Stiffness and excluded volume effects on conformation and dynamics of polymers: A simulation study

This work investigates the effects of the excluded volume and especially those of the chain stiffness on the structural and dynamical properties of a model polymer chain.The theoretical framework is the same as in the recent works by Steinhauser et al.,where a Rouse approach is adopted.Our model differs in that our chains have a finite average bending angle.As in the works by Steinhauser et al.,Langevin dynamic simulations were performed without hydrodynamic interactions.Whereas this doesn’t impact the static properties we obtain,it also allows us to compare our results on dynamic properties to those predicted by Rouse theory,where hydrodynamic interactions are also neglected.Our results show that the structural properties are very sensitive to the chain stiffness,whereas the dynamic scaling laws remain the same as those by Rouse theory,with the prefactor depending on the persistence length.     

Analyzing excited-state processes and optical signatures of a ratiomeric fluorine anion sensor: a quantum look

Recently, the spectroscopic signatures of a benzoselenadiazole derivative have been investigated in the framework of designing a new ratiometric fluoride sensor(Saravanan et al., Org Lett, 2014, 16: 354–357). It was suggested that this sensor is undergoing excited-state intramolecular proton transfer. In this work, we provide a new look at these experimental data, using a state-of-the-art time-dependent density functional theory approach to mimic the spectroscopic signatures. New insights about the nature of the excited-state processes are obtained.     

Sol-Gel Distribution of Polycondensation Reaction

A theoretical approach to polycondensation reaction of Aa-Bb type Involving intramolecular cycllzatlon has been proposed by Tang Au-chin et al.. In this paper, the theoretical sol-gel distribution is tested by polycondensation of adipic acid with trimethylol propane in a sto-ichiometric ratio of 0. 8, and the effect of intramolecular cyclization is discussed in detail.     

The Effect of the Chain Length on MMA Free Radicl Polymerization

It has been known that the propagation rate coefficient (kp) will change with monomer conversion, Friss, Hamielec et al.1-5 have proposed many formulas about kp to fit this change. In order to obtain the relation between Arrhenius parameters and monomer conversion, a group of kp values at the same monomer conversion must be measured. We adopted a new method through analysis of kp values measured by usual experimental techniques for this purpose. Firstly, we chose a value of monomer convers…     

RELATIONSHIPS BETWEEN X—H STRETCHING OVERTONE FREQUENCIES AND BOND LENGTHS/BOND ANGLES

A relationship between the X-H (X = N, O, C, and so on) equilibrium bond length in a Morse oscillator and the X-H stretching overtone frequency shifts is obtained theoretically. We use the equation to discuss the empirical linear relationships that have been proposed for heterocyclics, alkanes and fluorinated benzenes. On the other hand, a unified relationship between the X-H bond angles and the experimental quantities (ω(?) and the coupling strength λ) is also presented for XH2, XH, and XH4 molecules or molecular fragments. Calculations of X-H bond angles for a number of molecules show that the results from our equations are in excellent agreement with the experimental values. Also we can extract the information of relative magnitude of bond coupling force field.     

Software Package: An Advanced Theoretical Tool for Inhomogeneous Fluids(Atif)

In spite of the impending flattening of Moore’s law,the complexity and size of the systems we are interested in keep on increasing.This challenges the computer simulation tools due to the expensive computational cost.Fortunately,advanced theoretical methods can be considered as alternatives to accurately and efficiently capture the structural and thermodynamic properties of complex inhomogeneous fluids.In the last decades,classical density functional theory(cDFT)has proven to be a sophisticated,robust,and efficient approach for studying complex inhomogeneous fluids.In this work,we present a pedagogical introduction to a broadly accessible open-source density functional theory software package named"an advanced theoretical tool for inhomogeneous fluids"(Atif)and of the underlying theory.To demonstrate Atif,we take three cases as examples using a typical laptop computer:(i)electric double-layer of asymmetric electrolytes;(ii)adsorptions of sequencedefined semiflexible polyelectrolytes on an oppositely charged surface;and(iii)interactions between surfaces mediated by polyelectrolytes.We believe that this pedagogical introduction will lower the barrier to entry to the use of Atif by experimental as well as theoretical groups.A companion website,which provides all of the relevant sources including codes and examples,is attached.     

Theoretical Evaluation of the Global and Local Electrophilicity Patterns to Characterize Hetero Diels-Alder Cycloaddition in the Synthesis of Isoxazolo[4,5-e](1,2,3,4-tetrazine)

A series of substituted dienophiles 1—10 and three 1,2-diazadienes namely D-1, D-2, D-3 were chosen to understand the reactivity and selectivity of Diels-Alder cycloaddition. The global and local electrophilicity patterns have been evaluated on a series of cycloaddition reactions to assess the reaction pathways (NDAC/IEDDAC) using the absolute scale of electrophilicity proposed by Parr et al. Regional electrophilicity at the active sites of the reagents involved in Diels-Alder processes has been described on a quantitative basis using local or regional electrophilicity index i.e. Fukui function. Good qualitative/quantitative comparison was found between molecular energy gaps and global electronic parameters, which has been employed to assess the cycloaddition pathways successfully.     

An analysis of the rate of isomerization in molecules with an asymmetric double well potential

The classical theory of the rate of unimolecular isomerization developed by Gray andRice as extended by Zhao and Rice is applied to the calculation of the rate of isomerization in modelsystems which have linear asymmetric double well potentials. We are interested in this system fortwo reasons. First, we are interested in the detailed dynamical processes for the mentioned systembecause it is widely related to practical chemical reactions. Second, the present model systemhas an asymmetric double well potential, which provides a different test of the accuracy of theapproximations used in the Gray-Zhao-Rice theory than posed by previous applications. We havecalculated relaxation rates and relaxation times for the model systems on different time scales.We find that for the systems under studies the Gray-Zhao-Rice version of the classical theory ofisomerization rate yields values in good agreement with those generated from trajectory calculationsand from the Reactive Island theory of De Leon et al.     

Comparison and application of different empirical correlations for estimating the hydrate safety margin of oil-based drilling fluids containing ethylene glycol

As the oil and gas industries continue to increase their activity in deep water, gas hydrate hazards will become more serious and challenging, both at present and in the future. Accurate predictions of the hydrate-free zone and the suitable addition of salts and/or alcohols in preparing drilling fluids are particularly important both in preventing hydrate problems and decreasing the cost of drilling operations. In this paper, we compared several empirical correlations commonly used to estimate the hydrate inhibition effect of aqueous organic and electrolyte solutions using experiments with ethylene glycol (EG) as a hydrate inhibitor. The results show that the Najibi et al. correlation (for single and mixed thermodynamic inhibitors) and the Ostergaard et al. empirical correlation (for single thermodynamic inhibitors) are suitable for estimating the hydrate safety margin of oil-based drilling fluids (OBDFs) in the presence of thermodynamic hydrate inhibitors. According to the two correlations, the OBDF, composed of 1.6 L vaporizing oil, 2% emulsifying agent, 1% organobentonite, 0.5% SP-1, 1% LP-1, 10% water and 40% EG, can be safely used at a water depth of up to 1900 m. However, for more accurate predictions for drilling fluids, the effects of the solid phase, especially bentonite, on hydrate inhibition need to be considered and included in the application of these two empirical correlations.     

The Ring-Chain Competition in Nonlinear Polycondensation (Ⅱ)——The Intramolecular Reaction Equations of the System Containing Two Dynamic Parameters

Flory considered that the discrepancy between classical gelation theory and real reaction system was mainly attributed to the intramolecular reaction.On the basic principles of the probability we analysed the growing-disappearing of the groups in the even generations against a certain group statistically when three different reaction systems containing two dynamic parameters reached some degree of reaction.Furthermore,we obtained the intramolecular reaction equations in the form of analysis by means of introducing probability factor of cyclization bond.Using the equations obtained from this paper to deal with the critical condition of gelation,we found that the theoretical values agreed with the experimental ones.     

Anchoring Properties of Nematic Liquid Crystals Studied Using the Attenuated Total Reflection Method

By using the attenuated total reflection method associated with the excitation of surface plasmons, the tilt angle of the liquid crystal director and its gradient at the surface are measured in a planar nematic cell as a function of the applied voltage. The surface anchoring anisotropy δπ of the liquid crystal and the surface elastic constant k_s, are found to be δπ = 0.288 erg/cm and k_s, = 9·12 × 10^-11 erg, respectively, when the boundary condition suggested by Barbero et al is used. The theoretical and experimental values obtained with this boundary condition and that of Mada are discussed. The results show that the boundary condition proposed by Barbero et al is in better agreement with the experiment.     

Study of degraded neutron spectra through metal matrix composites using CR-39 films

Considerable interest has grown in the last decade in the use of CR-39 films in routine neutron monitoring and dosimetry. In addition, work in neutron spectrometry has been undertaken by various workers (Decossas et al., 1984; Faermann et al., 1983; Fews et al., 1984; Turner et al., 1984). In the present study metal matrix composites (MMC) samples of Pb-Li, Pb-Cd, Al-Li and Al-B₄C were prepared by using powder metallurgy and alloying elements techniques. The density was obtained for these MMC samples in the range of 80-90% of the metal density. Degraded neutron spectra were obtained by exposing these samples to a ²⁵²Cf source housed in a camera and the detector used was CR-39 films covered with a 1 mm PE radiator. These spectra have also been validated through Monte Carlo Neutron and Photon Transport Code (1983).     

On the K13 surface-like elastic constant An experimental method to test different theoretical models

Oldano and Barbero showed that, due to the presence of the surface-like elastic constant K₁₃ in the expression of the elastic free energy density, F₂, in a nematic liquid crystal, the functional F₂ is unbounded from below and thus it is impossible to find an equilibrium director distortion. In particular, they showed that the surface-like elastic constant favours a discontinuity of the director-field at the interfaces. In recent years two quite different theoretical approaches have been proposed to eliminate the mathematical difficulties related to the K₁₃ problem. Barbero et al., expanded the free energy functional F up to the fourth order in the director derivatives and showed that the minimization problem becomes mathematically well posed. A strong subsurface director distortion on a length scale of the order of the molecular length is predicted by using this approach. This point has been critized by V. Pergamenshchik who considers the subsurface strong distortion as an artefact of theory and proposes an alternative method to account for the effect of K_{1 3}. This method is virtually coincident with that already proposed by Hinov on the basis of an a priori assumption. In this paper we discuss some direct consequences of these two different approaches and we propose two simple experimental measurements which should lead to different results depending on which model is the correct one, allowing in this way a test of the different theoretical models.     

Alkaloids of Glycosmis arborea—III Structure of arborinine

Arborinine is identical with the 2,3-dimethoxy-1-hydroxy-10-methylacridone of Hughes et al.     

基于MX_4结构的配位化合物析氢反应催化性能

具有MX4结构的(M=Fe,Co,Ni,Cu等,X=N,S,Se等)小分子配位化合物及配位聚合物(MX4催化剂),由于其新颖的结构和优异的电催化析氢性能受到研究人员越来越多的关注。本文综述了MX4催化剂的研究进展,其中MX4催化剂活性受金属中心、配位原子、配体的结构以及材料的形貌、尺寸等因素的影响。理论计算有助于分析催化剂中这些因素对催化剂活性的影响,也有助于通过理论模拟,设计更合理的催化剂分子结构。     

Electric dipole moments and molecular structure of aliphatic nitro compounds and oximes

Using the treatment of Smith et al. charge distributions in and consequently the dipole moments of some aliphatic nitro compounds and oximes have been evaluated. The mesomeric moment derived as a difference between the calculated and the observed values gives a clear picture as to how the positive (+M) and the negative (−M) mesomeric effects operate in such systems.     

Precise Functionalization of Remote C―H Bonds

Remote C-H bond functionalization of arenes with precise control is a recognized extraordinary challenge in organic synthesis. Recently, Yu and Houk et al. developed an elegant strategy to distinguish and functionalize remote C_sp²-H bonds of (hetero)arenes within one-bond distance by the interplay of a remote directing template and a transient norbornene-type mediator. A wide range of medicinally important benzoazines are well compatible with this method. The chemistry significantly expands the toolbox for site-selective functionalization of remote C_sp²-H bond of (hetero)arenes. This work has been published in Nature Chemistry in March, 2020.     

Comparison between experimental and theoretical scales of electrophilicity in benzhydryl cations

The theoretical electrophilicity pattern of a series of benzhydryl cations as described in terms of a model based on global reactivity indexes is compared with an experimental scale recently proposed by Mayr et al. [J. Am. Chem. Soc. 2001, 123, 9500]. A good correlation between both theoretical and experimental quantities is found. The effect of chemical substitution on the electrophilic power of these charged electrophiles may also be assessed as local responses at the active site, in terms of a global contribution described by changes in global electrophilicity with reference to the dianisylcarbenium ion, and a local contribution described by the variations in electrophilic Fukui function at that site.     

Encapsulation and Release of Circulating Tumor Cells Using 3D DNA Hydrogels

To minimize the concomitant cell damage in the capture process of circulating tumor cells(CTCs) in conventional methods, Zuo et al. proposed to use aptamer-trigger-clamped hybridization chain reaction method to encapsulate CTCs by in-situ forming 3D DNA hydrogel. The 3D porous DNA hydrogel on cell membrane minimizes cell damage, and the CTCs can be readily released by the ATP triggered dis-assembly of DNA hydrogel for subsequent culture and live cell analysis. This work has been published online in the Nature Protocols in June 22, 2020.