A computer simulation — Statistical procedure for predicting complexation equilibrium constants

The complexing properties of macrocyclic ligands have been quantitatively studied by the combined use of molecular mechanics, molecular dynamics, and multiple linear regression. The dependent variables in the regression equations are experimental equilibrium constants for known macrocycle complexes in various solvents. The independent variables are theoretical simulation results on the solvent-free ligand and its complex and additional physically motivated empirical variables to describe solvent and other important effects.The systems studied were: (a) 314 metal ion-macrocycle-solvent (including 3 mixed solvents) combinations; (b) 88 ammonium ion-crown ether-solvent (including 1 mixed solvent) combinations; (c) 24 hydrogen ion-crown ether-H₂O combinations; (d) 26 Na⁺ ion-spherand-CDCl₃ combinations; (e) 78 ammonium ion-spherand-CDCl₃ combinations; and (f) 73 complicated host-guest-solvent (including 1 mixed solvent) combinations.For each system, we report the best regression equation obtained using the AMBER force field. The standard errors in logK range from 1.42 in the largest system to 0.36 in the smallest. Regression equations were determined for several of the systems using the MMP2 force field as well, and the equations are shown to be relatively insensitive to the force field.The predictive ability of the method was tested by predicting logK for 20% of the cases chosen at random using equations derived from the remaining 80%. The errors in the predicted values are shown to be consistent with the statistical assumptions of the model.Regression equations obtained with this method can be used to predict the equilibrium constants for new complexes involving some combination of new, possibly unknown macrocycle, new host and, in certain cases new solvent. No X-ray or other structural data for the macrocycle is needed.     

Graph model for calculating the properties of saturated monoalcohols based on the additivity of energy terms

A 16-constant additive scheme was derived for calculating the physicochemical properties of saturated monoalcohols CH₄O-C₉H₂₀O and decomposing the triangular numbers of the Pascal triangle based on the similarity of subgraphs in the molecular graphs (MGs) of the homologous series of these alcohols. It was shown, using this scheme for calculation of properties of saturated monoalcohols as an example, that each coefficient of the scheme (in other words, the number of methods to impose a chain of a definite length i ₁, i ₂, … on a molecular graph) is the result of the decomposition of the triangular numbers of the Pascal triangle. A linear dependence was found within the adopted classification of structural elements. Sixteen parameters of the schemes were recorded as linear combinations of 17 parameters. The enthalpies of vaporization L _{298 K}⁰ of the saturated monoalcohols CH₄O-C₉H₂₀O, for which there were no experimental data, were calculated. It was shown that the parameters are not chosen randomly when using the given procedure for constructing an additive scheme by decomposing the triangular numbers of the Pascal triangle.     

Dilute solution properties of branched polymers

For calculating the ratio of the intrinsic viscosities of branched and linear polymers of the same molecular weight, [η]_B/[η]_L, a new theory taking into account the excluded volume effect is presented. By using the modified Flory equation, the excluded volume effect of branched polymers is predicted with the aid of the first-order perturbation theory. The linear expansion factor α_s is converted to the hydrodynamic expansion factor α_η by using the Kurata-Yamakawa theory. Our calculated results, i.e., [η]_B/[η]_L and 〈s²〉_B/〈s²〉_L, agree well with experiment for various type branched polymers, i.e., randomly branched and comb-shaped polymers of poly(vinyl acetate).     

An atom-in-molecules and electron-localization-function study of the interaction between O2 and VxOy+/VxOy (x = 1, 2, y = 1–5) clusters

 The most stable structures of V _x O _y ⁺/V _x O _y (x=1, 2, y=1–5) clusters and their interaction with O₂ are determined by density functional calculations, the B3LYP functional with the 6-31G* basis set. The nature of the bonding of these clusters and the interaction with O₂ have been studied by topological analysis in the framework of both the atoms-in-molecules theory of Bader and the Becke–Edgecombe electron localization function. Bond critical points are localized by means of the analysis of the electron density gradient field, ∇ρ(r), and the electron localization function gradient field, ∇η(r). The values of the electron density properties, i.e., electron density, ρ(r), Laplacian of the electron density, ∇²ρ(r), and electron localization function, η(r), allow the nature of the bonds to be characterized, and linear correlation is found for the results obtained in both gradient fields. Vanadium-oxygen interactions are characterized as unshared-electron interactions, and linear correlation is observed between the electron density properties and the V–O bond length. In contrast, O₂ units involve typical shared-electron interactions, as for the dioxygen molecule. Four different vanadium–oxygen interactions are found and characterized: a molecular O₂ interaction, a peroxo O₂ ²⁻ interaction, a superoxo O₂ ⁻ interaction and a side-on O₂ ⁻ interaction. Received: 15 October 2001 / Accepted: 30 January 2002 / Published online: 24 June 2002     

Molecular graphs as topological objects in space

Stereochemistry deals primarily with distinctions based on rigid geometry, e.g., bond angles and lengths. But some chemical species have molecular graphs (such as knots, catenanes, and nonplanar graphs K₅ and K_3.3) that reside in space in a topologically nontrivial way. For such molecules there is hope of using topological methods to gain chemical information. Viewing a molecular graph as a topological object in space makes it unrealistically flexible; but if one proves that a certain graph is “topologically chiral” or that two graphs are “topological diastereomers,” then one has ruled out interconversion under any physical conditions for which the molecular graph still makes sense. In this paper, we consider several kinds of topological questions one might ask about graphs in space, methology and results available, and specific topological properties of various molecules.     

Influence of Temperature on Molecular Dipole Orientation in the Binary Mixture of Polar-Nonpolar Liquids

Abstract

The excess correlation factor in the binary mixture of polar liquid (i.e., i-butanol and t-butanol) in the nonpolar solvent (i.e., benzene, tetrachloromethane, n-heptane) has been evaluated at 301°K,307°K,313°K and 318°K. An extensive study has been made on the influence of temperature on the orientation of molecular dipoles in the solution with the help of the excess correlation factor.     

Intrinsic viscosity of polymers of low molecular weight

Experimental evidence concerning the dependence of the intrinsic viscosity [η] on molecular weight M in the low molecular weight range (from oligomers to M = 5 × 10⁴) has been collected in a variety of solvents for about ten polymers, i.e., polyethylene, poly(ethylene oxide), poly(propylene oxide), polydimethylsiloxane, polyisobutylene, poly(vinylacetate), poly(methyl methacrylate), polystyrene, poly-α-methylstyrene, and some cellulose derivatives. In theta solvents, the constancy of the ratio [η]Θ/M^0.5 extends down to values of M much lower than those predicted by current hydrodynamic theories. In good solvents, and on decreasing M, the polymers examined, with the exception of polyethylene and some cellulose derivatives, show a decrease in the exponent a of the Mark-Houwink equation [η] = KM^a. This upward curvature gives rise to the existence of a more or less extended linear region where the equation [η] = K₀M^0.5 is obeyed. Below the linear range, i.e., for even shorter chains, the exponent a can increase, i.e., polydimethylsiloxane, or decrease below 0.5, i.e., poly(ethylene oxide), depending on the particular chain properties. These different dependences have been discussed in terms of: (a) variations of thermodynamic interactions with molecular weight; (b) variations of conformational characteristics (as for instance the ratio) 〈r0²/nl²〉, where 〈r0²〉 is the unperturbed mean square end-to-end distance and n is the number of bonds each of length l; (c) hydrodynamic properties of short chains.     

Cyclical Edge-Connectivity of Fullerene Graphs and (k, 6)-Cages

It is shown that every fullerene graph G is cyclically 5-edge-connected, i.e., that G cannot be separated into two components, each containing a cycle, by deletion of fewer than five edges. The result is then generalized to the case of (k,6)-cages, i.e., polyhedral cubic graphs whose faces are only k-gons and hexagons. Certain linear and exponential lower bounds on the number of perfect matchings in such graphs are also established.     

Thermodynamics of a Fluid of Hard D-dimensional Spheres: Percus-Yevick and Carnahan-Starling-like Results for D = 4 and 5

Abstract

The equations of state (EOS) of four and five dimensional hyperspheres have been calculated using Leutheusser's ansatz. In five dimensions these, and the correlation functions, are compared with the results obtained from the analytic solution of the Percus—Yevick (PY) approximation. It is shown that the ansatz reproduces extremely well the PY results. However, in both approximations neither the virial, Z_v , nor the compressibility, Z_c , EOS reproduce well the available molecular dynamics (MD) results. Yet a linear combination of Z_v and Z_c , following the Carnahan—Starling EOS for hard spheres, are in excellent agreement with the MD results in four and five dimensions.     

Domain‐Averaged Exchange‐Correlation Energies as a Physical Underpinning for Chemical Graphs

A novel solution to the problem of assigning a molecular graph to a collection of nuclei (i.e. how to draw a molecular structure) is presented. Molecules are universally understood as a set of nuclei linked by bonds, but establishing which nuclei are bonded and which are not is still an empirical matter. Our approach borrows techniques from quantum chemical topology, which showed for the first time the construction of chemical graphs from wave functions, shifting the focus on energetics. This new focus resolves issues surrounding previous topological analyses, in which domain‐averaged exchange‐correlation energies (V_xc), quantities defined in real space between each possible atom pair, hold the key. Exponential decay of V_xc in non‐metallic systems as the intercenter distance increases guarantees a well‐defined hierarchy for all possible V_xc values in a molecule. Herein, we show that extracting the set of atom pairs that display the largest V_xc values in the hierarchy is equivalent to retrieving the molecular graph itself. Notably, domain‐averaged exchange‐correlation energies are transferable, and they can be used to calculate bond strengths. Fine‐grained details resulted to be related to simple stereoelectronic effects. These ideas are demonstrated in a set of simple pilot molecules.     

Adsorption of carbon dioxide on montmorillonites of different ion exchange forms

The adsorption isotherms of carbon dioxide are measured at 303, 343, 373, and 400 K and pressure P ranged from 0.1 to 6 MPa for three montmorillonites, i.e., the initial sample in the sodium form and two ion exchanged samples in pyridinium and polyhydroxyaluminum forms. It is shown that the linear character of the isosteres of CO₂ adsorption is retained for polyhydroxyaluminum and sodium montmorillonites, a deviation of the adsorption isosteres from linearity is observed for pyridinium montmorillonite at high temperatures (373–400 K) and high surface fillings. The differential excess entropies of CO₂ adsorbate are calculated for different fillings of montmorillonites. Using various approaches, the adsorption volumes for the systems studied are calculated.     

The BERKELEY system. III. General configuration-interaction methods for open-shell molecular electronic states

A system of general “open-ended” configuration-interaction (CI ) programs, specifically designed for the Harris Corporation Slash Four minicomputer, is described. These methods are general in the sense that an arbitrary list of configurations (linear combinations of Slater determinants) may be used, and open ended in that peripheral (i.e., disk) storage capacity determines the maximum size problem that can be solved. The largest variational calculations carried out to date using BERKELEY involve 7064 open-shell singlet configurations (31,898 Slater determinants). Detailed timing breakdowns are presented for four test cases, two of which involve the lowest ππ* singlet state of ethylene. The other two examples are the orthogonal or bisected singlet state of trimethylenemethane and the ⁸B₁ state of the MnCH₂ complex. In the latter case, it is found that the predicted Mn? CH₂ dissociation energy is only slightly increased by electron correlation effects.     

Biomimetic oxovanadium(IV) and (V) complexes with a tridentate (N,N,O)-donor hydrazonic ligand. Two X-ray crystal structure modifications of (2-acetylpyridine-benzoylhydrazonato)dioxovanadium(V)

Two oxovanadium(IV) and (V) complexes with 2-acetylpyridine-benzoylhydrazone have been prepared and characterized. The analytical methods used included elemental analysis, i.r., FAB⁺ m.s., ⁵¹V-n.m.r. and e.p.r. X-ray diffractometry from single crystals as well as from microcrystalline material were also performed. Molecular modeling was used to calculate the complex structures in a vacuum and their vibrational frequencies. Octahedral coordination is suggested for the complex acetylacetonato(2-acetylpyridine-benzoylhydrazonato)-oxovanadium(IV) (1), for which good agreement was verified between calculated and observed i.r. data. Two crystal structure modifications of (2-acetylpyridine-benzoylhydrazonato)dioxovanadium(V) (2) have been determined by X-ray diffraction methods. In both crystalline modifications the molecular structure of the complex shows a distorted trigonal bipyramidal VN₂O₃ coordination. The molecular structure, found experimentally for (2), was compared with the theoretically calculated one. The results validate the theoretical method.     

Recent developments in living carbocationic polymerization of alkenes

The recent discovery of living cationic polymerization of alkenes has been followed up in three areas: 1) Demonstration of the validity of the DP_n = [M_o]/[I_o] relationship and the synthesis of high molecular weight polyisobutylenes M_n_>100 000) of narrow molecular weight distribution (M_w/M_n ≈︁1.1) 2) Discovery of electrophilic quenching and the use of this method for the preparation of allyl-telechelic polyisobutylenes by quenching with trimethylallylsilane and 3) Synthesis of novel linear and three-arm star radial thermoplastic elastomers comprising rubbery polyisobutylene central sequences connected to glassy polystyrene or a polystyrene derivative (i.e., p-methylstyrene, p-tert-butylstyrene, indene) outer sequences. Some physical-mechanical properties of these materials have been investigated.     

Investigations on structure and properties of blends of polypropylene with polyamide 1010 by Rayleigh scattering

The phase structure and morphology of polypropylene （iPP） blends with poly（decamethylene sebacamide （polyamide 1010, PA 1010） and the part-compatible alloys （iPP/PA 1010/iPP-g-MAH） are investigated by Rayleigh scattering, i.e. small angle light scattering （SALS）. The structure parameters of SALS, i.e. correlation distance, ac and average chord lengths, /, are calculated. Their variation with the composition of the blends and alloys is discussed. The morphology and structure of fracture surfaces in the blends and alloys are studied by SEM images. The average diameter of dispersed phase in the blends and alloys is calculated by graph processing. The variation of average diameter is similar as that of average chord lengths of dispersed phases. The mechanical properties of the blends and alloys are analyzed and the relation of modulus and yield stress with the composition is studied. It is confirmed that the correlation between the modulus and the structure parameters is non-linear, while the correlation between the yield stress and the composition or structure parameters is linear.