Showing papers on "Tetrahedral molecular geometry published in 2001"

Adsorption Kinetics and Size Exclusion Properties of Probe Molecules for the Selective Porosity in a Carbon Molecular Sieve Used for Air Separation

Abstract: The adsorption characteristics of a series of planar (ethylene, benzene, and pyridine) and tetrahedral (methane, chloromethane, dichloromethane, chloroform, and carbon tetrachloride) molecules on a carbon molecular sieve used for air separation (CMS A) were investigated over a range of temperatures as a function of pressure, to study the selective porosity. The size-exclusion characteristics of planar and tetrahedral molecules indicate that the selective porosity behaves as though it has spherical-shaped structural characteristics and, therefore, two minimum dimensions need to be considered in relation to size exclusion. The partial exclusion of the probe molecule adsorptives from the microporous structure for sizes >360 pm allowed the selective and nonselective microporosity and the meso/macroporosity to be quantified. Adsorption kinetics obey a linear-driving-force mass transfer, combined barrier resistance/diffusion, or Fickian kinetic models depending on the adsorptive and experimental conditions. Com...

Bis-Salicylaldiminato Complexes of Zinc. Examination of the Catalyzed Epoxide/CO2 Copolymerization

Abstract: A series of salicylaldimine ligands of the general formula (NR2C7H5-x(R1)xOH) [x = 1 or 2; R1 = Me, tBu, Cl, OMe; R2 = 2,6-iPr2C6H3, or 3,5-(CF3)2C6H3] have been synthesized and characterized via 1H and 13C NMR, elemental analysis, and X-ray crystallography. The concomitant series of zinc bis(salicylaldiminato) complexes of the general formula have been synthesized and characterized in the solid state by X-ray crystallography. All complexes crystallized as four coordinate monomers with distorted tetrahedral geometry about the zinc center. The O−Zn−O angles range between 105 and 112.5°, and the N−Zn−N bond angles were more obtuse spanning the range 122.9−128.9°. The only deviation from distorted tetrahedral geometry occurred when R2 = 3,5-(CF3)2C6H3 which crystallized as a distorted trigonal bipyramidal dimeric species with Oax−Zn−Oax bond angles of 165.00(15)°. The equatorial angles approach 120° except for the Neq−Zn−Neq angle of 110.54(16)° which is attributed to the strain of the bridging ligands. The ...

Liquid-liquid phase transitions of phosphorus via constant-pressure first-principles molecular dynamics simulations.

Abstract: Pressure-induced phase transitions in liquid phosphorus have been studied by constant-pressure first-principles molecular dynamics simulations. By compressing a low-pressure liquid which consists of the tetrahedral P4 molecules, a structural phase transition from the molecular to polymeric liquid (a high-pressure phase) observed in the recent experiment by Katayama et al. [Nature (London) 403, 170 (2000)] was successfully realized. It is found that this transition is caused by a breakup of the tetrahedral molecules with large volume contraction. The same transition is also realized by heating. This indicates that only the polymeric liquid can stably exist at high temperature.

Dinuclear double helicates with a twist: synthesis, structure and supramolecular entanglement in [M2L2] metallo-helices {M = Co(II), Cu(II), H2L = bis(N-salicylidene-4,4′-diaminodiphenyl)methane}

Abstract: Self-assembly of the tetradentate ligand bis(N-salicylidene-4,4′-diaminodiphenyl)methane, (H2L), around Co(II) and Cu(II) gives rise to the dinuclear double helicates [Co2L2]·1.7CH2Cl2·0.6H2O, (1) and [Cu2L2]·CH2Cl2 (2), which have been characterised by a single crystal X-ray diffraction study. Complexes 1 and 2 are structurally similar with two four coordinate metal centres in slightly distorted tetrahedral geometries in 1 and severely distorted tetrahedral geometry in 2. The different geometric preferences of Co(II) (tetrahedral) and Cu(II) (square planar) contort the ligands such that extended planarity is absent in 1 whilst present in 2. This distortion results in a tighter helical twist in 1 compared with 2, which consequently has a larger metal–metal separation. Complexes 1 and 2 each exhibit three-dimensional supramolecular entanglement brought about by elaborate inter-molecular π⋯π (face-to-face) and C–H⋯π interactions.

Cobalt(II) complexes of 1,2-(diimino-4′-antipyrinyl)ethane and 4-N-(4′-antipyrylmethylidene)aminoantipyrine

Abstract: Cobalt(II) complexes of the Schiff bases 1,2-(diimino-4′-antipyrinyl)ethane (GA) and 4-N-(4′-antipyrylmethylidene)aminoantipyrine (AA) have been prepared and characterised by elemental analysis, electrical conductance in non-aqueous solvents, i.r. and electronic spectra, as well as by magnetic susceptibility measurements. The complexes have the general formulae [Co(GA)X]X (X = ClO−4 or NO3−), [Co(GA)X2] (X = Cl−, Br− or I−), [Co(AA)2]X2 (X = ClO4−, NO3−, Br− or I−) and [Co(AA)Cl2]. GA acts as a neutral tetradentate ligand, coordinating through both carbonyl oxygens and both azomethine nitrogens. In the perchlorate and nitrate complexes of GA one anion is coordinated in a bidentate fashion, whereas in the halide complexes both anions are coordinated to the metal, generating an octahedral geometry around the Co ion. AA acts as a neutral bidentate ligand, coordinating through the carbonyl oxygen derived from the aldehydic moiety and the azomethine nitrogen. Both anions remain ionic in the perchlorate, nitrate, bromide and iodide complexes of AA, whereas both anions are coordinated to the metal ion in the chloride complex, resulting tetrahedral geometry around the Co ion.

On the fragment ion angular distributions arising from the tetrahedral molecule CH3I

Abstract: The mass spectra for both horizontal and vertical polarizations and the angular distributions of fragment ions arising from Coulomb explosion of tetrahedral methyl iodide (CH3I) ions, obtained at a laser intensity of 1016 W cm-2 are presented. All fragment ion distributions are peaked along the direction corresponding to collinearity of the laser electric field with the time-of-flight mass spectrometer axis. The In + ion (n≤7) angular distributions from the dissociation of the parent ions are all of similar widths, which would imply a geometric, as opposed to dynamic, alignment. Additionally, the lower-charged I ions have an isotropic component that decreases as the charge state increases. Measurements of the CHm+ (m≤3), Cp + (p≤4) and H+ ion distributions show that these are also maximal along the polarization direction. Furthermore, there is also a CH22+ ion peak present in the CHm group, which has a distribution similar to those measured for the other ions. This mass peak is the prominent multi-charged ion in this group. As the CH3I molecule is initially tetrahedral, these results suggest that the molecular structure undergoes a change such that the H-C and C-I bonds tend to lie along the field. Several authors have described work which first aligned CH3I molecules with a nanosecond laser and then photodissociated with a femtosecond laser, to produce fragment ion distributions. This is the first time that the angular distributions from a tetrahedral molecule have been presented using femtosecond laser pulses only and in the case of CH3I, for fragments other than CH3+ and I+. The fragment energetics from the single CH3I molecule have been compared with those from recent work dealing with the Coulomb explosion of CH3I clusters.

Weak magnetic coupling of coordinated verdazyl radicals through diamagnetic metal ions. Synthesis, structure, and magnetism of a homoleptic copper(I) complex.

Abstract: The synthesis and characterization of a copper(I) complex containing two-coordinated chelating verdazyl radicals are described. The structure (shown) consists of a distorted tetrahedral geometry at the copper ion; the angle between the planes defined by the two verdazyl rings is nearly 116°. Variable-temperature magnetic-susceptibility data for the complexes were obtained, the analysis of which revealed very weak intramolecular magnetic coupling between the two-coordinated radicals.

Bis(melaminium) sulfate dihydrate.

Abstract: The crystals of a new melaminium salt, bis(2,4,6-tri­amino-1,3,5-triazin-1-ium) sulfate dihydrate, 2C3H7N6+·SO42−·2H2O, are built up from monoprotonated melaminium(1+) residues, sulfate(2−) anions and water mol­ecules. The SO42− ion has a slightly distorted tetrahedral geometry. The melaminium residues are interconnected by N—H⋯N hydrogen bonds, forming chains. The chains of melaminium residues develop a three-dimensional network through multiple donor–acceptor hydrogen-bond interactions with sulfate anions and water mol­ecules.

Study with density functional theory method on methane dehydro-aromatization over Mo/HZSM-5 catalysts I: Optimization of active Mo species bonded to ZSM-5 zeolite

Abstract: A study of the structure of active Mo species on HZSM-5 zeolite was carried out by applying the density functional theory (DFT). The interaction of the MoO2+ unit with the zeolite framework had been verified by an optimization procedure based on the DFT. The result of calculation reveals a dicoordinated bridging geometry in which the molybdenum coordinated simultaneously to both of the two lattice oxygen atoms, leading to a quadrangular geometry with an O(24)–Al(12)–O(20) bridge structure. The Mo ions having O(24)–Al(12)–O(20) bridges exist in identical planes, with equal bond distances of 1.72 A to both the O(24) and O(20) atoms and an interatomic distance of 2.86 A to lattice Al. The molybdenum has a tetrahedral geometry and a formal charge of +5. This kind of active Mo center possesses a typical C2v symmetry. Approximation vibrational analysis indicates that the vibrations of Mo–O (framework oxygen) bonds occur at the frequencies between 580 and 928 cm−1, while the vibrations of the Mo–O (extra-framewo...

Structural anomaly in sodium germanate glasses by molecular dynamics simulation

Abstract: The structures of GeO2–Na2O glasses with different Na2O contents, from 0–30 mol%, have been simulated by molecular dynamics (MD) and were found to be in good agreement with the existing experimental results. The Ge–O coordination number was found to increase initially with the addition of alkali oxide. The results indicate that this increase in coordination number was due to the conversion of some of the tetrahedral germanate units into 5-fold coordinated trigonal bipyramidal units. Further additions of alkali oxide, beyond the glass composition range where maxima (or minima) in certain physical properties are experimentally observed (∼18 mol% Na2O), led to the formation of non-bridging oxygen (NBO) atoms and to a decrease of the average Ge–O coordination number. The reconversion to tetrahedral geometry was found to be responsible for the observed decrease in Ge–O coordination. A simple scheme was used to analyse the populations of various structural entities present in the glass system. It is shown that the conversion of 4-fold to 5-fold coordinated units and their back-conversion, with increasing alkali oxide contents, are feasible and consistent.

Distorted cage structures of Sin (n = 20, 24, 26, 28, 30, 32) clusters

Abstract: We have performed calculations on the fullerene cage structures and the binding energies of Sin (n = 20, 24, 26, 28, 30, 32) clusters by the full-potential linear-muffin-tin-orbital molecular dynamics (FP-LMTO-MD) method. It is found that the fullerene cages are not stable, and relax into structures which are severely distorted. Except for Si20, their atomic arrangement tends towards tetrahedral geometry. After the structural distortion, about two silicon atoms can still be filled into the inside spaces of the distorted cages for Sin (n = 26-32).

Rotational-vibrational relative equilibria and the structure of quantum energy spectrum of the tetrahedral molecule P4

Abstract: We find relative equilibria (RE) of the rotating and vibrating tetrahedral molecule P4 and study the correspondence of these RE's to the extremal quantum states in the vibration-rotation multiplet and to the extrema of the semi-quantum rotational energy surfaces obtained for a number of excited vibrational states. To compute the energy of RE's we normalize the full rotation-vibration Hamiltonian H of P4 in the approximation of nonresonant modes ν E 2 and ν F_2 3 and find the stationary points of the resulting normal form (known as reduced effective Hamiltonian H eff ) which is defined on the reduced phase space CP 2 × CP 1 × S 2 . Most of these points are fixed points of the symmetry group action on CP 2 × CP 1 × S 2 . To explain our results in more detail we introduce numerical values of the parameters of H, such as the cubic force constants, using an atom-atom harmonic potential with one adjustable parameter. This simple model gives correct qualitative description of the rotational structure of the lowest excited vibrational states ν 2 , ν 3 and ν 2 + ν 3 of P4.

Metal–nucleobase interaction: di­bromo­bis­(cytosine)­cadmium(II)

Abstract: In the crystal structure of the title compound, [CdBr2(C4H5N3O)2], two N atoms at the 3-position of the cytosine ligands and the two bromide ions complete the distorted tetrahedral geometry around the CdII atom. The cytosine ligands of one type are paired through N—H⋯O hydrogen bonds while the cytosine ligands of another type are chained through N—H⋯O hydrogen bonds.

Metal-selenium interactions. synthesis and crystal structure of [diiodo{1,1′-methylene-bis (diphenyl phosphine selenide)}] zinc(ii)

Abstract: Reaction of 1,1′-methylenebis(diphenylphosphine selenide) {Ph2P(Se)—CH2—P(Se)Ph2, dpmSe2} with zinc(II) diiodide in dry diethylether-benzene mixture formed Znl2(dpmSe2) (1), which has been characterised with the help of analytical data, IR spectroscopy and single crystal X-ray crystallography. Zinc(II) is bonded to two iodine atoms {Zn—I, 2.5732(9), 2.5584(7) A} and two selenium atoms {Zn—Se, 2.5135(8), 2.4906(10) A}. The angles about Zn vary from 103.56(3) to 114.69(3) A, with Se(1)—Zn—Se(2) and I(1) bond angles being 104.57(3) and 114.69(3) A, respectively. Thus dpmSe2 chelates to Zn(II) forming a distorted tetrahedral geometry about the metal centre. In the six membered metallacyclic ring formed by Zn(1), Se(1), P(1), C(13), P(2) and Se(2) atoms, the angles at Se atoms, namely, Zn(1)—Se(1)—P(1) and Zn(1)—Se(2)—P(2) {96.86(4) and 100.67(5)°, respectively} are the lowest. This reveals higher angular flexibility at Se atom vis-a-vis angles at P or C atoms. Complex 1 is the first example of a comp...

AuCuSe4: A Mixed Polychalcogenide with Se32- and Se2- Anions

Abstract: The polychalcogenide compound AuCuSe(4) crystallizes in the monoclinic space group P2(1)/m with a = 4.318(2) A, b = 7.447(1) A, c = 8.099(1) A, beta = 93.33(2)(o), and Z = 2. The structure of AuCuSe(4) is a unique three-dimensional framework assembled from formally Au(3+) and Cu(+) ions and Se(3)(2-) and Se(2-) fragments. Conceptually, the framework is composed of one-dimensional [Au(Se(3))Se](n)(n-) chains linked by Cu(+) atoms. The Au atoms adopt a square planar geometry while Cu atoms adopt a severely distorted tetrahedral geometry. The Se(3)(2-) ligand in this structure shows an unprecedented ligation mode using both its terminal and internal selenium atoms. The material is a p-type narrow gap semiconductor with a band gap of approximately 0.52 eV.

Static and Dynamic Stereochemistry of N-Alkyl- and N,O-Dialkyl- N-9-triptycylhydroxylamine Derivatives

Abstract: Stereochemistry of N-9-triptycylhydroxylamine and its N-alkyl and N,O-dialkyl derivatives was studied. X-ray crystallographic analysis of N,O-diethyl-N-9-triptycylhydroxylamine revealed (a) the chiral nature of the molecule, (b) an almost tetrahedral geometry of the nitrogen atom, and (c) a nearly eclipsing arrangement of the O–Et bond with the nitrogen lone-pair orbital. In solution, enantiomerization takes place rapidly at room temperature on the NMR time scale but slows down at low temperatures. The stereomutation proceeds by way of two processes, “R-passing,” and “O-passing,” in which the alkyl (R) and oxy (OH or OR) groups attached to the nitrogen atom, respectively, pass over a benzene ring, accompanied by nitrogen inversion and partial rotation of the N–O bond. The energy barriers to these processes were obtained by dynamic NMR spectroscopy.

Bis­(di­thio­acetato‐κ2S)­bis­(tri­phenyl­phosphine‐κP)­copper(I) 1.5‐toluene solvate

Abstract: The crystal structure of the title compound, [Cu(C2H3­S2)­(C18­H15­P)2]·1.5C7H8, is reported. The compound is monomeric with the copper metal atom assuming a distorted tetrahedral geometry.

Di­chloro­bis­(tri­phenyl­phosphine oxide)­zinc(II) tetra­hydro­furan disolvate

Abstract: In the crystal structure of the title compound, [ZnCl2{(C6H5)3PO}2]·2THF (THF is tetra­hydro­furan, C4H8O), the ZnII ion is four-coordinated by two Cl atoms and two O atoms in a distorted tetrahedral geometry. Additionally, two solvent THF mol­ecules occupy the open space in the unit cell, not bonding to the Zn atom. The Zn—Cl distances are 2.2108 (6) and 2.2165 (6) A, and the Zn—O distances are 1.9851 (15) and 1.9751 (15) A. Within the tetrahedral coordination sphere around the central Zn atom, the bond angles are O1—Zn—O2 98.16 (6)° and Cl1—Zn—Cl2 120.55 (2)°; the four Cl—Zn—O bond angles range from 107.43 (4) to 110.55 (4)°.