Marco Antonio Chaer Nascimento

The polarization dependence of two-photon absorption rates for randomly oriented molecules

The method and results are presented for the analysis of the two-photon absorption experiment for randomly oriented molecules (in a single-beam regime) that can be easily extended to higher-order processes without any assumption regarding the nature of the initial state. Some known results were recovered and some new ones presented. The ratio of the averaged transition rate for circularly polarized light to the one for linearly polarized light, Ω, can be obtained from the following rules: (a) for any transition involving two states of the same symmetry (on any point group), Ω < 1: (b) exceptions to rule (a) are found in T, Th, Td, O and Oh groups for which A ↔ A transitions give Ω = 0 and E ↔ E gives Ω = 2431: and (c) for any transition involving two states of different symmetry, Ω = 32. The results of these predictions are in excellent agreement with the available experimental data.

The effects of the presence of an alkaline atomic cation in a molecular hydrogen environment

Ab initio calculations were performed for X+(H2)n clusters [X=Li, Na (n=1–7) and K (n=1–3)]. For n=1–6, the equilibrium geometries correspond to spherically symmetrical distributions of H2 units around the X+. The binding energies and the geometric parameters indicate that the seventh H2 unit opens a new shell of ligands for the cluster with X=Li but not for X=Na.

A generalized multistructural wave function

A generalized multistructural (GMS) wave function is presented which combines the advantages of the self‐consistent‐field molecular orbital (SCF‐MO) and valence bond (VB) models, preserving the classical chemical structures but optimizing the orbitals in a self‐consistent way. This wave function is particularly suitable to treat situations where the description of the molecular state requires localized wave functions. It also provides a very convenient way of treating the electron correlation problem, avoiding large configuration interaction (CI) expansions. The final wave functions are much more compact and easier to interpret than the ones obtained by the conventional methods, using orthogonal orbitals. Applications of the GMS wave functions to a variety of systems are presented.

The dehydrogenation and cracking reactions of isobutane over the ZSM-5 zeolite

Abstract The dehydrogenation and cracking reactions of isobutane over zeolite HZMS-5 were studied at the DFT/B3LYP level of calculation. The zeolite was represented by the‘double-ring’ 20T cluster. The activation energies for the reactions were 9–12 kcal/mol lower than those obtained with the linear 5T cluster. In both cases the attack of the acid site proton was directly on a carbon atom of the substrate, and not on the C–H and C–C bonds, evidencing carbonium-ion-type transition states. The results suggest that the reactions should be competitive, although the more hindered acid sites should favor the dehydrogenation over the cracking reaction.

The Rydberg states of trans-butadiene from generalized valence bond and configuration interaction calculations

Self-consistent ab initio generalized valence bond (GVB) and configuration internction (Cl) cnlculutians are presented for the Rydberg states of Irans-13-butadiene. Five Rydberg series were identifie_d, three optically allowed (np.. ,tp. and nf,‘) and two optically forbidden (ns and nd,*). It is shown that, except for the A and F bands (which correspond to rhc non-valence 1 ‘B. and the vntcnce 2 ‘A, states. respectively) all the transitions observed in the ultra-violet (UV) and electron-impact (EI) spectra, and in the two-photon spectra. can be assigned to members of these Rydberg seric:;.

A density-functional study of the dehydrogenation reaction of isobutane over zeolites

Abstract The dehydrogenation reaction of isobutane over zeolites was investigated at the B3LYP/6-31G ∗∗ and 6-311G ∗∗ levels of calculation, and with T3 and T5 clusters representing the zeolite. The transition state (TS) exhibits a carbenium ion-like character, and the activation energy, at the best level of theory, is 53.4 kcal/mol. Contrary to what has been previously proposed, IRC calculations show that the mechanism does not involve the formation of alkoxide, but rather the carbocation collapses directly into isobutene while the eliminated proton, restores the zeolites acid site. Increasing the size of the cluster and of the basis set does not change the mechanism.

Isomeric structures and energies of Hn+ clusters (n=13, 15, and 17)

Ab initio calculations have been performed for the Hn+ clusters (n=3–17; odd) at Moller–Plesset second order (MP2)/6-311G(mp), Moller–Plesset complete fourth order (MP4)/6-311G(mp), and coupled-cluster single-double-triple [CCSD(T)/6-311G(1p)] levels of calculations. Such hydrogen clusters are constituted by an H3+ core in which H2 units are bound. In order to understand the features of these bindings, enthalpy and entropy variations upon cluster formation, binding energies, and charge distributions have been computed, and a molecular orbital analysis, based on localized orbital, was performed. Our results show that the way the first three H2 units bind to the H3+ core is fundamentally different from the others, providing an explanation for the binding energies observed for these molecules. For the H13+, H15+, and H17+ clusters, the way in which the external H2 units are distributed around the H3+ plane leads to the formation of different isomers with very close energies, but with a rotational barrier lar...

Membrane-disruptive properties of the bioinsecticide Jaburetox-2Ec: Implications to the mechanism of the action of insecticidal peptides derived from ureases

Jaburetox-2Ec, a recombinant peptide derived from an urease isoform (JBURE-II), displays high insecticidal activity against important pests such as Spodoptera frugiperda and Dysdercus peruvianus. Although the molecular mechanism of action of ureases-derived peptides remains unclear, previous ab initio data suggest the presence of structural motifs in Jaburetox-2Ec with characteristics similar to those found in a class of pore-forming peptides. Here, we investigated the molecular aspects of the interaction between Jaburetox-2Ec and large unilamellar vesicles. Jaburetox-2Ec displays membrane-disruptive ability on acidic lipid bilayers and this effect is greatly influenced by peptide aggregation. Corroborating with this finding, molecular modeling studies revealed that Jaburetox-2Ec might adopt a well-defined beta-hairpin conformation similar to those found in antimicrobial peptides with membrane disruption properties. In addition, molecular dynamics simulations suggest that the protein is able to anchor at a polar/non-polar interface. In the light of these findings, for the first time it was possible to point out some evidence that the peptide Jaburetox-2Ec interacting with lipid vesicles promotes membrane permeabilization.

A generalized multi-structural wavefunction. The He+2 molecule as an example

Abstract A generalized multi-structural wavefunction is presented which combines the advantages of the SCF-MO and VB models, preserving the classical chemical structures but optimizing the orbitals in a self-consistent way. The He + 2 molecule is used as an illustration of the method. The potential energy surface for the 2 Σ + u state is constructed and the resulting internuclear equilibrium distance and dissociation energy are compared to the results of more elaborated calculations and to the experiments.

Experimental and Theoretical Studies of (CsI)nCs + Cluster Ions Produced by 355 nm Laser Desorption Ionization

Collision cross-sections of gas-phase (CsI)n = (1-7)Cs(+) cluster ions formed by pulsed-UV laser (355 nm) desorption ionization are measured by ion mobility-mass spectrometry. Experimental collision cross-sections are compared with calculated cross sections of candidate structures generated from a search for the lowest energy structures at the DFT/B3LYP/LACV3P** and MP2/LACVP3P** levels. The relative stabilities of these candidate structures are examined by IM-CID-MS, and the experimental results are compared to theoretical predictions. Analysis of (CsI)n = (1-7)Cs(+) cluster ion dissociation energies shows that the lower fragmentation thresholds are observed for cluster ions with the lower predicted stability.