Papers by Marco Antonio C Nascimento
HAL (Le Centre pour la Communication Scientifique Directe), 2007
The Hartree-Fock equations describe atomic and molecular eletronic wave functions, based on the m... more The Hartree-Fock equations describe atomic and molecular eletronic wave functions, based on the minimization of a functional of the energy. This can be formulated as a constrained global optimization problem involving nonconvex polynomials exhibiting many local minima. The traditional method of solving the Hartree-Fock problem does not provide a guarantee of global optimality and is very sensitive to the initial starting point. In this paper we show how to use a deterministic global optimization method to solve Hartree-Fock systems. The validity of the proposed approach was established by successfully computing the ground-state of the He and Be atoms.
EPL, Feb 23, 2007
The Hartree-Fock equations describe atomic and molecular eletronic wave functions, based on the m... more The Hartree-Fock equations describe atomic and molecular eletronic wave functions, based on the minimization of a functional of the energy. This can be formulated as a constrained global optimization problem involving nonconvex polynomials exhibiting many local minima. The traditional method of solving the Hartree-Fock problem does not provide a guarantee of global optimality and is very sensitive to the initial starting point. In this paper we show how to use a deterministic global optimization method to solve Hartree-Fock systems. The validity of the proposed approach was established by successfully computing the ground-state of the He and Be atoms.
Journal of Chemical Physics, Nov 12, 2019
Users may download and print one copy of any publication from the public portal for the purpose... more Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.
Journal of Applied Polymer Science, Feb 15, 2022
Biobased flame-retardant polylactic acid composites were prepared using ammonium polyphosphate (A... more Biobased flame-retardant polylactic acid composites were prepared using ammonium polyphosphate (AP), lignin, and a raw montmorillonite (ANa) as the intumescent formulation. The concentration of AP and of ANa was varied in order to study its influence on the flammability properties of the composites. The samples were submitted to cone calorimeter test, thermogravimetric analysis coupled to Fourier-transform infrared spectroscopy (TGA-FTIR), limiting oxygen index (LOI), and UL-94 vertical burn. The cone calorimeter residues were analysed through scanning electronic microscopy, X-ray diffraction, and FTIR. The results show that the combined addition of the intumescent formulation and the ANa leads to an improvement in the fire behavior of the composites, compared with that of the neat polymer. The best fire-retardant performance was achieved by using the highest AP concentration (17%) and the lowest ANa concentration (1.2%), reaching a LOI value of 39%.
Polymers, Apr 21, 2022
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
Polymers, Nov 25, 2020
A raw montmorillonite (Mt) was submitted to different acidic activation times in order to investi... more A raw montmorillonite (Mt) was submitted to different acidic activation times in order to investigate the influence of the strength and the nature (Brønsted and Lewis) of acidic sites on the synergistic action with an intumescent formulation (IF) composed of ammonium polyphosphate (APP) and pentaerythritol (PER) when incorporated into a polypropylene (PP) matrix. The acidity of the Mt samples was quantified by ammonia temperature-programmed desorption (TPD-NH3) and Fourier transform infrared spectroscopy (FTIR) with pyridine adsorption. The mineral clays were also characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), nitrogen adsorption analysis and particle size distribution. Thermogravimetric analysis (TGA), limit oxygen index (LOI) and UL-94 were performed to evaluate the flame-retardant properties and the thermal stability. The TGA results show that the final residue increased 2 to 3 fold in comparison to the values predicted theoretically. The flammability properties achieved a maximum for the system containing an excess of moderate-strength Brønsted sites relative to the Lewis ones, reaching 38% in the LOI test. This result suggests that the presence of these Brønsted acidic sites is important, as they take part in the esterification reaction between APP and PER which gives rise to the char formation. The FTIR-Pyr adsorption and flammability results indicate that both the nature and strength of the acidic sites influence the flame-retardant properties.
The Quantum Interference Energy Analysis: A Tutorial Introduction
Elsevier eBooks, 2022
Three-centre two-electron bonds from the quantum interference perspective
Physical Chemistry Chemical Physics
The stability of chemical systems with 3c2e bonds derives from quantum interference, while other ... more The stability of chemical systems with 3c2e bonds derives from quantum interference, while other factors are always overall destabilizing. The interference energy of a 3c2e bond is about 3 times higher than that of a comparable 2c2e bond.
Molecular Photoionization and Photodetachment Cross Sections Based on L$$^2$$ Basis Sets: Theory and Selected Examples
Advances in Methods and Applications of Quantum Systems in Chemistry, Physics, and Biology, 2021
A detailed description of a method proposed to compute total photoionization and photodetachment ... more A detailed description of a method proposed to compute total photoionization and photodetachment cross sections of atoms and molecules relying just on L\(^2\) basis sets is presented. The method consists of an analytical continuation procedure based on Pade approximants. It has the advantage of treating the discrete and continuum part of the spectra at the same level with a flexible choice of the correlated electronic structure approximations. Herein, we present results obtained at the coupled cluster singles and doubles (CCSD) and time-dependent density functional theory (TDDFT) level. Examples of L\(^2\) basis sets adequate and inadequate to reproduce the continuum region of the spectrum are given for the CCSD photoionization cross section of He. Also, a brief review of recent applications of the method is presented, together with original TDDFT results for the total photoionization cross section of formic acid, HCOOH, and the photodetachment cross section of the formate anion, HCOO\(^-\).
One-electron bonds are not “half-bonds”
Physical Chemistry Chemical Physics, 2019
The formation of one-electron bonds follows the same mechanism as the regular covalent bonds. It ... more The formation of one-electron bonds follows the same mechanism as the regular covalent bonds. It is the same phenomenon, quantum interference, that rules the stabilization of systems containing such bonds and there is no distinction between one- and two-electron bonds from the conceptual point of view. The difference in the BDEs of the one- and two electron bonds has no relation with bond order. One-electron bonds are not “half-bonds”.
Computational and Theoretical Chemistry, 2017
The nature of the chemical bond in the pentagonal-pyramidal benzene dication and related species ... more The nature of the chemical bond in the pentagonal-pyramidal benzene dication and related species was described by Modern Valence Bond calculations and the Generalized Product Function Energy Partitioning method. The results suggest that the π space of C 6 H 6 2+ is composed of a donor-acceptor bond from a cyclopentadienyl anion moiety, described as a 5c-6e π bonding, to a triply charged Lewis acid CH structure. Similar results were found for isoelectronic species, such as C 5 H 5 BH + , C 5 H 5 BeH, and C 5 H 5 LiH-, leading to different main-group hexa-coordinate atom-bearing structures. A pictorial mechanism for the bonding of the studied molecules was constructed, and different geometrical and electronic features could be explained by this straightforward model. Finally, the results suggest that several poly-coordinate atom-containing molecules, sandwich-type compounds, oligomers, and polymers could be formed by the interaction of a π system and Lewis acid moieties.
Transesterification reactions of dimethoxycarbonylethyltin- morpholinodithiocarbamate, [(MeO 2 CCH 2 CH 2 ) 2 Sn(MDTC) 2 ]: Synthesis, spectroscopy, X-ray structural characterization and DFT calculations of new diestertin dithiocarbamate complexes – Part II
Polyhedron, 2017
Abstract The scarcity of studies concerning diestertin(IV) dithiocarbamate complexes led us to in... more Abstract The scarcity of studies concerning diestertin(IV) dithiocarbamate complexes led us to investigate the spectroscopic and structural properties of [(RO2CCH2CH2)2Sn(MDTC)2] (R = Me (1) or Et (2); MDTC = morpholinodithiocarbamate). The reaction of [(MeO2CCH2CH2)2SnCl2] with sodium morpholinodithiocarbamate (NaMDTC) in CHCl3 afforded (1), while the transesterification reaction of (1) with EtOH in the presence of dmso yielded (2). Both diestertin(IV) complexes were characterized by elemental analysis, FTIR and multinuclear (1H, 13C and 119Sn) NMR spectroscopy, and single-crystal X-ray diffraction. Our X-ray structural analysis revealed that (1) and (2) exhibit a distorted pentagonal bipyramidal coordination geometry. In both cases, the apical positions are occupied by ester groups while the equatorial plane displays two bidentate morpholinodithiocarbamate ligands and one intramolecular C O⋯Sn interaction. The O⋯Sn distances amount to 2.632 (1) and 2.618 A (2). According to our supramolecular analysis, the crystal arrangement of each product is assembled by weak C–H⋯O and C–H⋯S hydrogen bonds. Finally, a DFT study of the six- and seven-coordinate forms of (1) and (2) allowed an upper-bound estimate of the intramolecular C O⋯Sn interaction energy, −3.7 kcal mol−1.
Applied Catalysis A: General, 2016
ONIOM calculations have been carried out to determine geometries, adsorption energies and vibrati... more ONIOM calculations have been carried out to determine geometries, adsorption energies and vibrational frequencies of CO on Au 2 and Au 4-exchanged mordenite catalysts (Au 2 /MOR and Au 4 /MOR) using DFT with the B3LYP and ωB97X-D functionals. We considered structures with two Al atoms, MOR(TT), with at least one of them at the preferential site T4. To investigate the CO adsorption on Au 2 /MOR(TT) we examined three different situations: the adsorption of one single CO molecule bridged to two Au atoms (case I), one CO molecule adsorbed on a single Au atom (case II), and two molecules of CO absorbed on two Au atoms, one CO per Au atom (case III). From all the previous theoretically and experimentally proposed species of Au inside the MOR channels, including the ones considered in this work, only the 2-CO-Au 2 /MOR configuration, with one CO adsorbed in a bridge form on two Au atoms, exhibits (CO) frequencies within the experimental range of 1949-2120 cm −1 (low frequencies). Increasing the size of the Au particles shifts the (CO) to higher frequencies. The values of H ads decrease when the number of adsorbed CO molecules increases, similarly to what is experimentally observed when CO is adsorbed on Au/TiO 2 and Au/Al 2 O 3. Charge analysis shows that the Au 2 in the Au 2 /MOR and the Au 4 cluster in the Au 4 /MOR are cationic. Based on the present results we conclude that the use of (CO) as a probe to determine the Au species in the channels of MOR can only clearly identify Au 2 ␦+ species inasmuch as the 2-CO-Au 2 /MOR configuration is the only one among several others to exhibit (CO) frequencies within the experimental low frequency region.
The Journal of Physical Chemistry C, 2015
The adsorption of NH 3 and alkylamines (MeNH 2 , Me 2 NH, and Me 3 N) on H-mordenite was studied ... more The adsorption of NH 3 and alkylamines (MeNH 2 , Me 2 NH, and Me 3 N) on H-mordenite was studied using density functional theory (DFT) and the ONIOM method. The adsorption of the probe molecules was investigated with a set of functionals (B3LYP, B3LYP-D3, CAM-B3LYP, LC-ωPBE, and ωB97X-D) properly chosen to verify the importance of noncovalent interaction on the adsorption process. The adsorption energy increases with the size of the amine and with the type of functional used, according to the trend B3LYP-D3, ωB97X-D > CAM-B3LYP, LC-ωPBE > B3LYP. The noncovalent contribution to the adsorption energy is higher for Me 3 N, reaching 92 kJ/mol when B3LYP-D3 is used and 70 kJ/mol for ωB97X-D. Although the adsorption energies for each amine span a wide range of values depending on the functional used, the geometries of equilibrium are quite similar. The adsorption energies obtained by full geometry optimization of the complexes do not differ appreciably from those of single-point calculations using the geometries optimized at the B3LYP:UFF level of calculation, suggesting that for these systems the single-point strategy may be a reasonable alternative to reduce the computational cost of including noncovalent corrections in DFT results. A comparison was also made with the experimental differential heats of adsorption. At low coverage the computed E ads at the T4 site corrected for the basis set superposition (BSSE) and zero-point vibration energy errors exhibit the same trend of the experimental ΔH ads with the degree of methylation observed by Lee et al. (J. Am. Chem. Soc. 1996, 118, 3262−3268) as opposed to the one by Chen et al. (J. Catal. 1994, 146, 257−267). Also, the results obtained at the B3LYP-D3:UFF level of calculation including BSSE corrections strongly suggest that the falloff observed on the ΔH ads x coverage diagram, normally attributed to site heterogeneity, may be due to a combination of two factors: the number of molecules adsorbed on a given site and the acid site local configuration.
Searching for quarks: a quantum chemical assisted approach
Molecular Physics, 2002
ABSTRACT The evidence for quarks as the basic blocks of matter come only from very expensive expe... more ABSTRACT The evidence for quarks as the basic blocks of matter come only from very expensive experiments, performed in very large particle accelerators. An alternative method of detection is to look for changes in spectroscopic properties that could be influenced by fractionally charged nuclei formed with the &#39;wrong&#39; number of quarks. Although this proposal has already been made by many authors, they were not able to provide feasible and easily interpretable experiments to detect quarks in matter. In this paper, we present the results of configuration interaction (CI) calculations for the transition energies and optical oscillator strengths associated with inner shell electronic spectra of molecular nitrogen made of fractionally charged nuclei. It is shown that these bands can be used as fingerprints characterizing the presence of quarks in matter.
Molecular properties calculations using the q‐integral method
International Journal of Quantum Chemistry, 2008
The q‐Integral method, based on the q‐Exponential function, has been recently introduced as an al... more The q‐Integral method, based on the q‐Exponential function, has been recently introduced as an alternative approach to calculate the two‐electron integrals which appear in ab initio atomic and molecular quantum mechanical calculations. The advantage of this procedure is that the CPU time for calculating of two‐electron integrals is substantially reduced when compared with the usual one. The objective of this work is to investigate the accuracy of the q‐Integrals as a function of the internuclear distance for molecular systems. To this purpose we employed the q‐Integrals to construct the potential energy surface (PES) for the H2 molecule and used the PES to determine the rovibrational levels and spectroscopic constants of the molecule, which are properties very sensitive to the form of the PES. The results obtained are in good agreement with the ones obtained through the standard procedure of calculating the two‐electron integrals, implying that the q‐Integral method is accurate enou...
Molecules, Jul 27, 2021
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
Reactivity properties of the HOSO and HSO2 isomers in liquid medium: a sequential Monte Carlo/quantum mechanics study
Journal of Molecular Modeling
Catalysis Research
We synthesized and spectroscopically characterized a new heterotrimetallic {Ti2Ni} ethylene (co)p... more We synthesized and spectroscopically characterized a new heterotrimetallic {Ti2Ni} ethylene (co)polymerization precatalyst containing one (α-diimine)NiBr2 and two (phenoxy-imine)TiCl4 scaffolds. Its calculated structure was investigated at the DFT B3LYP/LACVP** level. Our calculations showed that the titanium(IV) centers were in a slightly distorted trigonal bipyramidal environment, and the average Ti···Ni distance was 8.76 Å. The precatalyst was used for synthesizing polyethylene and ethylene copolymers. The results of GPC analyses showed that the obtained polyethylenes had the desired bimodal molecular weight distributions. The FTIR spectra revealed that polydispersity decreased as the vinyl end-group content increased. These results suggested that high mechanical resistance can increase the mechanical energy needed for processing the material. All 13C NMR signals were assigned to short-chain branches with specific spatial arrangements along the polymer backbone. The chain walking...
CCDC 601016: Experimental Crystal Structure Determination
An entry from the Cambridge Structural Database, the world's repository for small molecule cr... more An entry from the Cambridge Structural Database, the world's repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
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Papers by Marco Antonio C Nascimento