Theoretical study of the SN2 reaction of Cl−(H2O)+CH3Cl using our own N-layered integrated molecular orbital and molecular mechanics polarizable continuum model method (ONIOM, PCM)

The effects of solvation in the S_N2 reaction Cl^–(H₂O)+CH₃Cl were investigated using our own N-layered integrated molecular orbital and molecular mechanics (ONIOM) polarizable continuum model (PCM) method [Vreven T, Mennucci B, da Silva CO, Morokuma K, Tomasi J (2001) J Chem Phys 115:62–72], which surrounds the microsolvated ONIOM system with a polarizable continuum. The microsolvating water molecule tends to stay in the vicinity of the original chloride ion. In the ONIOM calculations, Cl^–+CH₃Cl was considered as the model system and was handled with the high-level method, while the explicit water molecule in the microsolvated complex was treated at the low-level. The molecular orbital (MO) and ONIOM(MO:MO) calculations allow us to assess the errors introduced by the ONIOM extrapolation, as well as the effects of microsolvation on the potential-energy surfaces. We find that ONIOM[B3LYP/6-31+G(d,p):HF/6-31+G(d,p)] and ONIOM[B3LYP/6-31+G(d,p):HF/6-31+G(d,p)]-PCM methods are good approximations to the target B3LYP/6-31+G(d,p) and B3LYP/6-31+G(d,p)-PCM methods. In addition, several approximate (computationally less expensive) schemes in the ONIOM-PCM method have been compared to the exact scheme, and all are shown to perform well.Contribution to the Jacopo Tomasi Honorary Issue     

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.     

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...     

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.     

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.     

A theoreticalab initio study of the reaction of formation of 2-fluoroethanol from oxirane and HF

The reaction of ring opening of epoxides under the action of a halogenic acid has been investigated using as model the system C₂H₄O + HF → CH₂OHCH₂F. The physical conditions chosen for the model more directly correspond to the gas phase reaction and under these conditions the preferred mechanism leads to the formation of a halohydrine having the same configuration at the C atom as the reagent. Parallel investigations have been performed on other mechanisms which postulate the preliminary formation of the conjugate acid of the oxirane (C₂H₄OH⁺) and proceed via the well known mechanismsA ₁ orA ₂. In this case the best mechanism corresponds to the so-called “borderlineA ₂” mechanism. This last type of mechanism probably is the dominant one in protic solutions, but by coupling the present calculations with experimental conductometric measurements in anhydrous aprotic media one could consider the first concerted mechanism as a possible candidate also for the reaction in “inert” media. A qualitative analysis of the transition state indicates, in addition, that the propension for the retention path, is probably emphasized by the use of HF as reactant, and that with other acids, like HC1, or even by assuming the presence of dimers like HF·HF, the inversion path could be preferred. The investigations have been done by determining the geometry of the transition state and the reaction coordinate withab initio SCF STO-3G calculations on the whole nuclear configuration space (21 dimensions). These calculations have been supplemented by a few CI calculations on the same basis set and by a few SCF calculations with a larger basis set.     

Counterpoise corrections to the components of bimolecular energy interactions: An examination of three methods of decomposition

Counterpoise corrections for the basis set superposition error to the components of the bimolecular interaction energy are defined for three methods of decomposition. The results for the case of the NH₃ + BH₃ interaction are presented and discussed.     

Optical necklace states in Anderson localized 1D systems

We report on the observation of nonlocalized modes or necklace states of light waves in disordered systems in the Anderson localized regime. The samples consist of positional-disordered binary multilayer systems. Anderson localized modes manifest themselves as narrow high-transmission peaks in the transmission spectrum, whereas the average of the logarithm of the transmission coefficient decreases linearly with thickness. Optical necklace states are observed as modes with a characteristic multiresonance time response and relatively fast decay time.