Valence charge concentrations, electron delocalization and beta-agostic bonding in d0 metal alkyl complexes

In this paper we describe a range of model d(0) metal ethyl compounds and related complexes, studied by DFT calculations and high resolution X-ray diffraction. The concept of ligand-opposed charge concentrations (LOCCs) for d(0) metal complexes is extended to include both cis-and trans-ligand-induced charge concentrations (LICCs) at the metal, which arise as a natural consequence of covalent metal-ligand bond formation in transition metal alkyl complexes. The interplay between locally induced sites of increased Lewis acidity and an ethyl ligand is crucial to the development of a beta-agostic interaction in d(0) metal alkyl complexes, which is driven by delocalization of the M-C bonding electrons. Topological analysis of theoretical and experimental charge densities reveals LICCs at the metal atom, and indicates delocalization of the M-C valence electrons over the alkyl fragment, with depletion of the metal-directed charge concentration (CC) at the alpha-carbon atom, and a characteristic ellipticity profile for the C(alpha)-C(beta) bond. These ellipticity profiles and the magnitude of the CC values at C(alpha) and C(beta) provide experimentally observable criteria for assessing quantitatively the extent of delocalization, with excellent agreement between experiment and theory. Finally, a concept is proposed which promises systematic control of the extent of C-H activation in agostic complexes.     

Crystal and molecular structures and experimentally determined charge densities of fluorinated ethenes

Crystals of various fluorinated ethenes were grown by in situ crystallization from their melts on a diffractometer, allowing the structures of tetrafluoroethene (C2F4), trifluoroethene (C2HF)3, 1,1-difluoroethene (C2H2F2), (E)-1,2-difluoroethene (C2H2F2), and (Z)-1,2-difluoroethene (C2H2F2) to be determined by X-ray crystallography. Unexpectedly, the C=C bond lengths show only small variations arising from fluorine substitution. These findings are supported by ab initio calculations at a DFT level of theory and the results of topological analyses of the experimentally determined and theoretically calculated charge densities.     

Molecular structures and conformations: Experiment and theory

Theoretical calculations in combination with experimental gas phase structure research can be performed in two ways. The first is to support and improve experimental analyses by including additional data from theoretical calculations. This is to the advantage of the experiment. The second way is a comparison of geometric structures and conformational properties obtained with different theoretical methods with the experimental result. This comparison indicates which theoretical method or methods are suitable for a specific compound. This approach is to the advantage of the theory. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 123–128, 1998     

Experimental,experimental–theoretical and theoretical estimates of intermolecular interaction energies in η6-[(5-methyl-1,3-oxazolidin-3-yl)benzene]tricarbonylchromium(0)

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Chemical bonding in view of electron charge density and kinetic energy density descriptors

Stalke's dilemma, stating that different chemical interpretations are obtained when one and the same density is interpreted either by means of natural bond orbital (NBO) and subsequent natural resonance theory (NRT) application or by the quantum theory of atoms in molecules (QTAIM), is reinvestigated. It is shown that within the framework of QTAIM, the question as to whether for a given molecule two atoms are bonded or not is only meaningful in the context of a well‐defined reference geometry. The localized‐orbital‐locator (LOL) is applied to map out patterns in covalent bonding interaction, and produces results that are consistent for a variety of reference geometries. Furthermore, LOL interpretations are in accord with NBO/NRT, and assist in an interpretation in terms of covalent bonding. © 2008 Wiley Periodicals, Inc.J Comput Chem, 2009.     

Molecular and supramolecular structures of complexes formed by polyelectrolyte‐surfactant interactions: effects of charge density and compositions

The process of complex formation and the structure of the complexes were studied for the surfactant binding of the N‐alkylpyridinium chlorides (C_nPyCl, n = 12, 16) to the sodium salt of poly(styrenesulfonate) (NaPS) and its copolymer with styrene. Both the NaPS and the amphiphilic copolymer form non‐stoichiometric complexes with an excess of the cationic surfactants. The NaPS‐complex with pronounced short‐range (d₁) and long‐range (d₂) orderings is insoluble, and the amphiphilic copolymer‐complex is water‐soluble when bound with extra charges. The mechanism of the complex formation was discussed in terms of the charge density and chemical composition of the polymers. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 635–644, 1999     

New La2/3TiO3 Derivatives: Structure and Impedance Spectroscopy

New phases which arise from inserting Na cations on the vacant A-sites of the compound La_2/3TiO₃ have been obtained, giving rise to the series La_4/3−xNa_3xTi₂O₆ (for x=0.16 and 0.28). These phases adopt a perovskite-type structure as deduced from their characterization by electron microscopy and neutron diffraction. Rietveld analyses show that the symmetry is orthorhombic (S.G. Ibmm). Electrical conductivity was determined by impedance spectroscopy, as a function of temperature. A similar behavior is observed for both samples, which behave as ionic conductors with activation energies of 0.92(3) and 0.92(5) eV, respectively.     

Ultrafast electron diffraction: oriented molecular structures in space and time.

The technique of ultrafast electron diffraction allows direct measurement of changes which occur in the molecular structures of isolated molecules upon excitation by femtosecond laser pulses. The vectorial nature of the molecule-radiation interaction also ensures that the orientation of the transient populations created by the laser excitation is not isotropic. Here, we examine the influence on electron diffraction measurements--on the femtosecond and picosecond timescales--of this induced initial anisotropy and subsequent inertial (collision-free) molecular reorientation, accounting for the geometry and dynamics of a laser-induced reaction (dissociation). The orientations of both the residual ground-state population and the excited- or product-state populations evolve in time, with different characteristic rotational dephasing and recurrence times due to differing moments of inertia. This purely orientational evolution imposes a corresponding evolution on the electron scattering pattern, which we show may be similar to evolution due to intrinsic structural changes in the molecule, and thus potentially subject to misinterpretation. The contribution of each internuclear separation is shown to depend on its orientation in the molecular frame relative to the transition dipole for the photoexcitation; thus not only bond lengths, but also bond angles leave a characteristic imprint on the diffraction. Of particular note is the fact that the influence of anisotropy persists at all times, producing distinct differences between the asymptotic "static" diffraction image and the predictions of isotropic diffraction theory.     

Valence state parameters of all transition metal atoms in metalloproteins—development of ABEEMσπ fluctuating charge force field

To promote accuracy of the atom‐bond electronegativity equalization method (ABEEMσπ) fluctuating charge polarizable force fields, and extend it to include all transition metal atoms, a new parameter, the reference charge is set up in the expression of the total energy potential function. We select over 700 model molecules most of which model metalloprotein molecules that come from Protein Data Bank. We set reference charges for different apparent valence states of transition metals and calibrate the parameters of reference charges, valence state electronegativities, and valence state hardnesses for ABEEMσπ through linear regression and least square method. These parameters can be used to calculate charge distributions of metalloproteins containing transition metal atoms (Sc‐Zn, Y‐Cd, and Lu‐Hg). Compared the results of ABEEMσπ charge distributions with those obtained by ab initio method, the quite good linear correlations of the two kinds of charge distributions are shown. The reason why the STO‐3G basis set in Mulliken population analysis for the parameter calibration is specially explained in detail. Furthermore, ABEEMσπ method can also quickly and quite accurately calculate dipole moments of molecules. Molecular dynamics optimizations of five metalloproteins as the examples show that their structures obtained by ABEEMσπ fluctuating charge polarizable force field are very close to the structures optimized by the ab initio MP2/6–311G method. This means that the ABEEMσπ/MM can now be applied to molecular dynamics simulations of systems that contain metalloproteins with good accuracy. © 2014 Wiley Periodicals, Inc.     

常压下能量与电荷在分子间的传递机制

离子源是质谱仪的心脏,制备待测物的离子是质谱分析的首要前提.如何有效地电离含有复杂基体的实际样品中的痕量待测物分子是制约实际样品直接质谱分析的重要瓶颈.本文提出了用电子云模型帮助理解能量与电荷传递和中性分子电离过程的关系,从分子的基本物性参数、能量耦合方式等角度讨论了能量与电荷在不同相态、组成和维度的复杂基体样品分子间传递的过程,着重阐述了能荷传递过程中分子的电子云变化规律及其对分子电离的影响,并简要讨论了这些影响在质谱分析中的应用.     

Chlorobis(pentafluoroethyl)phosphane: improved synthesis and molecular structure in the gas phase

(C₂F₅)₂PCl is now accessible through a significantly improved synthesis protocol starting from the technical product (C₂F₅)₃PF₂. (C₂F₅)₃PF₂ was reduced in the first step with NaBH₄ in a solvent‐free reaction at 120 °C. The product, P(C₂F₅)₃, was treated with an excess of an aqueous sodium hydroxide solution to afford the corresponding phosphinite salt Na⁺(C₂F₅)₂PO^? selectively under liberation of pentafluoroethane. Subsequent chlorination with PhPCl₄ resulted in the selective formation of (C₂F₅)₂PCl, which was isolated by fractional condensation in an overall yield of 66 %. The gas electron diffraction (GED) pattern for (C₂F₅)₂PCl was recorded and found to be described by a two‐conformer model. A quantum chemical investigation of the potential‐energy surface revealed the possible existence of many low‐energy conformers, each with a number of low‐frequency vibrational modes and therefore large‐amplitude motions. The conformer calculated to be most stable was also found to be most abundant by GED and comprised 61(5) % of the total. The molecular structure parameters determined by GED were in good agreement with those calculated at the MP2/TZVPP level of theory; the only significant difference was a discrepancy of about 3° in the C‐P‐C angle, which, for the lowest‐energy conformer, was refined to 98.2(4)° and was calculated to be 94.9°.     

ChemInform Abstract: The Gas Phase Structures of CF3NBr2 and (CF3)2NBr.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.     

X-ray diffraction and quantum-chemical study of 3-(1,3-dioxolan-2-yl)-4,6-dinitrobenzo[d]isoxazole

X-ray diffraction study of the molecular structure of 3-(1,3-dioxolan-2-yl)-4,6-dinitrobenzo[d]isoxazole in the crystal, and quantum-chemical calculations of the isolated molecule demonstrated that the differences in the reactivity of the nitro groups at positions 4 and 6 are not associated with the electronic structure of the molecule but are, apparently, attributed to the properties of the corresponding ipso- complexes.     

Structural characterization of TaMe3Cl2 and Ta(PMe3)2Me3Cl2, a pair of five and seven-coordinate d0 tantalum methyl compounds

The trimethylphosphine complex Ta(PMe(3))(2)Me(3)Cl(2) has been synthesized by addition of PMe(3) to TaMe(3)Cl(2). The molecular structures of both TaMe(3)Cl(2) and Ta(PMe(3))(2)Me(3)Cl(2) have been determined by X-ray diffraction thereby demonstrating that, in the solid state, these complexes respectively adopt trigonal bipyramidal and capped trigonal prismatic geometries.