Two-Dimensional Brownian Random Interlacements

Potential Analysis - We introduce the model of two-dimensional continuous random interlacements, which is constructed using the Brownian trajectories conditioned on not hitting a fixed set...     

Synthesis and Crystal Structure of [Fe(Cp)2]3(Bi2Cl9)·thf

Single crystals of [Fe(Cp)₂]₃(Bi₂Cl₉)·thf were obtained from a thf solution containing ferrocene and BiCl₃. The structure shows disorder at room temperature which disappears upon cooling, coupled with a decrease in symmetry. The title compound crystallizes in the orthorhombic space group P2₁2₁2₁ [a = 1698.64(2), b = 2318.69(3), c = 1085.66(2) pm] with three ferrocenium ions, one nonachlorodibismutate ion and one molecule of thf in the asymmetric unit.     

Asymptotically sharp uniform estimates in a scalar spectral stiff problemEstimations asymptotiques uniformes pour le spectre d'un problème scalaire raide

Estimates of convergence rates for rescaled eigenvalues of the stiff Neumann problem for the Laplacian are obtained. The bounds are expressed in terms of the stiffness ratio and properties of the limit spectrum both for low and middle frequency ranges. To cite this article: M. Lobo et al., C. R. Mecanique 331 (2003).     

Synthesis of selectively deuterated fulvenes and indenes from enediynes

A facile enediyne--> fulvene--> indene transformation provides a route to all possible isotopomers of substituted fulvenes and indenes.     

Bicyclic and acyclic diamides: comparison of their aqueous phase binding constants with Nd(III), Am(III), Pu(IV), Np(V), Pu(VI), and U(VI)

This report describes affinity measurements for two, water-soluble, methyl-alkylated diamides incorporating the malonamide functionality, N,N,N',N' tetramethylmalonamide (TMMA) and a bicyclic diamide (1a), toward actinide metal cations (An) in acidic nitrate solutions. Ligand complexation to actinides possessing oxidation states ranging from +3 to +6 was monitored through optical absorbance spectroscopy, and formation constants were obtained from the refinement of the spectrophotometric titration data sets. Species analysis gives evidence for the formation of 1, 4, 1, and 2 spectrophotometrically observable complexes by TMMA to An(III, IV, V, and VI), respectively, while for 1a, the respective numbers are 3, 4, 2, and 2. Consistent with the preorganization of 1a toward actinide binding, a significant difference is found in the magnitudes of their respective formation constants at each complexation step. It has been found that the binding affinity for TMMA follows the well-established order An(V) < An(III) < An(VI) < An(IV). However, with 1a, Np(V) forms stronger complexes than Am(III). The complexation of 1a with Np(V) and Pu(VI) at an acidity of 1.0 M is followed by reduction to Np(IV) and Pu(IV), whereas TMMA does not perturb the initial oxidation state for these dioxocations. These measurements of diamide binding affinity mark the first time single-component optical absorbance spectra have been reported for a span of actinide-diamide complexes covering all common oxidation states in aqueous solution.     

How important is the dispersion interaction for cyclobis(paraquat‐p‐phenylene)‐based molecular “shuttles”? A theoretical study

Inclusion complexes of cyclobis(paraquat‐p‐phenylene) and various aromatic molecules in their neutral and oxidized form were studied at the LMP2/6‐311+G**//BHandHLYP/6‐31G* level of theory, which represents the highest level theoretical study to date for these complexes. The results show that it is dispersion interaction that contributes most to the binding energy. One electron oxidation of a guest molecule leads to complete dissociation of inclusion complex generating strong repulsion potential between guest and host molecules. Electrostatic interactions also can play an important role, provided the guest molecule has a dipole moment; however, dispersion interactions always dominate in binding energy. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Quantum chemical study of three polymorphs of the mononuclear spin-transition complex [Fe(DPPA)(NCS)2

The calculations of the high spin (HS) and low spin (LS) states of the [Fe(II)(DPPA)(NCS)(2)] complex have been performed at three experimentally observed geometries corresponding to three synthesized polymorphs with different spin-transition behavior. The structure optimization leads to a single molecular structure, suggesting that the existence of three geometries is not an intrinsic phenomenon but is induced by the crystal lattice. The structural difference between three forms can be reproduced by introducing the Madelung field of the crystal lattice. However, the calculations show that the differences in magnetic behavior of the three polymorphs cannot be attributed only to variations of the energy gap between two spin states.     

Exchange interactions in oxovanadium phosphates: towards the understanding of the magnetic patterns

We report the calculations of exchange magnetic constants in a series of vanadium phosphorus oxides for which structural and magneto-chemical data are available. Four different types of dimers have been extracted from the crystal lattices in all solids studied. On the basis of a combined density functional theory broken symmetry approach, our calculations on molecular models allow us to define schemes of magnetic interactions. The largest absolute magnetic interaction is provided by the double O–P–O and di-μ-oxo bridges, suggestive of an alternating dimer chain model and an isolated dimer one for the VO(HPO₄)·0.5H₂O and α-VO(HPO₄)·2H₂O phases, respectively. Conversely, VO(HPO₄)·4H₂O is consistent with a bi-dimensional magnetic pattern whereas VO(H₂PO₄)₂ and α-VO(PO₃)₂ with three-dimensional magnetic schemes. The use of dimer models has been justified by the analysis of higher-nuclearity clusters.     

Gauge invariance of the spin-other-orbit contribution to the g-tensors of electron paramagnetic resonance

The spin-other-orbit (SOO) contribution to the g-tensor (DeltagSOO) of electron paramagnetic resonance arises due to the interaction of electron-spin magnetic moment with the magnetic field produced by the orbital motion of other electrons. A similar mechanism is responsible for the leading term in nuclear magnetic-shielding tensors sigma. We demonstrate that analogous to sigma, paramagnetic DeltagSOO contribution exhibits a pronounced dependence on the choice of the magnetic-field gauge. The gauge corrections to DeltagSOO are similar in magnitude, and opposite in sign, to the paramagnetic SOO term. We calculate gauge-invariant DeltagSOO values using gauge-including atomic orbitals and density-functional theory. For organic radicals, complete gauge-invariant DeltagSOO values typically amount to less than 500 parts per million (ppm), and are small compared to other g-tensor contributions. For the first-row transition-metal compounds, DeltagSOO may contribute several thousand ppm to the g-tensor, but are negligible compared to the remaining deviations from experiment. With popular choices for the magnetic-field gauge, the individual gauge-variant contributions may be an order of magnitude higher, and do not provide a reliable estimation of DeltagSOO.     

Exchange interactions and theoretical analysis of (31)P NMR spectra in VO(HPO(4)).0.5H(2)O

Based on combined DFT/broken symmetry approach, a theoretical analysis of the exchange interactions in the VO(HPO(4)).0.5H(2)O solid is performed. Depending on the crystallographic structures reported in the literature, two very different spin models are formulated. In addition, a complete fit of the temperature-dependent (31)P NMR chemical shift is performed to determine exchange and hyperfine constants. The magnetic models used in the fit are those obtained by our theoretical calculations. The comparison between the calculated and fitted exchange constants confirms the adequacy of an isolated dimer model and rules out the alternating antiferromagnetic chain model for VO(HPO(4)).0.5H(2)O.