Molecular structure of hydrogen bonded N,N′-diethylurea in non-polar solvents

IR spectroscopy and static dielectric polarization methods are used for studies on self-association of N,N′-diethylurea (DEU) in non-polar solvents. A model of the aggregation in sym-dialkylureas in which the molecules are linked by four hydrogen bonds, is proposed. The model gives satisfactory prediction of the spectroscopic and dielectric behaviour of DEU-organic solvents systems.     

Jahn-Teller distortion,ferromagnetic coupling,and electron delocalization in a high-spin Fe–Fe bonded dimer

Fe₂(N,N’-diphenylformamidinate)₄, 1, first synthesized in 1994, is one of very few non-organometallic compounds with Fe–Fe distances, (2.46 Å) suggestive of an Fe–Fe bond. The electronic structure of 1 has been unclear because of its distorted D₂ geometry, as well as its reported S = 4 ground state. Computational investigations using DFT methods have shown that the D₂ geometry is the result of a Jahn-Teller distortion away from D₄ symmetry, in which the ground state would be orbitally degenerate. Broken symmetry methods have shown that ferromagnetic coupling between the two high-spin Fe(II) ions in 1 is a consequence of spin delocalization caused by a three-electron σ bond and a weaker three-electron δ bond between the Fe atoms. The relationship between ferromagnetic coupling and an Fe–Fe bond is established from results using hybrid functionals having variable amounts of Hartree-Fock exchange, which is found, surprisingly, to mitigate Fe–Fe bonding.     

Complexes of transition metals bonded to silica via β-diketonate groups — synthesis, structure, and catalytic activity

Transition metal complexes bonded to silica via silanes with β-diketonate groups can be used as packings for complexation gas chromatography or as immobilized homogenous metal complex catalysts. On basis of elemental analysis and the determination of surface area, possible structures of the complexes formed on the silica surface have been proposed. The possibility of using the immobilized complexes as catalysts has been indicated. Especially nickel complexes were taken into consideration. These immobilized complexes were used previously as packings for complexation gas chromatography.     

Novel domino reactions in β-carbolines with triple bonded dienophiles

Vinylpyrrolo-[2,1-a]-β-carbolines 1 give different products upon reaction with dienophiles. With dimethyl acetylenedicarboxylate (DMAD), a novel domino process takes place, involving Michael attack and rearrangement, affording complex polycycles like 3, 4, and 5. Diels-Alder cycloadditions are favored in the presence of Lewis acids and are the only reactions with dimethyl maleate. When 3-butyn-2-one is used as dienophile, a Stevens rearrangement is observed giving product 9.     

Highly conducting π-conjugated molecular junctions covalently bonded to gold electrodes

We measure electronic conductance through single conjugated molecules bonded to Au metal electrodes with direct Au-C covalent bonds using the scanning tunneling microscope based break-junction technique. We start with molecules terminated with trimethyltin end groups that cleave off in situ, resulting in formation of a direct covalent σ bond between the carbon backbone and the gold metal electrodes. The molecular carbon backbone used in this study consist of a conjugated π system that has one terminal methylene group on each end, which bonds to the electrodes, achieving large electronic coupling of the electrodes to the π system. The junctions formed with the prototypical example of 1,4-dimethylenebenzene show a conductance approaching one conductance quantum (G(0) = 2e(2)/h). Junctions formed with methylene-terminated oligophenyls with two to four phenyl units show a 100-fold increase in conductance compared with junctions formed with amine-linked oligophenyls. The conduction mechanism for these longer oligophenyls is tunneling, as they exhibit an exponential dependence of conductance on oligomer length. In addition, density functional theory based calculations for the Au-xylylene-Au junction show near-resonant transmission, with a crossover to tunneling for the longer oligomers.     

Syntheses and structures of metal–metal triply bonded M2R6 compounds: consideration of starting materials,stability, and structural parameters

The syntheses and structures of four homoleptic metal–metal triply-bonded M₂R₆ compounds [Mo₂(CH₂CMe₂Ph)₆, 1; Mo₂(CH₂SiMe₂Ph)₆, 2; W₂(CH₂SiMe₂Ph)₆, 3; and W₂(CH₂Ph)₆, 4] are reported. The synthetic effort suggests that ditungsten compounds are inherently more difficult to prepare and more thermally sensitive than dimolybdenum compounds, probably as a result of the larger dimetal core the ligands must protect. The structural data confirm that dimetal hexaalkyls exhibit shorter M≡M distances than do dimetal hexaalkoxides, even in a matched pair case where steric differences are minimal.     

Computational identification of a global carbon–sulfur triply bonded isomer SCBO

Searching for stable sulfur–carbon triply bonded molecules has been of great interest from both the fundamental and applied viewpoints. The known polyatomic sulfur–carbon triply bonded molecules are usually not the global minima. Here, we report a potential energy surface investigation of a tetra-atomic molecule [S,C,B,O] in both doublet and quartet states. The B3LYP and M06-2X methodologies with 6-311+G(3df,2p) and aug-cc-pVTZ basis sets were applied for geometrical optimization and CCSD(T)/aug-cc-pVTZ for single-point energy calculations. The thermodynamically most stable isomer is the linear SCBO 01 (0.0 kcal/mol). Kinetically, SCBO 01 is separated from the other isomers and fragments by the rather high barriers of at least 44.7 kcal/mol. In particular, isomer SCBO 01 contains a typical carbon–sulfur triple bond based on the systematic analysis from the structure, vibrational frequency, molecular orbital, Wiberg bond index, and adiabatic bond dissociation energy. In addition, there exists a second low-lying isomer, i.e., linear SBCO 02 (7.3 kcal/mol) with S≡B triple bonding, whose kinetic stability is governed by its fragmentation to ²SB+¹CO (30.4 kcal/mol). The remaining isomers are either kinetically unstable with low conversion barriers or energetically very high lying. We propose that the simple two-body association between SC and BO, SB and CO pairs can preferentially lead to the formation and stabilization of SCBO 01 and SBCO 02, respectively. The isomer SCBO 01, which is the global structure and extraordinarily stable against both isomerization and fragmentation, strongly deserves future laboratory studies.     

Preparation and characterization of epoxy–silica networks chemically bonded through aminophenyl-trimethoxysilane

Epoxy–silica hybrids with interfacial bonding using aminophenyl-trimethoxysilane (APTMOS) have been prepared by the sol–gel process. In a sequential polymerization procedure the amine groups present on the APTMOS were used to partially cure diglycidyl ether of bisphenol-A (DGEBA) whereas the methoxy groups created silica-network simultaneously, through the sol–gel process. Complete curing and cross-linking were carried out later using curing agent jeffamine D-400 at higher temperature. The nature of silica network structure chemically bonded with the epoxy chains was studied by Fourier transformed infrared spectroscopy and the morphology of the hybrid through scattering electron and atomic force microscopies. The visco-elastic properties of the resulting hybrids were measured through dynamical thermal mechanical analysis. The effect of inter-phase bonding of the resulting hybrids and their thermal mechanical properties are compared with the similar DGEBA epoxy matrix where un-bonded silica network was produced from tetraethoxysilane. The properties of the hybrids using APTMOS show considerable improvement in thermal mechanical properties and the coefficient of thermal expansion is reduced in contrast to the un-compatiblized system.     

Hydrogen bonded С–H···Y (Y = O,S, Hal) molecular complexes: A natural bond orbital analysis

Hydrogen bonded C–H···Y complexes formed by H₂O, H₂S molecules, hydrogen halides, and halogen-ions with methane, halogen substituted methane as well as with the C₂H₂ and NCH molecules were studied at the MP2/aug-cc-pVDZ level. The structure of NBOs corresponding to lone pair of acceptor Y, n_Y, and vacant anti-σ-bond C–H of proton donor was analyzed and estimates of second order perturbation energy Е⁽²⁾ characterizing donor–acceptor n_Y → σ_C-H^* charge-transfer interaction were obtained. Computational results for complexes of methane and its halogen substituted derivatives show that for each set of analogous structures, the Е_nY→σ*C-H⁽²⁾ energy tends to grow with an increase in the s-component percentage in the lone pair NBO of acceptor Y. Calculations for different C···Y distances show that the equilibrium geometries of complexes lie in the region where the E⁽²⁾ energy is highest and it changes symbatically with the length of the covalent С–H bond when the R(C···Y) distance is varied. The performed analysis allows us to divide the hydrogen bonded complexes into two groups, depending on the pattern of overlapping for NBOs of the hydrogen bridge.     

A Theoretical Study on the Hydrogen—bonded Dimers of HNCO Molecules

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Is fullerene C60 large enough to host a multiply bonded dimetal?

Some dimetal fullerenes M 2@C 60 (M = Cr, Mo, W) have been studied with computational quantum chemistry methods. The transition metal diatomic molecules Cr 2, Mo 2, W 2 form exohedral complexes with C 60, while U 2 forms a highly symmetric endohedral compound and it is placed in the center of the C 60 cavity. This highly symmetric structure is an artifact due to the small size of the C 60 cavity, which constrains U 2 at the center. If a larger cavity is used, like C 70 or C 84, U 2 preferentially binds the internal walls of the cavity and the U-U bond no longer exists.     

Retention properties of novel β-CD bonded stationary phases in reversed-phase HPLC mode

With the given special structures, the CD bonded stationary phases are expected to have complementary retention properties with conventional C18 stationary phase, which will be helpful to enhance the polar selectivity in RP mode separation. In this work, two β-cyclodextrin (β-CD) bonded stationary phases for reversed-phase HPLC, including 1, 12-dodecyldiol linked β-CD stationary phase (CD1) and olio (ethylene glycol) (OEG) linked β-CD stationary phase (CD2), have been synthesized via click chemistry. The resulting materials were characterized with FT-IR and elemental analysis, which proved the successful immobilization of ligands. The similarities and differences in retention characteristics between the CD and C18 stationary phases have been elucidated by using comparative linear solvation energy relationships (LSERs). The force related to solute McGowan volume has no significant difference, while the hydrogen bonding and dipolar interactions between solutes and CD stationary phases are stronger than between solutes and C18, which is attributed to the special structures (CD and triazole groups) of CD stationary phases. Chemical origins are interpreted by comparison between CD1 and CD2. Similar dispersive interactions of CD1 and CD2 are attributed to their similar length of spacer arms. CD2 which contains OEG spacer arm has relative weaker HBD acidity but stronger HBA basicity. CD stationary phases display no serious different methylene selectivity and higher polar selectivity than in the case of C18. Higher acid selectivity and lower basic selectivity are observed on CD2 than on CD1. Distinctive retention properties and good complementary separation selectivity to C18 make the novel CD bonded stationary phases available for more application in RPLC.     

Ultraviolet photoelectron spectroscopy of the hydrogen bonded heterodimer H2S⋯HCl

The HeI photoelectron spectrum of the hydrogen bonded hetero-dimer H₂S⋯HCl shows two vertical ionization energies at 10.91 and 12.16 eV. Ab initio MO calculations reveal that these features are due to the sulphur and chlorine lone pair ionizations respectively. Results show that while the ground ionic state is repulsive the first excited ionic state is strongly bound. The photoelectron spectrum of the diethyl sulphide⋯HCl complex is similar to that of H₂S⋯HCl     

A bonded–riveted technique of fabrication and machine maintenance using hot-melt adhesives

Results of mechanical tests of bonded–riveted joints using two types of adhesives represented by epoxy adhesive and hot-melt adhesives derived from ethylene vinyl acetate are given. Removal time of bonded–riveted joints is evaluated and elastic characteristics of the employed adhesive materials are measured. It is shown that the use of hot-melt adhesives is more processable, because it provides the removal of riveted connections in short period without mechanical damages.     

Triply bonded stannaacetylene (RC≡SnR): theoretical designs and characterization

The effect of substitution on the potential energy surfaces of RC≡SnR (R = F, H, OH, CH(3), SiH(3), Tbt, Ar*, SiMe(SitBu(3))(2), and SiiPrDis(2)) was explored using density functional theories (B3LYP/LANL2DZdp and B3PW91/Def2-QZVP). Our theoretical investigations indicate that all the triply bonded RC≡SnR molecules prefer to adopt a trans-bent geometry, which is in good agreement with the theoretical model (mode B). In addition, we demonstrate that the stabilities of the RC≡SnR compounds bearing smaller substituents (R = F, H, OH, CH(3), and SiH(3)) decrease in the order R(2)C═Sn: > RC≡SnR > :C═SnR(2). On the other hand, the triply bonded R'C≡SnR' molecules with bulkier substituents (R' = Tbt, Ar*, SiMe(SitBu(3))(2), and SiiPrDis(2)) were found to possess the global minimum on the singlet potential energy surface and are both kinetically and thermodynamically stable. Further, we used the B3LYP computations to predict the stability of stannaacetylene bearing the very bulky phosphine ligand. Our theoretical observations strongly suggest that both the electronic and the steric effects of bulky substituents play an important role in making triply bonded stannaacetylene (RC≡SnR) an intriguing synthetic target.     

Do pentaurea calix[5]arenes form hydrogen bonded dimeric capsules?

Calix[5]arenes substituted at their wider rim by five urea residues do not form hydrogen bonded dimeric capsules in aprotic, apolar solvents, in contrast to their calix[4]arene analogs, although molecular dynamic simulations predict very similar geometrical parameters for both cases. The reason for this different behavior is most probably the higher energetic cost which must be paid to arrange the calix[5]arene skeleton in the C₅-symmetrical conformation required for the dimerization.     

Chemically resistant microfluidic valves from Viton® membranes bonded to COC and PMMA

We present a reliable technique for irreversibly bonding chemically inert Viton? membranes to PMMA and COC substrates to produce microfluidic devices with integrated elastomeric structures. Viton? is widely used in commercially available valves and has several advantages when compared to other elastomeric membranes currently utilised in microfluidic valves (e.g. PDMS), such as high solvent resistance, low porosity and high temperature tolerance. The bond strength was sufficient to withstand a fluid pressure of 400 kPa (PMMA/Viton?) and 310 kPa (COC/Viton?) before leakage or burst failure, which is sufficient for most microfluidic applications. We demonstrate and characterise on-chip pneumatic Viton? microvalves on PMMA and COC substrates. We also provide a detailed method for bonding fluorinated Viton? elastomer, a highly chemically compatible material, to PMMA and COC polymers. This allows the production of microfluidic devices able to handle a wide range of chemically harsh fluids and broadens the scope of the microfluidic platform concept.     

Synthesis and Characterization of H‐bonded Side‐Chain Liquid‐Crystalline Polysiloxanes

Smectic Liquid‐crystalline (LC) polysiloxanes P1～P7 were prepared using cholesteryl 6‐undec‐10‐enoyloxy‐naphthalene‐2‐carboxylate and cholesteryl 3‐sulfo‐4‐undec‐10‐enoyloxy‐benzoate in a one‐step reaction with sulfonic acid group contents ranging between 0 and wt 4.39%. With an increase of sulfonic acid groups, the glass transition temperature rose slightly; while the temperature of clear point decreased. As sulfonic groups increased, H‐bonding interaction strengthened, resulting in an increase of glass transition temperature. On the other hand, aggregates of H‐bond derived from sulfonic acids would destroy the homogeneous rigid moieties and the high‐ordered structure, resulting in a temperature of clear point decreased. In X‐ray measurement, all the polymers displayed sharp strong peaks around 2θ≈2.6° and broad peaks around 2θ ≈16.6°. The broad peaks at wide‐angle are similar, but there is great different at low angles. For the polymer without sulfonic acid, the only one strong peak at low angle indicates high‐ordered lamellar structure due to homogeneous rigid moieties. For the polymers containing more sulfonic acid, two sharp peaks appeared at low angles, and the intensities of these two peaks varied. With increase of sulfonic acid groups in the polymer systems, the hydrogen‐bonding aggregates in domains would divide the homogeneous rigid mesogens into two kinds of nanophases, that is, one containing non H‐bond mesogens and another involving H‐bonding aggregated mesogens. These two different nanophases result in different lamellar spacings.     

On the weakly C-H···π hydrogen bonded complexes of sevoflurane and benzene

A vibrational assignment of the anaesthetic sevoflurane, (CF(3))(2)CHOCH(2)F, is proposed and its interaction with the aromatic model compound benzene is studied using vibrational spectroscopy of supersonic jet expansions and of cryosolutions in liquid xenon. Ab initio calculations, at the MP2/cc-pVDZ and MP2/aug-cc-pVDZ levels, predict two isomers for the 1?:?1 complex, one in which the near-cis, gauche conformer of sevoflurane is hydrogen bonded through its isopropyl-hydrogen atom, the other in which the same conformer is bonded through a bifurcated hydrogen bond with the fluoromethyl hydrogen atoms. From the experiments it is shown that the two isomers are formed, however with a strong population dominance of the isopropyl-bonded species, both in the jet and liquid phase spectra. The experimental complexation enthalpy in liquid xenon, ΔH(o)(LXe), of this species equals -10.9(2) kJ mol(-1), as derived from the temperature dependent behaviour of the cryosolution spectra. Theoretical complexation enthalpies in liquid xenon were obtained by combining the complete basis set extrapolated complexation energies at the MP2/aug-cc-pVXZ (X = D,T) level with corrections derived from statistical thermodynamics and Monte Carlo Free Energy Perturbation calculations, resulting in a complexation enthalpy of -11.2(3) kJ mol(-1) for the isopropyl-bonded complex, in very good agreement with the experimental value, and of -11.4(4) kJ mol(-1), for the fluoromethyl-bonded complex. The Monte Carlo calculations show that the solvation entropy of the isopropyl-bonded species is considerably higher than that of the fluoromethyl-bonded complex, which assists in explaining its dominance in the liquid phase spectra.     

Complexation of two non-fully hydrogen bonded aromatic hydrazide heptamers toward n-octyl-α-L-glucopyranoside in chloroform

Two aromatic hydrazide haptamers have been prepared,with both consisting of two hydrogen bonded folded segments. Compared to their fully hydrogen bonded analogues,the flexibility of their backbones increases due to lack of one or two intramolecular hydrogen bonds at the middle aromatic unit. (2D) 1H NMR,circular dichroism and fluorescent studies revealed that both oligomers moderately complex n-octyl-α-L-glucopyranoside in chloroform.