Evaluation of the origin of conformational and tautomeric preferences in N‐formylformamide – A quantum chemical study

Quantum chemical study of N‐formylformamide (NFF) was carried out at various theoretical levels and the determinate equilibrium conformations were recomputed at the high level ab initio methods such as G2MP2, G2, G3, and complete basis set (CBS)‐QB3. The computational results reveal that the amide resonance and intramolecular hydrogen bonding are two superior factors in determining the most stable conformation of diamide (DA) and amide–imidic (AI) acid tautomers, respectively. The evaluation of hydrogen bond energies predicts that the hydrogen bond (HB( strength of NFF is weaker than the malonaldehyde (MA). But the results of atoms in molecules (AIM(, natural bond orbital (NBO), and geometrical parameters are given a different order, E_HB(NFF) > E_HB(MA). Although the bond average energies of tautomerization process emphasized on more stability of AI tautomer, but our theoretical calculations reveal that the DA conformers are more stable than the AI ones. The population analyses of equilibrium conformations by NBO method also predict that the origin of tautomeric preference is mainly because of the electron delocalization of amide functional group, especially LP(N)→ π*_C?O charge transfer. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Voltammetric determination of droxidopa in the presence of carbidopa using a nanostructured base electrochemical sensor

A novel carbon paste electrode modified with ZnO nanorods and 2-(4-oxo-3-phenyl-3,4-dihydroquinazolinyl)-N′-phenyl-hydrazinecarbothioamide (2PHCZNCPE) was fabricated and employed to study the electrocatalytic oxidation of droxidopa, using cyclic voltammetry, chronoamperometry and square wave voltammetry as diagnostic techniques. It has been found that the oxidation of droxidopa at the surface of modified electrode occurs at a potential of about 435 mV less positive than that of an unmodified carbon paste electrode. Square wave voltammetry exhibits a linear dynamic range from 7.0 × 10^–8 to 3.0×10^?4 M and a detection limit of 45.0 nM for droxidopa. Finally this modified electrode was used for simultaneous determination of droxidopa and carbidopa.     

A comparative study of open‐close and related rotamers methods to evaluate the intramolecular hydrogen bond energies in 3‐imino‐propen‐1‐ol and its derivatives

MP2 study of O? H…N intramolecular hydrogen bond (IMHB) in 3‐imino‐propen‐1‐ol and its derivatives were performed and their IMHB energies were obtained using the related rotamers and open‐close methods. Also the topological properties of electron density distribution and charge transfer energy associated with IMHB were gained by quantum theory of atoms in molecules and natural bond orbital theory, respectively. The computational results reveal that the related rotamers method energies are well correlates with geometrical parameters, topological parameters at hydrogen bond and ring critical points, integrated properties, proton transfer barrier and charge transfer energy of O? H…N unit. Surprisingly, it was found that the open‐close hydrogen bond energies cannot represent good linear correlations with these parameters. Consequently, we extrapolate a number of equations that can be used in estimation of O? H…N IMHB energy in complex biological systems. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Rheology of polypropylene/poly(ethylene-co-propylene) in-reactor alloy

The interfacial adhesion and molecular structure of an in-reactor polypropylene/ethylene propylene rubber alloy were studied with respect to the rheological behavior and final properties of the alloy. The polymer alloys have similar structural parameters but different impact properties. The samples were characterized by gel permeation chromatography, thermal analysis, rheological analysis and mechanical testing. Fractionation of samples showed that the quantities of components are the same. Gel permeation chromatography results showed that molecular weight distribution of all components were similar. Thermal analysis results showed that the crystal size of two samples was the same but the crystal contents were different. In studying the impact strength, it was revealed that a large difference exists between the two samples. The small amplitude oscillation rheometry indicates that the rheological parameters have a glaring difference in both samples that can be an evidence of interfacial adhesion in the in-reactor alloys having so many similar structural parameters.     

Mechanical properties,surface chemistry,and barrier characteristics of electron beam irradiated‐annealed LDPE/PA6/LDPE multi‐layer films at N2

The effects of electron beam irradiation in the nitrogen environment, on chain scission, crosslinking, crystallinity, mechanical performance, and barrier properties of LDPE/PA6/LDPE multi‐layer films were studied. The evaluation of radiation‐induced crosslinking effect by the gel content measurement and Charlesby–Pinner plot suggested more of crosslinking over chain scission, in all the layers, which was more pronounced in polyethylene phase. The FTIR analysis results showed good agreement with those observed by the gel content measurements. It is believed that the crosslinking reaction had occurred through the C? N bonds in polyamide‐6, and vinyl group in polyethylene layers. The evaluation of radiation effect on the crystallinity and crosslinking of films by FTIR technique showed that by increasing the applied doses, the crystallinity in all the layers was decreased and the crosslinking was increased. The differential scanning calorimetry of irradiated samples revealed that due to the crosslinking reaction, the crystallinity was decreased by the applied dose. The tensile strength of the films was increased and the percent elongation at break was decreased, by increasing the applied doses. This study was also indicated that the radiation‐induced crosslinking effect on the tensile properties was dominantly observed up to 50 kGy. The surface free energy analysis of the films using the contact angle measurement and geometric mean equation indicated that the surface polarity was decreased by increasing the absorbed doses. It was found that due to the decline in the surface polarity and the simultaneously formation of crosslinked network in these films, both water vapor transmission rate and oxygen permeability were significantly decreased. Copyright © 2009 John Wiley & Sons, Ltd.     

The effect of electron beam irradiation on dynamic shear rheological behavior of a poly (propylene‐co‐ethylene) heterophasic copolymer

In this study the effect of electron beam irradiation on rheological properties of a poly (propylene‐co‐ethylene) heterophasic copolymer is evaluated. Using dynamic viscoelastic measurement in the linear viscoelastic range of deformation, it is observed that the complex viscosity and dynamic modulus of polypropylenes were decreased by increasing the irradiation dose. Polypropylene heterophasic copolymers consist of ethylene propylene rubber phase dispersed in polypropylene homopolymer matrix. The high energy electron beams simultaneously affect both isotactic polypropylene (iPP) matrix and ethylene propylene dispersed phase. The molecular chains of polypropylene homopolymer phase breakdown to smaller species, those are prone to degradation and branching as well. Increase in the melt flow rate behavior and shifting the cross‐over point to higher frequencies and increase in melt strength are due to this phenomenon. At the same time, the ethylene propylene phase of the polypropylene copolymer cross‐links due to irradiation, and a significant effect on the rheological behavior of samples are observed. The mathematical modeling of complex viscosity behavior revealed the conformity of experimental data with modified Carreau equation. Copyright © 2010 John Wiley & Sons, Ltd.     

Degradation kinetics of electron beam irradiated poly(propylene-co-ethylene) heterophasic copolymer

This study considers the effects of electron beam radiation on degradation kinetics of a poly(propylene-co-ethylene) heterophasic copolymer. Polypropylene heterophasic copolymers are composed of ethylene–propylene rubbery phase dispersed in crystalline polypropylene homopolymer matrix. Electron beam radiation can affect both polypropylene homopolymer matrix and ethylene–propylene dispersed phases simultaneously. Both phases undergo degradation and crosslinking reactions, but degradation is more probable in the polypropylene homopolymer matrix. The aim of this work is to study kinetics of degradation in this material. A high power electron accelerator irradiated raw samples under nitrogen atmosphere. The samples are analyzed using TGA in non-isothermal mode, and the degradation kinetic parameters were determined using Kissinger, Flynn–Wall–Ozawa and Coats–Redfern methods. The kinetic parameters resulted from these methods are compared. Results of kinetics studies show that orders of degradation reactions occurring in nitrogen atmosphere are all less than one. It indicates degradation takes place due to thermal dissociation of the chemical bonds.