in Anionic Oxidized Phosphanes through a Chelated Proton
2015
Oxidation of the 1,2-(PR2 )2 -1,2-closo-C2 B10 H10 (R=Ph, iPr) platform with hydrogen peroxide in acetone is a two-step procedure in which partial deboronation of the closo cluster and oxidation of the phosphorus atoms occur. Based on NMR spectroscopic and kinetic data, we demonstrate that the phosphorus atoms are oxidized in the first step, followed by cluster deboronation. DFT calculations and natural-bond orbital (NBO) analysis were used to obtain insight into the electronic structures of diphosphane ortho-carborane derivatives.
Molecular orbital studies on the mechanism of catalytic isomerization of xylenes III. Protonation channels
1981
Abstract The isoenergetic lines of the interaction energy between a bare proton and ortho-, meta- , and para -xylene have been calculated in order to detect both the best approaching channel and the preferred protonation positions. The channels found direct the protonation to the following ring positions: 4 and 5 for o -xylene, 4 and 6 for m -xylene, and 2 equivalents for p -xylene. The overall interaction energy determines the protonation positions on each isomer and the most important contributions to it are the electrostatic term in o -xylene, the polarization term in p -xylene, and both terms equally in m -xylene. These results are in agreement with the experimental findings and appear …
A G2(MP2) theoretical study of substituent effects on H3BNHnCl3−n (n= 3-0) donor-acceptor complexes
2008
Abstract The complexation energies of H3BNHnCl3−n (n= 3-0) complexes and the proton affinities of NHnCl3−n compounds have been computed at the G2(MP2) level of theory. G2(MP2) results show that the successive chlorine substitution on the ammonia decreases both the basicity of the NHnCl3−n ligands and the stability of H3BNHnCl3−n complexes. The findings are interpreted in terms of the rehybridisation of the nitrogen lone-pair orbital. The NBO partitioning scheme shows that the variation of the N-H and N-Cl bond lengths, upon complexation, is due to variation of “s” character in these bonds.
Field-induced ferromagnetism due to magneto-striction in 1-D helical chains
2016
Two homochiral copper(II) helices, [Cu(μ1,3-N3)(L1)]n (1) and [Cu(μ1,3-NCO)(L2)]n (2), with end-to-end pseudohalide bridges, were synthesized using two N2O donor achiral Schiff bases via spontaneous chiral resolution. Field-induced ferromagnetic ordering due to magneto-striction in homochiral 1-D helix [Cu(μ1,3-N3)(L1)]n (1) is reported for the first time. At temperatures below 5.5 K, under a magnetic field of 1 T, orthogonality between the magnetic orbitals of copper centres increases significantly due to the contraction of lattice parameters, giving rise to long-range ferromagnetic ordering in the helical chain. The magneto-dielectric results are also indicative of the observed magneto-st…
The Transoid, Ortho, and Gauche Conformers of Decamethyl-n-tetrasilane, n-Si4Me10: Electronic Transitions in the Multistate Complete Active Space Se…
2003
Multistate complete active space second-order perturbation theory (MS-CASPT2) is used to improve earlier descriptions of the low-energy valence excited states of the transoid, ortho, and gauche conformers of decamethyl-n-tetrasilane, n-Si4Me10, using a generally contracted basis set of atomic natural orbitals (ANOs) at a ground-state geometry optimized in the second-order Moller−Plesset perturbation theory (MP2) approximation with Dunning's correlation consistent triple-ζ basis set (cc-pVTZ) on the silicon atoms and the 6-31G* and 6-31G basis sets on the carbon and hydrogen atoms, respectively. Relative energies, relative free energies, and mole fractions of the transoid, ortho, and gauche …