Search Results
Abstract
The search for the lowest energy conformation of complex {β-cyclodextrin (β-CD)+chlorambucil} were carried out by molecular mechanics method. Theoretical calculations of molecular interactions of complex were carried out using the molecular orbital method. The correlation between energy changes and molecular structures are discussed. The large interaction energies calculated by the molecular orbital method bears out the inclusion phenomenon.
Calorimetric study on inclusion of some alcohols into α-cyclodextrin cavities
Molecular mechanical calculation of hydration Gibbs energies
Abstract
The enthalpies of transfer 2-propanol, 1,2-butanediol (BD) and 1-hexanol from aqueous to aqueous α-cyclodextrin (CD) solutions have been determined by microcalorimetry at various mole fractions at 298.15 K. To clarify stabilities of inclusion complexes in aqueous solutions, hydration Gibbs energies calculation of inclusion complex of CD-alcohol were performed by using the molecular mechanics with the MMFF94s force field in the generalized born/surface area (GB/SA) model. The largest stabilization in Gibbs energy is obtained by the hydration (Δhyd H) of α-CD-1,2-butanediol complex among α-CD-butanediol isomers complexes.
Abstract
Computer-aided modeling has been very successful in the design of chelating ligands for the formation of selective metal complexes. We report herein preliminary efforts to extend the principles developed for ion-specific chelating ligands to the weaker, more diffuse electrostatic interactions between complex anions and dicationic sites of anion-exchange resins. We present formal- and partial-charge methodologies for determining calculated electrostatic affinity between plutonium(IV) hexanitrato dianions and free analogues of dicationic anion-exchange sites. Both approaches correlate well with empirically-determined distribution coefficients for our bifunctional pyridinium-based resins (0.65<r 2<0.98). This quantitative structure activity relationship (QSAR) will be useful in the determination of which structural modifications within a select series of bifunctional resins are most likely to be advantageous. Ultimately, we hope to refine this methodology to allow the a priori determination of ion-exchange behavior for a broad class of materials.
thermolysis of the above hydrate Ga(NO 3 ) 3 · x H 2 O, offering a different and more realistic scheme of events. Besides thermal analysis we have used the molecular mechanics method in order to build up the structural models, calculate their minimal potential
Abstract
Enthalpies of mixing of R- and S-enantiomers of liquid chiral compounds such as 2-aminohexane, 2-aminoheptane, 2-aminooctane, 2-aminononane, 1-(4-chlorophenyl)-ethylamine, 1-(4-fluorophenyl)ethylamine, 2-amino-butane-1-ol have been measured over the whole range of mole fractions at 298.15 K. Mixing of heterochiral liquids observed, realized enthalpic destabilization over entire compositions. The extreme values of enthalpies of mixing and the intermolecular interaction obtained by the molecular mechanics calculations showed a linear correlation, except the few compounds measured.
Abstract
Gamma-irradiated L-α-alanine used in EPR-coupled dosimetry has a complex EPR spectrum at room temperature. Changing the temperature or other conditions of the irradiated samples leads to varied EPR spectrum, i.e., some components disappear and/or new ones are formed. We used both molecular mechanics (MM+) and semiempirical (AM1) methods to perform a theoretical investigation of the seven radical species that have been experimentally detected. We established their order of priority in the given simulation conditions (at 0 K, in vacuo). The formation stages advanced for these long-lived radical species were characterized by a theoretical determination of the reaction enthalpies.
Abstract
The inclusion complex formation of riboflavin (RF) with hydroxypropyl-β-cyclodextrin (HP-β-CD) in water was investigated by 1H NMR, UV-vis spectroscopy, and solubility methods. A 1:1 stoichiometry and thermodynamic parameters of complex formation (K, Δc G 0, Δc H 0, and Δc S 0) were determined. Complexation was characterized by negative enthalpy and entropy changes due to prevalence of van der Waals interactions and hydrogen bonding between polar groups of the solutes. A partial insertion of RF into macrocyclic cavity was revealed on the basis of 1H NMR data and molecular mechanics calculation. Location of benzene ring of RF molecule inside the hydrophobic cavity of HP-β-CD results in an increase of aqueous solubility of the former.
Analysis of a symmetric neopolyol ester
II. Solid state13C NMR and X-ray measurements
The symmetric neopolyol ester tetra[methyleneoxycarbonyl(2,4,4-trirnethyl)pentyl]methane (MOCPM) has been studied by variable-temperature solid-state13C NMR and X-ray powder diffraction and compared to molecular mechanics calculations of the molecular structure. Between melting and glass transition temperatures the material is semicrystalline, consisting of two conformationally and motionally distinguishable phases. The more mobile phase is liquid-like and is, thus attributed to an amorphous phase (≈16%). The branches of the molecules in the crystal exhibit two conformationally distinguishable behaviors. In one, the branches are well ordered (≈56%), in the other, the branches are conformationally disordered (≈28%). Different branches of the same molecule may show different conformational order. This unique character of the rigid phase is the reason for the deficit of the entropy of fusion observed earlier by DSC. In the melt, solid state NMR can identify two bonds that are rotationally immobile, even though the molecules as a whole have liquid-like mobility. This partial rigidity of the branches accounts quantitatively for the observed increase in heat capacity at the glass transition. The reason for this unique behavior of MOCPM, a small molecule, is the existence of one chiral centers in each of the four arms of the molecule. A statistical model assuming that at least two of the chiral centers must fit into the order of the crystal can explain the crystallization behavior and would require 12.5% amorphous phase, 28.1% conformational disorder, and 59.4% crystallinity, close to the observed maximum perfection.
Abstract
Thermokinetic parameters of the solid-state of poly(o-toluidine) (POT) doped with perchloric (HClO4) acid was studied by thermogravimetric analysis (TG) and differential thermal analysis (DTA) under non-isothermal conditions. Molecular mechanics (MM) calculations suggest that the optimal geometric structure (OMG) of the HClO4-doped POT is at least four orders of magnitude more stable than the molecular geometric (MG) structure. These calculations indicate that the potential energy (PE/kJ mol−1) of the OMG is about four (1.09·104) orders of magnitude lower than the MG structure of the same matrix. The empirical formula of the doped polymer is best represented by [POT-2HClO4·2H2O]n as substantiate by elemental analysis and MM calculations. The full polymer decomposition and degradation were found to occur in three stages during the temperature increase. The decomposition activation energy (E d) of HClO4-doped POT matrix was calculated by employing different approximations. The heating rate (α) of the decomposition and the frequency factor (K o) were calculated. A number of equations were used to evaluate the kinetic parameters. The mechanism of the degradation of the conducting polymer is explained on the basis of their kinetic parameters. A remarkable heating rate dependence of the decomposition rate was observed.
. [5]. U. Burkert N.L. Allinger 1982 Molecular Mechanics, Vol. 99 American Chemical
