Biomedical subjects

Miquel Torrent-Sucarrat

Publications and source records attributed to Miquel Torrent-Sucarrat.

11 recordsLinked to original sources

PubMed2007

On the quality of the hardness kernel and the Fukui function to evaluate the global hardness.

An approximated hardness kernel, which includes the second derivative with respect to the density of the kinetic energy, the electron-electron coulomb repulsion, and the exchange density functionals, has been tested for the calculation of the global hardness. The results obtained for a series of 40 cations and neutral systems and 16 anions represent in most cases an improvement of the results obtained using the HOMO-LUMO gap approach and indicate the viability of this approach to evaluate global hardness. In addition, the relevance of the Fukui function approximation and the role of the three components of the hardness kernel in the evaluation of the global hardness have been analyzed.

Anions↗

PubMed2006

Analogies and differences between two ways to evaluate the global hardness.

The weight of the energetic components (electronic kinetic, electron-nucleus and electron-electron Coulombic, and correlation energies) of the ionization potential, electron affinity, chemical potential, and global hardness is evaluated and contrasted with the energetic components of the hardness kernel and the experimental values of these properties for 40 systems. The contrast of the hardness terms obtained from finite difference and hardness kernel gives some insight on the possible implications to differentiate the electronic energy with respect to the number electrons or the electron density.

Journal Article↗

PubMed2006

Gas-phase structures, rotational barriers, and conformational properties of hydroxyl and mercapto derivatives of cyclohexa-2,5-dienone and cyclohexa-2,5-dienthione.

The rotational barriers and conformational properties of the hydroxyl and mercapto groups attached to the alpha and beta positions of cyclohexa-2,5-dione and cyclohexa-2,5-dienthione have been studied at the B3LYP/ 6-311++G(d,p) level of theory. The results show that the conformational preferences of these studied systems are the result of a subtle interplay between different competing effects (conjugation, hyperconjugation, and steric repulsions). The applicability of the density functional theory reactivity indices and the maximum hardness principle for the present systems has been analyzed.

Journal Article↗

PubMed2005

The breakdown of the minimum polarizability principle in vibrational motions as an indicator of the most aromatic center.

The vibrational motions that disobey the minimum polarizability principle (MPP) in pi-conjugated molecules are distortions of the equilibrium geometry that produce a reduction in the polarizability due to the localization of pi electrons. For aromatic species, this electronic localization is responsible for the subsequent reduction in the aromaticity of the system. In the present work, we diagonalize the Hessian matrix of the polarizability with respect to the vibrational nontotally symmetric normal coordinates, to calculate the nontotally symmetric distortions that produce the maximum breakdown of the MPP in a series of twenty polycyclic aromatic hydrocarbons. It is shown that the nuclear displacements that break the MPP have larger components in those rings that possess the highest local aromaticity. Thus, these vibrational motions can be used as an indicator of local aromaticity.

Journal Article↗

PubMed2005

Stiffness and Raman intensity: a conceptual and computational DFT study.

A DFT-based reactivity descriptor, the nuclear stiffness, is related to the Raman scattering intensity, which is experimentally accessible. The application of this new relationship obtained within certain approximations has been checked in two different sets of molecules. First, we study a favorable case, where the contribution of the anisotropy to the Raman intensity is zero (symmetric stretching mode in 15 tetrahedral molecules). Second, we consider a "worst" case scenario, where the anisotropy contribution can be expected to be important (stretching mode in 32 diatomic molecules). The numerical results clearly show a relationship between stiffness and Raman intensity reflecting the expected anisotropy influence.

Journal Article↗

PubMed2005

Variational calculation of vibrational linear and nonlinear optical properties.

A variational approach for reliably calculating vibrational linear and nonlinear optical properties of molecules with large electrical and/or mechanical anharmonicity is introduced. This approach utilizes a self-consistent solution of the vibrational Schrodinger equation for the complete field-dependent potential-energy surface and, then, adds higher-level vibrational correlation corrections as desired. An initial application is made to static properties for three molecules of widely varying anharmonicity using the lowest-level vibrational correlation treatment (i.e., vibrational Møller-Plesset perturbation theory). Our results indicate when the conventional Bishop-Kirtman perturbation method can be expected to break down and when high-level vibrational correlation methods are likely to be required. Future improvements and extensions are discussed.

Algorithms↗

PubMed2005

Calculation of Franck-Condon factors including anharmonicity: simulation of the C2H4+X2B3u<--C2H4X1A(g) band in the photoelectron spectrum of ethylene.

Our new simple method for calculating accurate Franck-Condon factors including nondiagonal (i.e., mode-mode) anharmonic coupling is used to simulate the C2H4+X2B3u<--C2H4X1A(g) band in the photoelectron spectrum. An improved vibrational basis set truncation algorithm, which permits very efficient computations, is employed. Because the torsional mode is highly anharmonic it is separated from the other modes and treated exactly. All other modes are treated through the second-order perturbation theory. The perturbation-theory corrections are significant and lead to a good agreement with experiment, although the separability assumption for torsion causes the C2D4 results to be not as good as those for C2H4. A variational formulation to overcome this circumstance, and deal with large anharmonicities in general, is suggested.

Journal Article↗

PubMed2005

Generalizing the breakdown of the maximum hardness and minimum polarizabilities principles for nontotally symmetric vibrations to non-pi-conjugated organic molecules.

In previous works we have shown that certain pi-conjugated organic molecules possess nontotally symmetric vibrations that break the maximum hardness (MHP) and minimum polarizability principles (MPP). We have also derived a set of simple rules to determine a priori without calculations whether a particular pi-conjugated organic molecule violates these two principles. In the present work, we generalize these results, and we show that not only pi-conjugated organic molecules but also other molecules without pi-conjugated structure or even pi-bonds can exhibit nontotally symmetric molecular distortions that do not follow these two principles. We have also found that the breakdowns of the MHP and the MPP are not necessarily connected, since the polarizability is not always proportional to the softness. Finally, we also introduced a methodology based on the diagonalization of the hardness Hessian matrix with respect to the vibrational normal coordinates to determine the nontotally symmetric molecular displacements that do not follow the MHP.

Aluminum↗

PubMed2004

The hardness profile as a tool to detect spurious stationary points in the potential energy surface.

In the present work, we have computed the energy and hardness profiles for a series of inter and intramolecular conformational changes at several levels of calculation. All processes studied have in common the fact that the choice of a weak methodology or a poor basis set results in the presence of spurious stationary points in the energy profile. At variance with the energy profiles, the hardness profiles calculated as the difference between the vertical ionization potential and electron affinity always show the correct number of stationary points independently of the basis set and methodology used. For this reason, we have concluded that hardness profiles can be used to check the reliability of the energy profiles for those chemical systems that, because of their size, cannot be treated with high level ab initio methods.

Journal Article↗

PubMed2004

Basis set and electron correlation effects on initial convergence for vibrational nonlinear optical properties of conjugated organic molecules.

Using three typical pi-conjugated molecules (1,3,5-hexatriene, 1-formyl-6-hydroxyhexa-1,3,5- triene, and 1,1-diamino-6,6-dinitrohexa-1,3,5-triene) we investigate the level of ab initio theory necessary to produce reliable values for linear and nonlinear optical properties, with emphasis on the vibrational contributions that are known to be important or potentially important. These calculations are made feasible by employing field-induced coordinates in combination with a finite field procedure. For many, but not all, purposes the MP2/6-31+G(d) level is adequate. Based on our results the convergence of the usual perturbation treatment for vibrational anharmonicity was examined. Although this treatment is initially convergent in most circumstances, a problematic situation has been identified.

Electrons↗

PubMed2004

The gas-phase hydrogen bond complexes between formic acid with hydroxyl radical: a theoretical study.

We report a theoretical study on seven radical hydrogen bond complexes between syn-HCOOH and OH and eight radical hydrogen bond complexes between anti-HCOOH and OH, that have been carried out by using the B3LYP, MP2, QCISD, and CCSD(T) theoretical approaches with the 6-311 + G(2df,2p) basis set. In all cases, the bonding features were analysed using the atoms in molecules (AIM) theory by Bader and the natural bond orbital (NBO) partition scheme by Weinhold et al. We have found twelve complexes having a single hydrogen bond and three complexes presenting a cyclic structure with multiple bonds, pointing out the existence of a cooperative effect. One of them presents a bound O...O interaction producing a stabilisation effect. The stability of these complexes has been calculated to be in the -0.81 and -5.96 kcal mol-1 range and their possible implication in the HCOOH plus OH reaction is also discussed. Finally, we also report the computed harmonic vibrational frequencies of the two O-H stretching modes and the HOC out-of-plane wagging mode, along with the frequency red-shifts originated by the complex formation and the corresponding computed intensity ratio relative to the monomers.

Journal Article↗