Conformational analysis for hydrated ethylene oxide oligomer models by quantum chemical calculations

Hydrate effects on the conformations of ethylene oxide oligomers (EO-x, x = 1–8 mers) were examined using quantum chemical calculations (QCC). Conformational analyses were carried out by RHF/6-31G. The models were constructed by locating a water molecule to each ether–oxygen in the structures optimized for non-hydrate oligomers. Hydrate ratio, h (h = H₂O_mol/O_{mol in oligomer}), was set from 0 to 1.0. The six type conformations with repeated units of O–C, C–C and C–O bonds were examined. Conformational energy, E _c (HF), was calculated as difference between the energy of oligomer with water molecules and that of non-hydrogen and/or hydrogen bonding water molecules. Hydrate energies for each conformer, ∆μ _h (kcal/m.u., based on E _c in non-hydrate state), were negative and linearly decreased with the increase of h values, and such effects with the increase of h values were weaken with increasing x values. These results were consistent with our previous results calculated using the permittivity, ε (ε = 0–80.1), by QCC. In non-hydrate (h = 0), the (ttt) _x conformers were the most stable independent of x. However, in hydrate states (h = 0.44–0.67), the (tg⁺t) _x conformers were the most stable independent of x values, and in h = 1, the (tg⁺t)₈ conformer (8-mer) was most stable [∆E _c(g) = −1.3 kcal/m.u., ∆E _c(g): energy difference between a given oligomer and the (ttt) _x oligomer]. These results supported the experimental those based on NMR analyses using dimethoxyethane and triglyme solutions. Molecular lengths (l) of (tg⁺t) _x , (tg⁺g⁻) _x and (g⁺g⁺g⁺) _x conformers having higher x values significantly decreased with increasing h values. Such contraction with hydration, however, was independent of ΔE _c(g) values of each conformer.     

Mixing of ethylene oxide and propylene oxide oligomers: 2. Phase separation

Equilibrium phase compositions and cloud point curves have been determined for mixtures of ethylene oxide and propylene oxide oligomers. Large differences in miscibility are noted between α,ω-hydroxy- and α,ω-methoxy-oligomers. The data for the α,ω-methoxy-oligomers are simply interpreted in terms of the Flory—Huggins equation with concentration dependent X, i.e. X = X₀ + X₁ø₁ where ø₁ is the volume fraction of component 1. The approximate value of X for the α,ω-methoxy-oligomer mixtures is found to be 0.1.     

Mixing of ethylene oxide and propylene oxide oligomers: 1. Enthalpy and volume changes

Calorimetry and dilatometry have been used to determine the enthalpy and volume changes on mixing oligomers of ethylene oxide ($\text{M}\text{?}{}_{\mathrm{n}}\text{< 600}$) and propylene oxide ($\text{M}\text{?}{}_{\mathrm{n}}\text{< 2025}$). The mixing of α,ω-hydroxy-oligomers is markedly affected by end-group interactions. The enthalpies of mixing of α,ω-methoxy-oligomers are determined predominantly by differences in chain-segment interaction energies and surface areas. The volumes of mixing of α,ω-methoxy-oligomers can be explained only by accounting additionally for slight differences in their liquid properties.     

Synthesis and applications of heterobifunctional poly(ethylene oxide) oligomers

Poly(ethylene oxide) (PEO) oligomers are employed extensively in pharmaceutical and biomedical arenas mainly due to their excellent physical and biological properties, including solubility in water and organic solvents, lack of toxicity, and absence of immunogenicity. PEO can be chemically modified and reacted with, or adsorbed onto, other molecules and surfaces. Sophisticated applications for PEO have increased the demand for PEO oligomers with tailored functionalities, and heterobifunctional PEOs are often needed. This review discusses the synthesis and applications of heterobifunctional PEO oligomers possessing amine, carboxylate, thiol, and maleimide functional groups.     

Conformational analysis of aqueous pullulan oligomers: an effective computational approach

Three rotational isomeric state (RIS) models for pullulan, a polysaccharide defined as [→6)-α-d-glucopyranosyl-(1→4)-α-d-glucopyranosyl-(1→4)-α-d-glucopyranosyl-(1→]_n, are presented. These RIS models are based upon simple molecular mechanics and solvation calculations and differ in the complexity of the energy evaluations used to determine Boltzmann weighting factors for each isomeric state. Each model is evaluated for its ability to predict correctly experimental small angle X-ray scattering data and the radius of gyration (R_g) for the hexameric pullulan oligomer (G₃)₂. A simple model based upon only disaccharide conformational energies in conjunction with a correction for solvation energies, as determined using a continuum Poisson-Boltzmann solvation method, was best and gave an excellent fit to experimental measurements.     

Conformational stability of poly(trimethylene oxide) by the CNDO/2 calculations

The CNDO/2 calculations using the tight-binding approximation for polymers were carried out on poly(trimethylene oxide), (OCH₂CH₂CH₂)_n, assuming three crystal modifications, zigzag, orthorhombic, and trigonal forms. The relative stability among the three forms in the polymer was reasonably reproduced by the present calculations. The factors governing the conformational stability were demonstrated.     

Conformational analyses for alanine oligomer during hydration by quantum chemical calculation (QCC)

The structure and association energy of hydrated l-alanine pentamers at the hydration rate (h) of 0–1 were calculated by quantum chemical calculation (QCC) using the density functional theory [B3LYP/6-31G(d, p)] method for three kinds of stable conformers (β-extended: t?/t+, PP_II-like: g?/g+, α-helix: g?/g?) converged by convergent calculations from l-alanine pentamers in gas phase. Water molecules are mainly inserted between intramolecular hydrogen bond of CO–HN in PP_II-like and α-helix conformers and attached to the CO group in β-extended conformer. α-Helix conformer turns to PP_II-like conformer at higher hydration rate. The association energy decreased with the increase of the hydration rate, indicating that the conformers were stabilized by the hydration. It was found that PP_II-like conformer, which was the most stable in the anhydrate state, was also the most stable in the hydrate state. This structure corresponds to a middle structure of PP_II (g?/t+) and β-extended (t?/t+) structures. These results obtained by energy calculation using QCC support the experimental results by Eker et al. and Graf et al., who reported that alanine oligomer exhibited a mixture of PP_II and β-extended structure in aqueous solution.     

Comparison of reactive sites of different conjugated linoleic acid molecules by quantum chemical calculations

In this paper, quantum chemical calculations and Gaussian 09 software were used to investigate the similarities and differences in the reactive sites of different fatty acids. The molecule structures of linoleic acid (LA) and four conjugated linoleic acid (CLA) were analyzed at the level of density functional theory. Molecular geometries and energies, frontline molecular orbitals, and surface electrostatic potentials were also analyzed. The results showed that the reactive of LA and CLA among the five fatty acids were all near the –C = C– structure and the carboxyl hydrogen atom, 9c, 11t CLA and 10t, 12c CLA were more reactive compared with 9t, 11t CLA, 10t, 12t CLA. This study can provide a theoretical basis for revealing the relationship between different structures and properties of fatty acids and also provide a guidance for the selection of fatty acids in the oil and fat industry. Practical Applications: This experiment compares the differences between the reactive sites of CLA and the four CLAs at the molecular structure level, which can be used to deepen the understanding of the structure-property relationship of different fatty acids and also provides a theoretical basis for the selection of fatty acids in the food and oil industry.     

Molecular simulation, quantum chemical calculations and electrochemical studies for inhibition of mild steel by triazoles

The inhibition performance of three triazole derivatives on mild steel in 1 M HCl were tested by weight loss, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The adsorption behavior of these molecules at the Fe surface was studied by the molecular dynamics simulation method and the quantum chemical calculations. Results showed that these compounds inhibit the corrosion of mild steel in 1 M HCl solution significantly. Molecular simulation studies were applied to optimize the adsorption structures of triazole derivatives. The iron/inhibitor/solvent interfaces were simulated and the charges on the inhibitor molecules as well as their structural parameters were calculated in presence of solvent effects. Aminotriazole was the best inhibitor among the three triazole derivatives (triazole, aminotriazole and benzotriazole). The adsorption of the inhibitors on the mild steel surface in the acid solution was found to obey Langmuir's adsorption isotherm.     

Conformational characteristics of poly(ethylene oxide) (PEO) in methanol

The conformational characteristics of poly(ethylene oxide) (PEO) in methanol at 25 °C were investigated by static light scattering and viscometry for high molar mass (M_w) PEO fractions covering M_w = 3.42 × 10⁵-5.05 × 10⁶ g mol⁻¹. No trace of downturn in the plot of angular dependence of Kc/R_θ at low angle was found. Experimental scaling laws for the second virial coefficient (A₂), the third virial coefficient (A₃), the radius of gyration and the intrinsic viscosity ([η]) were determined. The exponents characterizing these scaling laws confirmed that the PEO chain in methanol has a flexible conformation with relatively large excluded volume, but methanol is not as good solvent as water. On the other hand, the low value of interpenetration function (Ψ) and the relatively higher order of the dimensionless parameter Π are considered to be an indication of local chain stiffness. All the results obtained in this study allow us to conclude that the overall chain conformation of PEO assumed in methanol is basically a random coil, but is intermittently mixed with helical structure.     

Conformational analyses of periplanone analogs by molecular mechanics calculations

Conformational parameters of pheromonally active analogs (1 and 2) of periplanones, the sex pheromones of the American cockroach, were investigated by molecular mechanics calculations. They existed in several conformers with small energy differences. These results were supported by NMR analysis. The structural features of the conformers of the analogs were compared with X-ray structures of periplanones.     

Molecular size evaluation of linear and branched paraffins from the gasoline pool by DFT quantum chemical calculations

A good approach of the critical molecular dimensions of 35 linear and branched C5-C8 paraffins by DFT quantum chemical calculations at B3LYP/6-31G** level of theory in gas phase is described. In this context, we found that either the determined molecular width or width-height average values can be used as critical measures in the analysis for selection of molecular sieves materials, depending on their pore size and shape. The molecular width values for linear and monosubstituted paraffins are 4.2 and 5.5 Å, respectively. In the case of disubstituted paraffins, the values are 5.5 Å for 2,3-, 2,4-, 2,5- and 3,4-disubstituted and for 2,2- and 3,3-disubstituted are 6.7-7.1 Å. The values for ethyl-substituted are 6.1-6.7 Å and for trisubstituted isoparaffins are 6.7. In order to select a porous material for selective separation of isoparaffins and paraffins, the zeolite diffusivity can be correlated with the critical diameter of the paraffins according to the geometry-limited diffusion concept and the effective minimum dimensions of the molecules. The calculated values of CPK molecular volume of the titled paraffins showed a good discrimination between the number of carbons and molecular size.     

Effects of solvent on structures and properties of electrospun poly(ethylene oxide) nanofibers

Poly(ethylene oxide) (PEO) nanofibers were prepared by electrospinning PEO solution with a mixed solvent of ethanol and deionized water. The results show that the mixed solvent system has noteworthy influences on structures and properties of electrospun PEO nanofibers, including molecular chain orientation, crystallinity degree, surface morphology, fiber diameter, diameter distribution, spinnability, and productivity. With increasing ethanol content in the mixed solvent, wrinkly morphologies appear on the surface of PEO nanofibers due to a high evaporation rate of ethanol during electrospinning process. The dielectric constant, dipole moment, conductivity, density, boiling point, and solubility parameter of the mixed solvent become lower with the ethanol content increasing. Besides, the hydrogen‐bonding interactions between PEO and solvents become weaker. As a result, PEO nanofibers with larger diameters, lower molecular chain orientation, and crystallinity degree are obtained. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45787.     

Copolymerization of ethylene oxide and propylene oxide by anhydrous potassium hydroxide

Ethylene oxide and propylene oxide have been copolymerized at low temperatures in the presence of anhydrous potassium hydroxide. The monomer reactivity ratios were found to be r(ethylene oxide) = 6.5 and r(propylene oxide) = 0.5.     

环氧乙烷化工设计安全事项分析

针对环氧乙烷在生产、储存、运输和使用环节中存在的安全问题,对环氧乙烷特殊的物理化学性质进行分析. 环氧乙烷化学性质非常活泼,易开环并释放大量热量,在空气中的爆炸极限浓度为3%~100%,引燃能最小只需0.06 mJ,易燃易爆. 结合其自身反应特性及易自聚、歧化或分解等特点,讨论了其在安全操作、化工设计中的注意事项,并提出了具体建议措施,包括相关装置布置方案设计、安全温度和压力设计、设备安全及紧急处理设计等,以期能提高环氧乙烷在生产、运输和使用过程中的安全性和可靠性,减少相关事故的发生.     

Two-dimensional solid-state NMR studies of crystalline poly(ethylene oxide): Conformations and chemical shifts

The torsion angles of the OC-CO bonds in crystalline poly(ethylene oxide), PEO, were investigated by solid-state nuclear magnetic resonance (NMR). Two-dimensional double-quantum (DOQSY) spectra indicate that the OC-CO bonds are all gauche with an average torsion angle of ψ=74±4° and a narrow torsion-angle distribution, σ_ψ<8°. This is contradictory to the wider range of gauche torsion angles in the distorted helical structure previously proposed based on X-ray fiber diffraction. The low-temperature magic-angle-spinning (MAS) ¹³C NMR spectrum of unlabeled PEO contains four maxima and several shoulders, over a range of 3.1 ppm. Deconvolution of this spectrum, together with two-dimensional ¹³C INADEQUATE NMR and exchange MAS spectra, suggests a possible assignment of chemical shifts to the 14 carbons in the 7₂ helical repeat unit. The small line widths of the individual peaks indicate that the helical repeat unit is accurately replicated throughout the crystals. The results show that packing effects or small conformational differences can change chemical shifts by amounts that had previously been ascribed only to trans/gauche differences.     

The interaction of thin polyaniline films with various H‐phosphonates: Spectroscopy and quantum chemical calculations

The conducting form of polyaniline (PANI) is formed by doping of non‐conducting PANI base usually by protonic acids. An alternative way of doping, consisting in an interaction via hydrogen bonding, has been previously proven for H‐phosphonates. In this study, PANI base films are exposed to various H‐phosphonates, changes in the structure of the films are analyzed by ultraviolet–visible, infrared, and Raman spectroscopies and spectroscopic data are compared with quantum chemical calculations. According to our results, the interaction of the PANI base films with H‐phosphonates is realized by hydrogen bonding between the imine nitrogen of PANI and the hydroxy group of the H‐phosphonate tautomeric form, i.e., phosphite. This is in the contrast to the PANI powders doped with H‐phosphonates where both interacting phosphite and non‐interacting H‐phosphonate forms were observed. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46728.     

Polarographic reduction mechanism and quantum chemical calculations of some α-arylhydrazononitriles

The polarographic reduction of 2-arylhydrazono-3-keto-3-phenylpropionitriles ( I _a–i) and 2-arylhydrazono-3-ketobutyronitriles ( II _a–i) over a wide pH range is reported and discussed. The mechanism of the electrode process is elucidated and confirmed via coulometric analysis, quantum chemical calculations and IR spectra of the electrolysis products. The molecular structures and molecular orbital energies were calculated and correlated with the redox potentials of these compounds.