Emilia Sicilia

Solvation effects on reaction profiles by the polarizable continuum model coupled with the Gaussian density functional method

An efficient version of the polarizable continuum model for solvation has been implemented in the Gaussian density‐functional‐based code called deMon. Solvation free energies of representative compounds have been calculated as a preliminary test. The hydration effects on the reaction profile of the Cl−+CH3Cl→ClCH3+Cl− reaction and the thermodynamics of the Menschutkin reaction have also been investigated. Finally, the conformational behavior of the 1,2‐diazene cis–trans isomerization process in water was examined. Comparisons between the results obtained and the available experimental data and previous theoretical computations have been made. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 290–299, 1998

Geometries, singlet‐triplet separations, dipole moments, ionization potentials, and vibrational frequencies in methylene (CH2) and halocarbenes (CHF, CF2, CCl2, CBr2, and CI2)

The geometrical structure, harmonic vibrational frequencies, ionization potentials, and singlet‐triplet gaps of simple substituted halocarbenes (CHF, CF2, CCl2, CBr2, and CI2) have been investigated by using the linear combination of Gaussian‐type‐orbital local‐spin‐density method. Optimized geometries, as well as vibrational frequencies, are in good agreement with available experimental data. The obtained values of singlet‐triplet splittings (ΔEST) computed taking into account the nonlocal corrections are very close to experimental and previous theoretical investigations employing extended configuration interaction contributions. Many of the calculated properties obtained here have not yet been determined both experimentally and theoretically.

LANL2DZ basis sets recontracted in the framework of density functional theory

In this paper we report recontracted LANL2DZ basis sets for first-row transition metals. The valence-electron shell basis functions were recontracted using the PWP86 generalized gradient approximation functional and the hybrid B3LYP one. Starting from the original LANL2DZ basis sets a cyclic method was used in order to optimize variationally the contraction coefficients, while the contraction scheme was held fixed at the original one of the LANL2DZ basis functions. The performance of the recontracted basis sets was analyzed by direct comparison between calculated and experimental excitation and ionization energies. Results reported here compared with those obtained using the original basis sets show clearly an improvement in the reproduction of the corresponding experimental gaps.

On the applicability of the HSAB principle through the use of improved computational schemes for chemical hardness evaluation

Finite difference schemes, named Compact Finite Difference Schemes with Spectral‐like Resolution, have been used for a less crude approximation of the analytical hardness definition as the second‐order derivative of the energy with respect to the electron number. The improved computational schemes, at different levels of theory, have been used to calculate global hardness values of some probe bases, traditionally classified as hard and soft on the basis of their chemical behavior, and to investigate the quantitative applicability of the HSAB principle. Exchange acid‐base reactions have been used to test the HSAB principle assuming the reaction energies as a measure of the stabilization of product adducts.

Newly developed basis sets for density functional calculations

Optimized contracted Gaussian basis sets of double‐zeta valence polarized (DZVP) quality for first‐row transition metals are presented. The DZVP functions were optimized using the PWP86 generalized gradient approximation (GGA) functional and the B3LYP hybrid functional. For a careful analysis of the basis sets performance the transition metal atoms and cations excitation energies were calculated and compared with the experimental ones. The calculated values were also compared with those obtained using the previously available DZVP basis sets developed at the local‐density functional level. Because the new basis sets work better than the previous ones, possible reasons of this behavior are analyzed. The newly developed basis sets also provide a good estimation of other atomic properties such as ionization energies.

First-principle time-dependent study of magnesium-containing porphyrin-like compounds potentially useful for their application in photodynamic therapy.

Geometry optimization, singlet-triplet energy gap, and electronic absorption spectra calculation of complexes formed by Mg ion and porphyrin, porphyrazin, chlorine, bacteriochlorine, texaphyrin, phthalocyanine, naphthalocyanine, and anthracocyanine ligands have been carried out to elucidate their potentiality as photosensitizers in photodynamic therapy (PDT). The study has been performed employing the density functional theory (DFT) and its time-dependent approach (TDDFT) in conjunction with the PBE0 exchange-correlation functional and extended TZVP all-electron basis sets. The solvent effects have been evaluated throughout the polarizable continuum model (PCM). Results show that, following the properties requirement for the drugs used in PDT, the Mg-Tex and Mg-Pc complexes are reliable candidates for their use as photosensitizers in this medical therapy.

On the Potential Use of Squaraine Derivatives as Photosensitizers in Photodynamic Therapy: A TDDFT and RICC2 Survey.

A time-dependent density functional theory (TDDFT) and the second-order approximated coupled-cluster model with the resolution of identity approximation (RICC2) studies are reported here for some classes of squaraine derivatives. These compounds have a sharp electronic band, ranging from the visible to near-red part of the spectrum, with an high molar absorption coefficient. These features make them potential photosensitizers in the photodynamic therapy of cancer (PDT), in which a light source, a photosensitizer, and molecular oxygen ((3)O2) are combined to give cytotoxic singlet oxygen ((1)O2) as a final result in a photochemical process. For the examined structures, the introduction of different substituents (electron donating, electron withdrawing, or fused rings) in the parent molecule, in order to give different squaraine derivatives, changes the maximum absorption wavelength (λmax) from 620 to 730 nm, giving a near-red absorbing photosensitizer that can better penetrate human tissue to damage tumor cells. Theoretical results, obtained from both TDDFT/PBE0 and RICC2, are able to reproduce qualitatively the substitution effect on λmax, resulting in a useful tool for testing different structure modifications and, in general, for the molecular design of PDT photosensitizers. Calculated vertical excitation energies (singlet-singlet transitions) generally agree with experimental data within 0.3 eV. The singlet oxygen generation ability of these compounds requires that their triplet energy, for a type II reaction mechanism, should be greater than 0.98 eV. Theoretical triplet energies from the RICC2 method suggests that this requisite is fulfilled for all compounds, though the results are generally overestimated with respect to experiment by 0.7 eV, whereas TDDFT/PBE0 triplet energies, which are underestimated within 0.2 eV in few cases, lie close to the above-mentioned limit and can be considered suitable for PDT applications.

Graphical Interactive Strategy for the Analysis of NMR Spectra in Liquid Crystalline Phases

Spectral analysis of lH NMR spectra of molecules dissolved in liquid crystalline phases to obtain spectral parameters (chemical shifts, Jv indirect and Di, direct couplings) is usually a difficult and time-consuming task due to the peculiar characteristic of this kind of spectra. A procedure that links together a simulation/ iteration program with graphic routines has been developed to be run on a Vax cluster. The procedure has proved to be very useful for the analysis of spectra due to molecules containing up to 11 interacting nuclei, needing reasonably low CPU times for the simulation/iteration step and providing an interactive, friendly graphic environment for such tasks as spectrum display and line assignment, whose importance increases quickly with the number of interacting nuclei.

Hydration of ionic species studied by the reference interaction site model with a repulsive bridge correction.

We have tested the reference interaction site model (RISM) for the case of the hypernetted chain (HNC) and the partially linearized hypernetted chain (PLHNC) closures improved by a repulsive bridge correction (RBC) for ionic hydrated species. We have analyzed the efficiency of the RISM/HNC+RBC and RISM/PLHNC+RBC techniques for decomposition of the electrostatic and the nonpolar hydration energies on the energetic and the enthalpic parts for polyatomic ions when the repulsive bridge correction is treated as a thermodynamic perturbation, and investigate the repulsive bridge effect on the electrostatic potential induced by solvent on solute atoms. For a number of univalent and bivalent atomic ions, molecular cations, and anions, the method provides hydration energies deviating only by several percents from the experimental data. In most cases, the enthalpic contributions to the free energies are also close to the experimental results. The above models are able to satisfactory predict the hydration energies as well as the electrostatic potential around the ionic species. For univalent atomic ions, they also provide qualitative estimates of the Samoilov activation energies.

Absorption Spectra of the Potential Photodynamic Therapy Photosensitizers Texaphyrins Complexes: A Theoretical Analysis †

A systematic study of a class of divalent transition-metal texaphyrin complexes (M-Tex(+), M = Mn, Fe, Co, Ni, Cu, Zn), recently proposed as active photosensitizers in photodynamic therapy (PDT), was undertaken for the ground and excited electronic states. Geometry optimizations were performed by using the PBE0 exchange-correlation functional coupled with the 6-31G(d) basis set, while electronic excitations energies were evaluated by means of time-dependent density functional response theory (TD-DFT) at the PBE0/6-31+G(d) // PBE0/6-31G(d) level of theory. Solvent effects on excitation energies were taken into account in two ways:  by considering solvent molecules explicitly coordinated to the metal center and as bulk effects, within the conductor-like polarizable continuum model (C-PCM). The influence of the metal cation on the so-called Q-band, localized in the near-red visible region of the spectrum, was carefully examined since it plays a basic role in the drug design of new photodynamic therapy photosensitizers. The differences between experimental and computed excitation energies were found to be within 0.3 eV.