The influence of the solvent on the conformational energy differences due to the anomeric effect

The solvation free energy ΔG^sol of molecules exhibiting the anomeric effect is computed in an approach that considers a continuous distribution for the solvent. A partition of ΔG^sol into separately evaluated contributions confirms that changes in the energy of the systems due to changes in conformation of the solute are ruled by the electrostatic contribution. A comparison with the “exact” values indicates that the approximate expression for the electrostatic contribution to ΔG^sol are not accurate enough to permit a proper modeling of the solvent influence on the anomeric effect. The systems are composed of methanediol, methoxymethanol, dimethoxymethane, and 2-methoxytetrahydropyran in carbon tetrachloride, chloroform, acetone, and water. The calculations have been performed at the SCF level with the STO -3G and 4–31G basis sets.     

Dispersion and repulsion contributions to the solvation energy: Refinements to a simple computational model in the continuum approximation

A computational method for the evaluation of dispersion and repulsion contributions to the solvation energy is here presented in a formulation which makes use of a continuous distribution of the solvent, without introducing additional assumptions (e.g., local isotropy in the solvent distribution). The analysis is addressed to compare the relative importance of the various components of the dispersion energy (n = 6, 8, 10) and of the repulsion term, to compare several molecular indicators (molecular surface and volume, number of electrons) which may be put in relation to the dispersion-repulsion energy, and to define simplified computational strategies. The numerical examples refer to saturated hydrocarbons in water, treated with the homogeneous approximation of the distribution function which for this type of solution appears to be acceptable.     

Friction-induced vibrations in the framework of dynamic substructuring

Nonlinear Dynamics - In complex vibrating systems, contact and friction forces can produce a dynamic response of the system (friction-induced vibrations). They can arise when different parts of the...     

Recognition of enantiomers with chiral molecular tweezers derived from (+)- or (−)-usnic acid

The synthesis of four stereoisomers of a chiral molecular tweezer using trans-1,2-diaminocyclohexane as spacer and two molecules of usnic acid as pincers is reported. The behavior of these chiral tweezers as chiral complexing agents was evaluated in NMR with various chiral esters containing an electron-poor aromatic ring to allow non-covalent aromatic–aromatic interactions.     

Universal properties of Ising clusters and droplets near criticality

Clusters and droplets of positive spins in the two-dimensional Ising model percolate at the Curie temperature in absence of external field. The percolative exponents coincide with the magnetic ones for droplets but not for clusters. We use integrable field theory to determine amplitude ratios which characterize the approach to criticality within these two universality classes of percolative critical behavior.     

Geometries and properties of excited states in the gas phase and in solution: theory and application of a time-dependent density functional theory polarizable continuum model

In this paper we present the theory and implementation of analytic derivatives of time-dependent density functional theory (TDDFT) excited states energies, both in vacuo and including solvent effects by means of the polarizable continuum model. The method is applied to two case studies: p-nitroaniline and 4-(dimethyl)aminobenzonitrile. For both molecules PCM-TDDFT is shown to be successful in supporting the analysis of experimental data with useful insights for a better understanding of photophysical and photochemical pathways in solution.     

The effect of changing the TiO2 content (0-6%) on the structure and thermal evolution of glasses obtained from porphyric sands and dolomite

Glasses have been obtained from mixtures of porphyric sands and dolomite. The influence of changes in the TiO₂ content (0-6%) on the glass structure and the formation of crystalline phases on reheating has been studied. The experimental results suggest that in the studied system TiO₂ promotes glass-in-glass phase separation and plays the role of a nucleating agent. The activation energy for crystal growth, E_C = 486 kJ mol⁻¹, and the Avrami parameter (m = 1) have been evaluated by means of thermoanalytical techniques in the case of the base glass (no TiO₂ added). The value of the Avrami parameter (m = 1) agrees well with SEM observations of dendritic crystal growth from surface nuclei. In the other glasses, lath-like crystals were observed, the microstructure being finer the greater the TiO₂%. The first addition of TiO₂ (2%) gives, on quenching, a partially devitrified product; subsequent additions of titania, surprisingly, allow glasses to be formed more easily. The experimental results suggest that Na₂O and K₂O, present in the porphyric sands and therefore in the glasses (to a total amount of ≈ 5.6%), segregate preferentially into the titania-rich phase with respect to MgO. Taking into account that Na₂O and K₂ are useful in lowering the liquidus temperature of glasses but are known to have a negative effect on the mechanical properties, this can be important in the production of glass ceramics.     

Radiative and nonradiative decay rates of a molecule close to a metal particle of complex shape

We present a model to evaluate the radiative and nonradiative lifetimes of electronic excited states of a molecule close to a metal particle of complex shape and, possibly, in the presence of a solvent. The molecule is treated quantum mechanically at Hartree-Fock (HF) or density-functional theory (DFT) level. The metal/solvent is considered as a continuous body, characterized by its frequency dependent local dielectric constant. For simple metal shapes (planar infinite surface and spherical particle) a version of the polarizable continuum model based on the integral equation formalism has been used, while an alternative methodology has been implemented to treat metal particles of arbitrary shape. In both cases, equations have been numerically solved using a boundary element method. Excitation energies and nonradiative decay rates due to the energy transfer from the molecule to the metal are evaluated exploiting the linear response theory (TDHF or TDDFT where TD--time dependent). The radiative decay rate of the whole system (molecule + metal/solvent) is calculated, still using a continuum model, in terms of the response of the surrounding to the molecular transition. The model presented has been applied to the study of the radiative and nonradiative lifetimes of a lissamine molecule in solution (water) and close to gold spherical nanoparticles of different radius. In addition, the influence of the metal shape has been analyzed by performing calculations on a system composed by a coumarin-type molecule close to silver aggregates of complex shape.