Origin of the X-Hal (Hal = Cl, Br) bond-length change in the halogen-bonded complexes

The origin of the X-Hal bond-length change in the halogen bond of the X-Hal...Y type has been investigated at the MP2(full)/6-311++G(d,p) level of theory using a natural bond orbital analysis, atoms in molecules procedure, and electrostatic potential fitting methods. Our results have clearly shown that various theories explaining the nature of the hydrogen bond cannot be applied to explain the origin of the X-Hal bond-length change in the halogen bond. We provide a new explanation for this change. The elongation of the X-Hal bond length is caused by the electron-density transfer to the X-Hal sigma* antibonding orbital. For the blue-shifting halogen bond, the electron-density transfer to the X-Hal sigma* antibonding orbital is only of minor importance; it is the electrostatic attractive interaction that causes the X-Hal bond contraction.     

Individual electrophoretic mobilities of liposomes and acidic organelles displaying pH gradients across their membranes

This report focuses on measuring the individual electrophoretic mobilities of liposomes with different pH gradients across their membrane using capillary electrophoresis with laser-induced fluorescence detection (CE-LIF). The results from the individual analysis of liposomes show that, using surface electrostatic theories and the electrokinetic theory as the first approximation, zeta potential contributes more significantly to the electrophoretic mobility of liposomes than liposomal size. For liposomes with an outer pH 7.4 (pH(o) 7.4) and a net negative outer surface charge, the most negative electrophoretic mobilities occur when the inner pH (pH(i)) is 6.8; at higher or lower pH(i), the electrophoretic mobilities are less negative. The theories mentioned above cannot explain these pH-induced electrophoretic mobility shifts. The capacity theory, predicting an induced electrical charge on the surface of liposomes, can only explain the results at pH(i) > 6.8. In this report, we hypothesize that there is a flip-flop process of phospholipids, which refers to the exchange of phospholipids between the outer and inner layers of the membrane. This flip-flop is caused by the pH gradient and membrane instability and results in the observed electrophoretic mobility changes when pH(i) is <6.8. Furthermore, it is found that the mobilities of acidic organelles are consistent with the predictions of liposome models we used here.     

Ultrasound Absorption Anisotropy in the Vicinity of Smectic A-Nematic Transition

The results of a low frequency ultrasonic study of 4-n-butyloxybenzylidene-4'-n-butylaniline (BBBA) in the vicinity of the smectic A-nematic transition are presented. The frequency and temperature dependence of the parameters describing the anisotropic ultrasound absorption are determined experimentally and compared with the predictions of fluctuation and relaxation theories of the smectic A-nematic transition. It is shown that no single theory could explain all of the experimental data.     

Theoretical study of the enthalpies of formation for C40H56 carotenes

Theoretical study of the enthalpies of formation (DeltaHf) for polyenes up to nine ethylene units and for several C40H56 carotenes including beta-carotene, alpha-carotene, lycopene, and prolycopene is presented. For polyenes and small branched alkenes, we used G2, G3, and G3MP2B3 theories, and the DeltaHf values were evaluated with the atomization, isodesmic bond separation, and homodesmic schemes. The applicability of six DFT functionals were evaluated by comparing their predictions with those obtained using G3 theory within the atomization scheme. Additivity approaches, including atom equivalents and group equivalents using DFT and semiempirical theories, were explored. We found that group equivalents associated with isodesmic reactions are able to provide the most accurate predictions within the test set. The predictions from the six functionals are in good agreement with the G3 results. Among them, B3LYP performs the best, with an average absolute deviation of only 0.30 kcal/mol. The application of DFT in the prediction for the DeltaHf value of C40H56 carotenes is promising.     

Activation of CS2 and CS by ML3 complexes

The aim of this study was to determine the best neutral ML3 metal complexes for activating and cleaving the multiple bonds in CS2 and CS. Current experimental results show that, so far, only one bond in CS2 can be cleaved, and that CS can be activated but the bond is not broken. In the work described in this paper, density functional theory calculations have been used to evaluate the effectiveness of different ML3 complexes to activate the C-S bonds in CS2 and CS, with M = Mo, Re, W, and Ta and L = NH2. These calculations show that the combination of Re and Ta in the L3Re/CS2/TaL3 complex would be the most promising system for the cleavage of both C-S bonds of CS2. The reaction to cleave both C-S bonds is predicted to be exothermic by about 700 kJ mol(-1) and to proceed in an almost barrierless fashion. In addition, we are able to rationalize why the breaking of the C-S bond in CS has not been observed experimentally with M = Mo: this reaction is strongly endothermic. There is a subtle interplay between charge transfer and pi back-donation, and it appears that the Mo-C and Mo-S bonds are not strong enough to compensate for the breaking of the C-S bond. Our results suggest that, instead, CS could be cleaved with ReL3 or, even better, with a combination of ReL3 and TaL3. Molecular orbitals and Mulliken charges have been used to help explain these trends and to make predictions about the most promising systems for future experimental exploration.     

Ultrasound Absorption Anisotropy in the Vicinity of Smectic A-Nematic Transition

Abstract

The results of a low frequency ultrasonic study of 4-n-butyloxybenzylidene-4'-n-butylaniline (BBBA) in the vicinity of the smectic A-nematic transition are presented. The frequency and temperature dependence of the parameters describing the anisotropic ultrasound absorption are determined experimentally and compared with the predictions of fluctuation and relaxation theories of the smectic A-nematic transition. It is shown that no single theory could explain all of the experimental data.     

Relation between Energy Contribution to Elastic Force and Strain Dependence of Modulus for Polybutadiene Vulcanizates

It is well known that the modulus G = r/(λ - λ^?2) varies with deformation, thus deviating from the predictions of statistical theories of rubber elasticity which require it to be constant. It has also been found that there is a nonnegligible energy contribution to the elastic force. It is postulated that these two phenomena are related because both arise from energetic interaction between chains.

Based on the lateral order of chains indicated by x-ray fiber diagrams of elongated noncrystallizable elastomers, it is suggested that energetic interaction of chains is induced by strain orientation. Proportionality between these two is assumed. The orientation distribution functions of end-to-end vectors and of statistical chain segments are considered. The proportionality constants are determined from the energy contribution to the strain dependence of the coefficient of thermal expansion. With the aid of these constants the modulus, corrected for energy contribution, is calculated. The observed and calculated elongation dependence of G agree reasonably well.

It is concluded that an energy interaction between aligned chains can account for the deviation of the observed stress elongation relation from the predictions of entropy elasticity theories.     

Interpreting the dynamics of nano-confined glass-formers and thin polymer films: Importance of starting from a viable theory for the bulk

The changes of the dynamic properties of the nanoconfined materials vary greatly depending on the nature of the interfaces, the chemical structure of the nanoconfined glass-former, the experimental methods used, and, in the case of polymers, the length-scale of the dynamics probed. Just for the glass transition temperature (T_g) alone, it can decrease, increase, or remain the same depending upon the experimental or simulation conditions. The conventional theories of T_g are unable to explain the range of behaviors seen at the nanometer size scale, and some of the theories give even conflicting predictions on the effect of small size or nanoconfinement on T_g. These problems of conventional theories orginate from the neglect or inadaquate treatment of the many-molecule relaxation, showing up already when applied to the bulk for not being able to explain some general properties of glass transition. Thus, it is not surprising to find the conventional theories fail to explain the range of behaviors of the more complicated case of materials in nanoconfinement. On the other hand, based on concepts and parameters that capture the essentials of many-molecule relaxation, the Coupling Model is not only consistent with the general properties of bulk glass-formers but can also explain the range of behaviors found in materials subjected to nanoconfinement. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2980–2995, 2006     

聚合物溶液体系玻璃化转变温度与组成之间的关系

大量的文献报道了聚合物溶液体系中聚合物玻璃化转变温度(Tg)与组成之间的关系,这些理论公式大多是基于经典热力学理论,自由体积等理论得到的,这些结果在一些聚合物溶液体系中与实验结果是符合的.在一些聚合物-溶剂体系中,Tg与组成的关系之间出现了反常的"cusp"点,两个玻璃化温度等现象,部分理论值与预测值出现较大偏差,可以用自由体积理论和自聚集理论加以定性的解释.但是依然缺乏普遍的理论来解释和预测上面现象的发生,需要进一步完善和发展聚合物溶液体系中玻璃化转变温度与组成之间的关系.     

Removing electrons can increase the electron density: a computational study of negative Fukui functions

Ab initio and density-functional theory calculations for a family of substituted acetylenes show that removing electrons from these molecules causes the electron density along the C-C bond to increase. This result contradicts the predictions of simple frontier molecular orbital theory, but it is easily explained using the nucleophilic Fukui function-provided that one is willing to allow for the Fukui function to be negative. Negative Fukui functions emerge as key indicators of redox-induced electron rearrangements, where oxidation of an entire molecule (acetylene) leads to reduction of a specific region of the molecule (along the bond axis, between the carbon atoms). Remarkably, further oxidization of these substituted acetylenes (one can remove as many as four electrons!) causes the electron density along the C-C bond to increase even more. This work provides substantial evidence that the molecular Fukui function is sometimes negative and reveals that this is due to orbital relaxation.     

Ab initio and density functional studies on bonding nature of the N?N bonds in 1,2,5‐trinitroimidazole and 1,2,4,5‐tetranitroimidazole

The bonding nature of the N N bonds in 1,2,5‐trinitroimidazole ( I ) and 1,2,4,5‐tetranitroimidazole ( II ) was examined with various levels of ab initio and density functional (DF) theories. The second‐order Møller–Plesset perturbation method (MP2) with the 6‐31G** basis set has predicted significantly long N N bond lengths in I and II , that is, 1.737 and 1.824 Å, respectively. Two DF theories, BLYP/6‐31G** and BP86/6‐31G**, provided similar results to those of MP2/6‐31G**. On the other hand, Hartree–Fock (HF) calculation with the 6‐311++G** basis set evaluated these bond lengths of I and II to be 1.443 and 1.414 Å, respectively. Bond properties including the bond critical density are strongly dependent on the equilibrium bond length. Thus, accurate prediction of geometric parameters is of particular importance to derive reliable bond properties. Especially, a substantial difference in bonding properties is observed when the electron correlation effect is included. According to our analyses with bonding natures and CHELPG charges at the MP2 level, (1) the N N bonds of I and II appear to have a significant ionic nature, and (2) the 1‐nitro group bears a considerable positive charge and has attractive electrostatic interactions with O atoms of adjacent nitro groups. Although all the theories utilized in this study predict that both I and II are stable in their potential‐energy surfaces, significantly long N N bond lengths calculated with MP2 and DF theories imply a strong hyperconjugation effect, which may explain a tendency to form a salt in these compounds easily. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 145–154, 1999     

A reexamination of the ice III/IX hydrogen bond ordering phase transition

Ice III is a hydrogen bond disordered crystal which when cooled 1 K / min or faster transforms to an antiferroelectric hydrogen bond ordered structure, ice IX. Throughout its region of stability, experiments indicate that the H bonds in ice III are, in fact, partially ordered, i.e., some proton arrangements are preferred. In addition, there has been evidence that the structure of ice IX retains some residual disorder after the transition. Diffraction experiments and calorimetry apparently conflict with regard to the degree of ordering at the ice III/IX transition. Mean field statistical mechanical theories have been used to link partial occupations from diffraction data with thermodynamics. In this work, we investigate the ice III/IX proton ordering phase transition using electronic density functional theory calculations for small unit cells, extended to simulate the phase transition in a large unit cell using graph invariants. In agreement with experiment, we observe partial ordering over a wide range of temperatures as ice III transforms to partially disordered ice IX, near 126 K, which becomes fully ordered at lower temperatures. We compare our results from full statistical mechanical simulations with mean field models, finding small errors for the low-temperature ice IX phase and much larger errors for the high-temperature ice III phase. The failure of mean field theories may explain the apparent conflict between diffraction experiments and calorimetry.     

Local relaxation of polymers in dense media: Cooperative kinematics theory and applications

Cooperative kinematics (CK) theory and its recent applications are presented. CK theory has been developed as an efficient approach for predicting the mechanism of segmental relaxation processes in bulk polymers. The theory aims at determining the most probable changes in atomic coordinates, occurring collectively in response to a given, external or localized, structural perturbation. The basic postulate is the minimization of the energy change involved in the overall conformational motion, which naturally yields the optimal pathway of cooperative relaxation. Attention has been confined here to the collective motions accompanying the rotational transitions of backbone bonds in polyethylene (PE) and polybutadiene (PB). The strong dependence of the mechanism of motions on the geometry of the repeat unit and on chain connectivity is emphasized. The differences in the types of correlated transitions operating in different structures, the effective conformational energy changes triggered by bond rotational jumps, and the correlation lengths for particular bond isomerizations are analyzed. The reorientations of C H bond vectors in cis- and trans-PB are also examined to explain the shorter correlation time of cis units, compared to trans, detected by NMR. A good agreement between various CK predictions and results from molecular dynamics (MD) simulations is obtained. The fact that CK calculations are at least two orders of magnitude faster than MD simulations invites attention to the utility of the CK method as an efficient tool for elucidating the pathway of motion in complex systems.     

The “fast” and “slow” mode theories of interdiffusion in polymer mixtures: Resolution of a controversy

The theory of interdiffusion of a pair of components in multicomponent polymer mixtures is reviewed from a statistical point of view, and the foundation of the “fast” and “slow” mode theories, as well as the more recent “ANK” theory of interdiffusion is critically examined. The ANK theory reproduces the results of the slow and fast mode theories as the two limits when the vacancy concentration is varied from zero to a large value, and shows that the interdiffusion coefficient in a binary compressible mixture at finite vacancy concentrations can not in general be expressed only in terms of the tracer diffusion coefficients of the components, but it involves in addition the cooperative diffusion coefficient which characterizes the relaxation of total density fluctuations. The predictions of the ANK expression for the molecular dependence of the kinetic factor is compared with recent scattering experiments.     

Effect of Sπ on the Polymerization of Liquid Sulfur and the Nature of Sπ

The Tobolsky-Eisenberg theory serves to explain the equilibrium polymerization of sulfur in terms of the dependence of S₈ (ring) concentration on temperature and the dependence of number-average degree of polymerization on temperature. The predictions of the theory are completely in accord with experiment. In this paper it is shown that it is possible to take the presence of another component in the melt, Sπ, into account, using a slightly modified version of the Tobolsky-Eisenberg theory. When this is done it is found that the earlier conclusions remain valid. The molecular complexity and structure of Sπ is also discussed.     

Universal properties of a single polymer chain in slit: Scaling versus molecular dynamics simulations

We revisit the classical problem of a polymer confined in a slit in both of its static and dynamic aspects. We confirm a number of well known scaling predictions and analyze their range of validity by means of comprehensive molecular dynamics simulations using a coarse-grained bead-spring model of a flexible polymer chain. The normal and parallel components of the average end-to-end distance, mean radius of gyration and their distributions, the density profile, the force exerted on the slit walls, and the local bond orientation characteristics are obtained in slits of width D=4/10 (in units of the bead diameter) and for chain lengths N=50/300. We demonstrate that a wide range of static chain properties in normal direction can be described quantitatively by analytic model-independent expressions in perfect agreement with computer experiment. In particular, the observed profile of confinement-induced bond orientation is shown to closely match theory predictions. The anisotropy of confinement is found to be manifested most dramatically in the dynamic behavior of the polymer chain. We examine the relation between characteristic times for translational diffusion and lateral relaxation. It is demonstrated that the scaling predictions for lateral and normal relaxation times are in good agreement with our observations. A novel feature is the observed coupling of normal and lateral modes with two vastly different relaxation times. We show that the impact of grafting on lateral relaxation is equivalent to doubling the chain length.     

The mesomeric effect of fluorine

A three-parameter model is used to explain the shifts in π orbital ionization potential of a range of fluorinated molecules. Using results from photoelectron spectroscopy, it appears that the shifts are not completely explicable in terms of inductive effects alone. A mesomeric term, derived from second-order perturbation theory, is found to be comparable in magnitude with inductive effects, contrary to the present theories based on ultraviolet absorption data. Values for some unperturbed π energy levels, at present the subject of discussion, are deduced from the results.