Rogério Custodio

Periodic study on the structural and electronic properties of bulk, oxidized and reduced SnO2(1 1 0) surfaces and the interaction with O2

The structural and electronic properties of bulk and both oxidized and reduced SnO2(1 1 0) surfaces as well as the adsorption process of O2 on the reduced surface have been investigated by periodic DFT calculations at B3LYP level. The lattice parameters, charge distribution, density of states and band structure are reported for the bulk and surfaces. Surface relaxation effects have been explicitly taken into account by optimizing slab models of nine and seven atomic layers representing the oxidized and reduced surfaces, respectively. The conductivity behavior of the reduced SnO2(1 1 0) surface is explained by a distribution of the electrons in the electronic states in the band gap induced by oxygen vacancies. Three types of adsorption approaches of O2 on the four-fold tin at the reduced SnO2(1 1 0) surface have been considered. The most exothermic channel corresponds to the adsorption of O2 parallel to the surface and to the four-fold tin row, and it is believed to be associated with the formation of a peroxo O 2� 2 species. The chemisorption of O2 on reduced SnO2(1 1 0) surface causes a significant depopulation of states along the band gap and it is shown to trap the electrons in the chemisorbed complex producing an electron-depleted space–charge layer in the inner surface region of the material in agreement with some experimental evidences. 2002 Elsevier Science B.V. All rights reserved.

Electronic and structural properties of SnxTi1−xO2 solid solutions: a periodic DFT study

The structural and electronic properties of selected compositions of Sn xTi1−xO2 solid solutions (x = 0, 1/24, 1/16, 1/12, 1/8, 1/6, 1/4, 1/2, 3/4, 5/6, 7/8, 11/12, 15/16, 23/24 and 1) were investigated by means of periodic density functional theory (DFT) calculations at B3LYP level. The calculations show that the corresponding lattice parameters vary non-linearly with composition, supporting positive deviations from Vegard’s law in the SnxTi1−xO2 system. Our results also account for the fact that chemical decomposition in SnxTi1−xO2 system is dominated by composition fluctuations along [0 0 1] direction. A nearly continuous evolution of the direct band gap and the Fermi level with the growing value of x is predicted. Ti 3d states dominate the lower portion of the conduction band of SnxTi1−xO2 solid solutions. Sn substitution for Ti in TiO 2 increases the oxidation–reduction potential of the oxide as well as it renders the lowest energy transition to be indirect. These two effects can be the key factors controlling the rate for the photogenerated electron–hole recombination. These theoretical results are capable to explain the enhancement of photoactivity in SnxTi1−xO2 solid solutions.

Regioselectivity of the nitration of phenol by acetyl nitrate adsorbed on silica gel

Abstract The reaction of phenol with acetyl nitrate in chloroform gives nitrophenol with an ortho/para ratio of 1.8. This ratio increase to 13.3 when the reaction was carried out with acetyl nitrate pre-adsorbed on dry silica gel. Silica may be acting as a template to bring phenol close to acetyl nitrate by hydrogen bonds forming a ternary complex, which undergoes a six-center rearrangement to o-nitrophenol. The formation of this ternary complex is evaluated by ab initio molecular orbital calculation.

Theoretical studies of Nile Red by ab initio and semiempirical methods

Abstract Ab initio and semiempirical calculations were carried out for the Nile Red (NR) molecule to study the possible occurrence of the twisted intramolecular charge transfer process. The results showed that NR is planar in the ground state (using the CEP-31g basis set) with a high barrier to rotation of the diethylamine group by 90° (0.334 and 0.381 eV with AM1 and CEP-31g, respectively). CIS calculations showed that the charge transfer decreases after the twisting, in contrast to the TICT prediction. The solvatochromic effect was justified through the dipole moments calculated for the first excited state.

Implementation of pseudopotential in the G3 theory for molecules containing first-, second-, and non-transition third-row atoms.

Compact effective pseudopotential (CEP) is adapted in the G3 theory providing a theoretical alternative referred to as G3CEP for calculations involving the first-, second-, and non-transition third-row elements. These modifications tried to preserve as much as possible the original characteristics of G3. G3CEP was used in the study of 247 enthalpies of formation, 22 atomization energies, 104 ionization potentials, 63 electron affinities, and 10 proton affinities, resulting in the calculation of 446 species for the first-, second-, and third-row atoms. The final average total absolute deviation was of 1.29 kcal mol(-1) against 1.16 kcal mol(-1) from all-electron G3 for the same calculations. The CPU time has been reduced by 7% to 56%, depending on the size of the molecules and the type of atoms considered.

A universal basis set to be used along with pseudopotentials

Abstract Universal basis sets from all-electron calculations can be adapted to be used along with pseudopotentials by the generator coordinate method (GCM). Calculations carriedout with Hay and Wadt or Steven, Bash and Krauss pseudopontentials along with a universal basis set are in agreement with all-electron calculations and also with improved basis sets specially developed to be used with pseudopotential. The support provided by GCM allows a simple procedure to enlarge or reduce the size of the universal basis set as well as a systematic procedure to include polarization functions.

A theoretical study of isomeric C6H4Br− ions

Abstract The structural stability of bromide o -benzyne complexes was studied. A new procedure was applied to adapt a basis set to be used with the Hay and Wadt pseudopotential for the Br atom. Calculations at the MP2, MP3 and MP4 levels established two stable structural isomers for the C 6 H 4 Br − ion: (a) a bromphenide anion and (b) an isomer with a loose hydrogen-bonded bromide-benzyne complex where the Br − ion is straddling the ortho and meta hydrogen of the ring. At the MP4 level the barrier between the two isomers is below the typical energies associated with the loose complexes formed as intermediates in ion/molecule reactions.

Theoretical investigation of the conformational behaviour of 3-monosubstituted 2-methylpropenes

Abstract The conformational behaviour of the some methallyl compounds (H2CC(CH3)CH2X; XF, Cl, Br, I, OH, OMe, OEt, SH, SMe, SEt, NMe2 and NEt2) has been investigated through MP2 and B3LYP levels of theory using the 6-31G(d,p) and/or 6-311+G(d,p) basis sets. For certain set of compounds the 3-21G and CEP-31G basis set were used. The calculations indicate the existence of s-cis and gauche conformers, considering the CC–C–X dihedral angle, and the skew and trans forms with respect to the C–C–X–R dihedral angle. Good agreement is obtained among the relative energies calculated at different levels of theory, except for the oxygen derivatives, and with respect to experimental data from the literature for some compounds. Similar conformational behavior is observed among the analogous methallyl, allyl and propanones derivatives.

The use of the generator coordinate method for designing basis set. Application to oxo-diperoxo molybdenum complexes

Abstract The molecular and electronic structures of MoO(O 2 ) 2 ( 1 ), MoO(O 2 ) 2 (OPy) ( 2 ) and MoO(O 2 ) 2 (OPy)(H 2 O) ( 3 ) complexes were investigated at the Hartree–Fock and density functional method (B3LYP) calculation levels. The generator coordinate method (GCM) has been used to design basis sets that properly represent the electronic density on the Mo and O atoms for all electron calculations, while a variant of the GCM method has been employed to design a valence basis set for the Mo atom for the pseudopotential calculations. Compound 1 adopts a distorted tetragonal pyramid structure, where the four peroxo oxygen atoms are located in the same plane, which is perpendicular to the axis defined by the Mo and oxo oxygen atoms. An analysis based upon the geometrical and electronic parameters and the vibrational frequencies renders that 1 can be described as two peroxide fragments bonded to the MoO moiety. 2 and 3 complexes are bipyramidal pentagonal structures, with the OPy ligand occupying a quasi-equatorial position in the same plane as the two peroxo triangles while the H 2 O ligand is situated trans to the oxo group in 3 . A comparison between theoretical and experimental results for the geometry and vibrational frequencies of 3 complex shows good agreement. The relationship between the reactivity of 1 , 2 and 3 complexes and their coordination number has been established by analyzing the values of the vibrational frequencies, frontier molecular orbitals and the values of electron affinities.

Polycarbonitrile : A semiempirical, ab initio and density functional study of molecular stability

Abstract The theoretical literature data with respect to the electronic properties of this compound is quite scarce and makes use of the planar all-trans structure as the most stable for the calculations. In this work semiempirical (AM1 and PM3), ab initio (at the Hartree-Fock level) and density functional theory (using the correlation functional of Vosko, Wilk and Nussair) were used to analyse the conformational stability of the all-trans and all-cis dimers, trimers and tetramers of polycarbonitrile. The semiempirical and ab initio calculations at the Hartree-Fock level showed in general that the all-trans structure with respect to other conformers is the most unstable structure. The inclusion of electronic correlation energy through the MP2 calculations or the VWN functional method suggest that the trans structure is the most stable. The relative energies calculated at the correlated level presented differences around 2 kcal/mol among the different conformers. While the all-cis compounds presented a planar structure for any of the three methods, the all-trans polymer showed a strong deviation of planarity with a set of local minima in its energy surface. These results suggest that further calculations on the electronic properties of this polymer can be significantly different of those actually available in the literature.