Effect of counterions on hypervalent interactions in prereaction complexes of S N 2 reactions of bisphenoid compounds of the second and third period elements

The electronic and molecular structures of the (LiF) _n XF _m complexes (X = C, N, O, F, Si, P, S, Cl; m = 1–4, n = 0, 1, 3) were studied by the ab initio (MP2(full)/6-311+G*) and density functional theory (B3LYP/6-311+G*) methods. All bisphenoid anionic structures XF _m ^? (X = C, N, O, F, Si, P, S, Cl; m = 2–5) of elements of the second and third periods, except carbon fluorides, are most stable in the hypervalent state of atoms with strongly elongated axial bonds. Carbon tetrafluoride forms a stable intermolecular F^?...CF₄ complex. In all cases of addition of the Li atom as a counterion, the most stable intermolecular complex of lithium fluoride with fluorides of elements is stabilized by the hypervalent interaction. In all cases when the counterion is complicated to lithium trifluoride, except for (LiF)₃CF₄ and (LiF)₃NF₃, the hypervalent structure with equal and elongated X-F axial bonds is stabilized. In the cases of the (LiF)₃CF₄ and (LiF)₃NF₃ complexes, the prereaction structures bound by the strong hypervalent interaction are stabilized.     

High-resoluttin 29Si NMR in solid silicates: Correlations between shielding tensor and Si-O bond length

²⁹Si NMR spectra of polycrystalline Ca₆[Si₂O₇¦(OH)₆] and [(CH₃)₄N] ₈Si₈O₂₀·69H₂O were measured using the cross polarization double-resonance technique. Observed shielding tensors are related to the known Si-O bond systems. The arrangement of the four Si-O bonds in the SiO₄ tetrahedra is reflected by the ²⁹Si shielding tensor. The most shielded direction corresponds to the shortest Si-O bond.     

Global Planar Tetra-, Penta- and Hexa-coordinate Silicon Clusters Constructed by Decorating SiO3 with Alkali Metals

The achievement of the rule-breaking planar hypercoordinate motifs (carbon and other elements) is mainly attributed to a practical electronic stabilization mechanism, where the bonding of the central atom p_z π electrons is a crucial issue. We have demonstrated that strong multiple bonds between the central atom and partial ligands can be an effective approach to explore stable planar hypercoordinate species. A set of planar tetra-, penta- and hexa-coordinate silicon clusters were herein found to be the lowest-energy structure, which can be viewed as decorating SiO₃ by alkali metals in the MSiO₃⁻, M₂SiO₃ and M₃SiO₃⁺ (M=Li, Na) clusters. The strong charge transfer from M atoms to SiO₃ effectively results in [M]⁺SiO₃²⁻, [M₂]²⁺SiO₃²⁻ and [M₃]³⁺SiO₃²⁻ salt complexes, where the Si−O multiple bonding and structural integrity of the Benz-like SiO₃ framework is maintained better than the corresponding SiO₃²⁻ motifs. The bonding between M atoms and SiO₃ motif is best described as M⁺ forming a few dative interactions by employing its vacant s, p, and high-lying d orbitals. These considerable M←SiO₃ interactions and Si−O multiple bonding give rise to the highly stable planar hypercoordinate silicon clusters.     

Synthesis,characterization, and X-ray crystal structures of zinc(II) complexes with schiff bases 2-[(2-isopropylaminoethylimino)methyl]-5-methoxyphenol and N,N-dimethyl-N′-(1-pyridin-2-ylethylidene)ethane-1,2-diamine

Two new zinc(II) complexes, [ZnBr₂L¹] (I) and [ZnBr₂L²] (II), where L¹ is 2-[(2-isopropylaminoethylimino)methyl]-5-methoxyphenol and L² is N,N-dimethyl-N′-(1-pyridin-2-yl-ethylidene)ethane-1,2-diamine, were prepared and characterized using elemental analysis, FT-IR spectroscopy, and X-ray single-crystal diffraction. In complex I, the Zn(II) atom is coordinated by one phenolic O and one imino N atoms of L¹ and two Br atoms, forming a tetrahedral coordination geometry. In complex II, the Zn(II) atom is in a trigonal bipyramidal coordination geometry with the equatorial plane formed by the imino N atom of L² and two Br atoms and with the two axial positions occupied by one pyridine N and one amino N atoms of L². In the crystal structure of I, the mononuclear zinc complex molecules are linked through intermolecular N-H…O and N-H…Br hydrogen bonds, forming chains running along the y axis. The chains are further linked via intermolecular C-H…Br hydrogen bonds. In the crystal structure of II, the mononuclear zinc complex molecules are linked through intermolecular C-H…Br hydrogen bonds, forming a 3D network.     

cis‐Dichloridobis(1,10‐phenanthroline‐5,6‐diol‐κ2N,N′)manganese(II): a supramolecular chain formed via O—H...Cl bonds and aromatic stacking

The title compound, [MnCl₂(C₁₂H₈N₂O₂)₂], displays a novel supramolecular chain formed by intermolecular O—H...Cl hydrogen bonds and aromatic stacking. The molecule has crystallographically imposed twofold symmetry with the Mn^II atom on the twofold axis. In the 1,10‐phenanthroline‐5,6‐diol ligand, each H atom of the two hydroxy groups is oriented towards the other hydroxy O atom. Both hydroxy groups form intermolecular O—H...Cl hydrogen bonds with a single Cl atom of an adjacent molecule. These hydrogen bonds connect the molecules via operation of the molecular twofold axis and the centre of inversion of the crystal lattice, forming a doubly‐bridged one‐dimensional structure with Mn atoms as the nodes. Strong aromatic π‐stacking between two antiparallel neighbouring 1,10‐phenanthroline‐5,6‐diol ligands also helps to stabilize the chain.     

Crystal and molecular structure of compounds containing a hypervalent O−Si(C3)−O fragment. Parametrization in the description of the coordination environment of the silicon atom

We have carried out an X-ray structural investigation of four pentacoordinated silicon compounds with a hypervalent O?Si(C₃)?O fragment. In their molecules, the axial Si?O bond lengths range from 1.711 to 2.785 Å. Analysis of the geometry of such fragments containing other atoms in axial positions shows that the main parameter determining the state of the hypervalent fragment is deviation of the Si atom from the plane of equatorial substituents. Some consequences of this study for structural modeling of nucleophilic substitution reactions are discussed.     

Diacetatobis­(tri­methyl­ene­thio­urea)­zinc(II) monohydrate

In the title compound, [Zn(CH₃COO)₂(C₄H₈N₂S)₂]·H₂O, the Zn atom is tetrahedrally coordinated in the ZnO₂S₂ form. N—H?O and O—H?O intramolecular and intermolecular hydrogen bonds are formed by the four N atoms and the water mol­ecule. N—H?O intermolecular hydrogen bonds and C—H?S and C—H?O intermolecular interactions interconnect columns formed by the mol­ecules into layers. Adjacent layers are then linked by other N—H?O and O—H?O intermolecular hydrogen bonds to form a three‐dimensional framework throughout the structure. The orientations of the acetate planes are such that the Zn atom lies within them.     

(1R,2S)‐2‐[N‐Methyl‐N‐(4‐toluene­sulfonyl)amino]‐1‐phenylpropan‐1‐ol

In the title compound, C₁₇H₂₁NO₃S, the S atom is in a distorted tetrahedral geometry and the N atom exhibits sp² character. The antiperiplanar conformation is observed for the N and hydroxyl‐O atoms and the torsion angle around the N—C linkage is ?136.3 (2)°. The mol­ecules are linked by O—H?O intermolecular hydrogen bonds to form an infinite one‐dimensional chains along the c axis.     

Hydrogen bonds in solid hydroxides, a bond valence approach

The bond valences s_OH due to the O-H bonds of OH⁻ ions in solids have been calculated indirectly from intermolecular H?O distances, viz. those within the Wigner Seitz cell around the respective hydrogen atom, by using the equation s_OH=1−∑s_H?O. The bond valences thus derived are an excellent measure of the strength of O-H bonds [J. Mol. Struct. 351 (1995) 205]. This is shown by their almost linear correlation with the wave numbers of the stretching modes of matrix isolated OD⁻ ions observed with IR or Raman experiments. In the case of very weak or lacking hydrogen bonds, this correlation fails because then other interionic bonding phenomena than hydrogen bonds as metal-oxygen interactions and hydrogen-hydrogen repulsion etc. gain in importance or dominate finally and, hence, partly or fully determine the wave numbers of the OD stretching modes, which, however, still remain a measure of the respective bond strengths. The relation of the distances r_OH, the bond valences s_OH, and the stretching modes ν_OD of both free, gaseous OH⁻ ions and H₂O molecules and those embedded in crystalline matrices is discussed.     

Synthesis,Crystal, and Molecular Structure of Bis(thiosemicarbazide)zinc(II) 1,5-Naphthalenedisulfonate Monohydrate [Zn(Tsc)<Subscript>2</Subscript>](1,5-Nds) · H<Subscript>2</Subscript>O

Complex [Zn(Tsc)₂](Nds) · H₂O (I) (Tsc is thiosemicarbazide, NH₂NHC(=S)NH₂, and 1,5-Nds₂–is the double-deprotonated anion of 1,5-naphthalenedisulfonic acid, C₁₀H₆(SO₃) ₂ ^2- ) has been synthesized and studied by IR spectroscopy, thermogravimetry, and X-ray diffraction analysis. The structural units of crystal I are complex cations [Zn(Tsc)₂]²⁺, anions (Nds)^2–, and crystallization water molecules. The Zn atom is coordinated along the vertices of the distorted tetrahedron by two sulfur atoms and two nitrogen atoms of two bidentate chelate (S,N) Tsc ligands. The structural units of crystal I are joined together by a branched network of the N–H···O hydrogen bonds involving independent donor hydrogen atoms of the NH₂ and NH groups of Tsc molecules (nine of ten, except for the H(6A) atom), all six independent acceptor oxygen atoms of the SO₃ groups of the Nds^2– anions, and two acceptor hydrogen atoms and one donor oxygen atom of the water molecule.     

Synthesis,crystal structure and magnetic properties of a nickel(II) complex with pyridine-substituted nitronyl nitroxide radicals

The complex [Ni(NIT2Py)(PDA)(H₂O)]·(MeOH)(H₂O) [NIT2Py = 2-(2-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide; PDA = 2,6-pyridine dicarboxylic acid] has been prepared and structurally characterized. The Ni^II ion is in a distorted octahedral environment: one nitrogen atom and one oxygen atom from the NIT2Py, one nitrogen atom from the PDA and one oxygen atom from the H₂O in the basal plane; two oxygen atoms from the PDA in the axial position. The units of [Ni(NIT2Py)(PDA)(H₂O)] were connected as a one dimension chain by intermolecular hydrogen bonds. The complex exhibits intramolecular antiferromagnetic interactions between the Ni^II ion and the NIT2Py.     

Mechanisms for H2O decomposition on the Si(111)‐7 × 7 surface: A DFT cluster model study

The mechanisms for the complete decomposition of water molecules on the Si (111)‐7 × 7 surface were investigated theoretically. The reaction pathways for dissociation of four water molecules over the adatom and rest atom sites were calculated using the density functional theory (DFT) in conjunction with the B3LYP functional. The calculated results demonstrated that the initial O? H bond dissociation from the first H₂O to form the adsorbed OH species is more preferential on the adatom site (Si_a) than the rest atom site (Si_r) of Si (111)‐7 × 7. Four water molecules dissociate successively over the adatom site, backbonds of adatoms which are saturated by OH species can reasonably be the place of insertion of oxygen atoms, yielding a tetrahedral SiO₄ structure with one on top and three inserted oxygen atoms. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Preparation and crystal structure of hydrated crystalline complex of ciprofloxacin and copper tetrachloride

Synthesis of a complex formed by ciprofloxacin (cfH) and copper(II) chloride is described; its crystal structure is reported and analyzed in comparison to related compounds. The obtained compound (cfH₃)CuCl₄·H₂O (cfH ₃ ²⁺ — double protonated cfH molecule) crystallizes as platelets of P2₁/c symmetry having the unit cell parameters a = 13.491(1) Å, b = 11.0459(7) Å, c = 16.299(1) Å; β = 111.392(7)°. Carbonyl oxygen O(1) is protonated, and hydrogen atom combined with it forms an intramolecular hydrogen bond with carboxylic O(2) oxygen (O(1)?O(2) = 2.642(5) Å). Terminal nitrogen atom N(3) of the piperazinyl group is also protonated, and two its hydrogen atoms participate in hydrogen bonds of N-H?Cl type. The structure also has hydrogen bonds O-H?O, O-H?Cl with the participation of water molecules which occupy hydrophilic channels. Molecular ions cfH ₃ ²⁺ make couples with intrapair π?π interactions.     

Cluster self-organization of Li- and TR-Containing silicate systems: Suprapolyhedral precursors and self-assembly of LiY<Superscript>[7]</Superscript>SiO<Subscript>4</Subscript> and LiTR<Superscript>[6]</Superscript>SiO<Subscript>4</Subscript> (TR = Y,Sc, Tm) dimorphic crystal structures

The combinatorial and topologic analysis (the TOPOS 4.0 program package, the coordination sequences method) is carried out for silicates LiY^[7]SiO₄ (a synthetic phase (SIN), space group P2₁/b) and LiTR^[6]SiO₄ (TR = Y, Sc, Tm; the olivine structure type (OLI), space group Pnma). The framework of these silicates is built of polyhedral structural units YO₇ + SiO₄ and YO₆(ScO₆, TmO₆) + SiO₄, respectively. The framework structures TRTO₄, built of Y,O- and Si,O-polyhedra in SIN and OLI, respectively, are represented as three-dimensional (3D) Y,Si-nets with distal oxygen atoms. The crystal-forming 2D Y,Si-net is the four-nodal net Y(1) (43333) + Y(2) (4334334) + Si(1) (434) + Si(2) (3343) for SIN and the two-nodal net Y 443333 + Si 4433 for OLI. In the 2D Y,Si, nets, equivalent chains of four-nodal precursor clusters Y-Si-Y-Si with three shared bonds are identified. In the dimorphic crystals structures of LiTRTO₄, polyhedral precursor clusters Li₂TR₂T₂ are linked into a ring with two Li atoms lying one above the other below the center of the ring, thus retaining the center of symmetry\(\bar 1\). Precursor clusters Li₂TR₂T₂ through the matrix self-assembly mechanism control the evolution of high-level crystal-forming clusters. Cluster coordination numbers in layers are equal to six.     

Structure and stability of oxygen adsorption on Si(n) (n = 5-10) clusters

The structures, binding energies, and electronic properties of one oxygen atom (O) and two oxygen atoms (2O) adsorption on silicon clusters Si(n) with n ranging from 5 to 10 are studied systematically by ab initio calculations. Twelve stable structures are obtained, two of which are in agreement with those reported in previous literature and the others are new structures that have not been proposed before. Further investigations on the fragmentations of Si(n)O and Si(n)O2 (n = 5-10) clusters indicate that the pathways Si(n)O --> Si(n-1) + SiO and Si(n)O2 --> Si(n-2) + Si2O2 are most favorable from thermodynamic viewpoint. Among the studied silicon oxide clusters, Si8O, Si9O, Si5O2 and Si8O2 correspond to large adsorption energies of silicon clusters with respect to O or 2O, while Si8O, with the smallest dissociation energy, has a tendency to separate into Si7 + SiO. Using the recently developed quasi-atomic minimal-basis-orbital method, we have also calculated the unsaturated valences of the neutral Si(n) clusters. Our calculation results show that the Si atoms which have the largest unsaturated valences are more attractive to O atom. Placing O atom right around the Si atoms with the largest unsaturated valences usually leads to stable structures of the silicon oxide clusters.     

Ab initio calculations of oxygen nuclear quadrupole coupling constants and oxygen and silicon NMR shielding constants in molecules containing SiO bonds

Ab initio coupled Hartree—Fock perturbation theory (CHFPT) calculations employing large gaussian basis sets have been used to evaluate the electric field gradient at O, q^o, and the NMR shieldings at O and Si, σ^o and σ^Si, in the molecules SiO, SiO₂, Si₂O₂, H₂SiO, SiO⁻⁴₄, Si(OH)₄ and (H₃Si)₂O. Species containing Si bonded to three or fewer atoms have small NMR shieldings at both O and Si while those with four-coordinate Si have systematically larger O and Si shieldings. A significant positive correlation is observed between calculated O and Si NMR shieldings. Reduction of the SiO distance in SiO⁴⁻₄ and H₃SiOSiH₃ gives a significant reduction in the magnitude of q^o and a small increase in σ. Anisotropies in σ^Si are large for two- and three-coordinate Si (200–900 ppm) but for (H₃Si)₂O the σ^Si anisotropy is only ≈60 ppm. Anisotropies in σ^o are generally larger than those in σ^Si, with values larger than 200 ppm for both SiO⁻⁴₄ and (H₃Si)₂O. Values of q^o for SiO⁴⁻₄ and (H₃Si)₂O are in qualitative agreement with experimentally determined values for nesosilicates and SiO₂ polymorphs, respectively, but all the q^o values appear to be exaggerated at the Hartree—Fock level. Also, q^o values are not greatly different for the exotic species SiO, Si₂O₂, etc. compared to the typical silicate models SiO⁴⁻₄ and (H₃Si)₂O. Calculated isotropic chemical shifts yield good values for the Si chemical shift differences of SiF₄, SiO⁴⁻₄ and (H₃Si)₂O and for the O chemical shift difference of SiO⁴⁻₄ and H₂O. For SiO⁴⁻₄ the paramagnetic contribution to the Si shift, σ^pSi is dominated by contributions from the t₂ symmetry SiO bonding orbital and σ^pO is dominated by contributions from the t₂ symmetry O 2p non-bonding orbital.     

Bis(5‐tert‐butyl‐2‐hydroxy‐3‐methylbenzyl)(6‐hydroxyhexyl)ammonium chloride monohydrate,an anion receptor complex

In the title compound, C₃₀H₄₈NO₃⁺·Cl⁻·H₂O, the cation acts with a water molecule as a chloride ion receptor. The chloride ion forms three strong intramolecular hydrogen bonds. The water molecule forms both an intramolecular bridge between one phenol H atom and the chloride ion, and an intermolecular link to the aliphatic alcohol O atom. Weak intermolecular C—H...Cl and C—H ...O hydrogen bonds provide additional packing interactions.     

Quantum-chemical study of the geometric and electronic structure of the chromate anion CrO 4 2− and a chromate group on the surface of finely divided silica by the CNDO/2 method

A comparative study of the geometric and electronic structure of the chromate anion CrO ₄ ^2– and a chromate group on the surface of finely divided silica (Si-O)₂-CrO₂, which was simulated by a CrO₉Si₆H₁₂ cluster, has been carried out by the SCF-MO-LCAO method in the all-valence-electron CNDO/2 approximation. The data obtained on the equilibrium geometry of the chromate group attest to the formation of a double bond between the Cr atom and each O atom (which is not bonded to Si). It has been shown that the support has a significant stabilizing influence on the energy of the MO's of the chromate group. The chromate group on an SiO₂ surface is characterized by partial delocalization of the frontier MO's among the skeletal bonds; however, the dominant contribution to the HOMO is made by the 2p AO of the oxygen atoms in the coordination shell of the Cr atom (70%), and the dominant contribution to the LUMO is made by the 3d AO of the chromium atom (50%). The positions and composition of the lowest unoccupied molecular orbitals point out the possibility of the display of electron-acceptor properties by a chromate group on an SiO₂ surface.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 5, pp. 602–606, September–October, 1988.