A.B.F. da Silva

Electrochemical Behavior of Nicotine Studied by Voltammetric Techniques at Boron‐Doped Diamond Electrodes

Abstract The electrochemical behavior of nicotine in alkaline media was studied using a boron doped diamond (BDD) surface as the working electrode. In order to establish the pH dependence and to gain information about the mass transport of the species, cyclic voltammetry studies were carried out in a 0.1 mol L−1 BR (Britton‐Robinson) buffer in the presence of 1.0×10−3 mol L−1 nicotine. The optimum pH value was 8 and the mass transport was controlled by diffusion of the species. The square wave voltammetry technique was used to determine the electroanalytical parameters such as frequency, amplitude, and scan increment. After optimization, an analytical curve was constructed. The limits of detection and quantification were 0.50 and 1.66 mg L−1, respectively. Theoretical calculations indicate that the probable oxidation site on the nicotine molecule was the nitrogen atom denoted “11 N” and a speculation about the reaction mechanism was proposed. Finally, an experiment using a real sample (cigarette tobacco) was carried out and a recovery study revealed a value of about 4.3 mg L−1 in 0.1 g of tobacco.

Accurate universal Gaussian basis set for hydrogen through lanthanum generated with the generator coordinate Hartree-Fock method

Abstract The generator coordinate Hartree-Fock method is applied to generate an accurate universal Gaussian basis set for the atoms H ( Z = 1) through La ( Z = 57). The Hartree-Fock energy results obtained with our universal Gaussian basis set are compared with numerical Hartree-Fock results and with Hartree-Fock energy results obtained by using optimized Gaussian basis sets. For the atoms B through Ca we have obtained, when compared to the corresponding numerical Hartree-Fock results, an energy value within the accuracy of 10 −5 –10 −4 E h , and for Sc through La this accuracy lies between 10 −4 and 10 −3 E h . The universal Gaussian basis set presented in this work is generated taking into account the sharing of exponential functions between all s, p and d atomic orbitals, i.e. the shell constraint.

A generator coordinate version of the closed‐shell Dirac–Fock equations

A generator coordinate version of the Dirac–Fock equations for relativistic closed‐shell atoms is presented. The integration of the Dirac–Fock equations is performed through the integral discretization technique so as to preserve the continuous character of the generator coordinate formalism. With the new formalism we generate a universal Gaussian basis set for relativistic closed‐shell atoms with d and f orbitals (zinc up to nobelium). The results obtained with the universal Gaussian basis set for Dirac–Fock–Coulomb self‐consistent‐field energies are compared with numerical‐finite‐difference results and Dirac–Fock–Coulomb energies obtained by using other Gaussian basis sets. The excellent performance of our universal Gaussian basis set is attributed to the integral discretization technique of the generator coordinate Dirac–Fock method, as with it we are capable of generating Gaussian‐type function exponents that are able to represent properly the relativistic kinematics of an electron inside the nucleus.

On the inclusion of the Breit interaction term in the closed‐shell generator coordinate Dirac–Fock formalism

The generator coordinate Dirac–Fock formalism with the inclusion of the frequency‐independent Breit interaction term is presented. The integration of the Dirac–Fock equations is performed through an integral discretization technique in order to preserve the continuous character of the original generator coordinate formalism. The inclusion of the Breit interaction term in the generator coordinate Dirac–Fock formalism is assessed through Dirac–Fock–Breit calculations for the relativistic closed‐shell atoms with d and f orbitals, i.e., from zinc (Z=30) up to nobelium (Z=102), by using a universal Gaussian basis set generated previously by employing the generator coordinate Dirac–Fock method. The Dirac–Fock–Breit energy results presented in this work are the best, so far, obtained for the relativistic closed‐shell atoms from Zn (Z=30) through No (Z=102) by using Gaussian‐type functions, and show the efficacy of the generator coordinate Dirac–Fock method in performing accurate Dirac–Fock–Breit calculations.

A structure–activity relationship study of HEPT-analog compounds with anti-HIV activity

The molecular orbital method PM3 is employed to calculate a set of molecular descriptors (variables) for 36 deoxy analogs of 1-[(2-hydroxyethoxy)methyl]-6-(phenylthio)thymine (HEPT) with anti-HIV-1 activity. Pattern recognition methods, principal component analysis (PCA) and stepwise discriminant analysis (SDA) were employed in order to reduce dimensionality and investigate which subset of variables should be more effective for classifying the HEPT-analog compounds according to their degree of anti-HIV-1 activity. The PCA showed that the variables log P (partition coefficient), MR (molecular refractivity),DHf (heat of formation), QN (net atomic charge on atoms 2 and 3), and x (Mulliken’s electronegativity) are responsible for the separation between compounds with higher and lower anti-HIV-1 activity. By using the SDA we have found the following descriptors as responsible for the separation between the active and less active compounds: log P (partition coefficient), x (Mulliken’s electronegativity), m (dipole moment), Q4 (net atomic charge on atom 4), and t2 (torsional angle). From the SDA we present a prediction rule for classifying new HEPT-analog compounds with anti-HIV-1 activity. q 2000 Elsevier Science B.V. All rights reserved.

The generator coordinate Hartree-Fock method for molecular systems. Formalism and first applications to H2, LiH and Li2

Abstract The generator coordinate Hartree-Fock method is extended to molecular systems. The resulting equations are integrated numerically through discretization. The method is then implemented with the use of the GAUSSIAN 86 program and applied to the H 2 , LiH and Li 2 diatomic species. The ground state Hartree-Fock energies (and dipole moment in the case of LiH) are calculated and compared with numerical Hartree-Fock values. The role of the molecular weight functions is discussed.

Generator coordinate Hartree–Fock method applied to the choice of a contracted Gaussian basis for the second-row atoms

The generator coordinate Hartree–Fock (GCHF) method is employed as a criterion for the selection of a 18s12p Gaussian basis for the atoms Na–Ar. The role of the weight functions in the assessment of the numerical integration range of the GCHF equations is shown. The extended basis is then contracted to (10s6p) by a standard procedure and in combination with the previously contracted (7s5p) Gaussian basis for the atoms Li–Ne is enriched with polarization functions. This basis is tested for AlF, SiO, PN, BCl, and P2. The properties of interest were HF total energies, MP2 dipolar moments, bond distances, and dissociation energies.

A multiple linear regression and partial least squares study of flavonoid compounds with anti-HIV activity

Abstract The molecular orbital semi-empirical method PM3 was employed to calculate a set of molecular properties (variables or descriptors) of 21 flavonoid compounds with anti-HIV activity. The correlation between biological activity and structural properties was obtained by using the multiple linear regression and partial least squares methods. The model obtained showed not only statistical significance but also predictive ability. The significant molecular descriptors related to the compounds with anti-HIV activity were: electronegativity (χ) and the charges on atoms C3 and C7 (Q3 and Q7, respectively). These variables led to a physical explanation of electronic molecular property contributions to HIV inhibitory potency.

A UNIVERSAL GAUSSIAN BASIS SET FOR ATOMS CERIUM THROUGH LAWRENCIUM GENERATED WITH THE GENERATOR COORDINATE HARTREE-FOCK METHOD

The generator coordinate Hartree–Fock method is applied to generate a universal Gaussian basis set for the heavy atoms from Ce (Z=58) through Lr (Z=103). The Hartree–Fock energies obtained with our universal Gaussian basis set are compared with the new numerical Hartree–Fock results of Koga et al., when available, and with geometrical Gaussian basis sets results available in the literature. The universal Gaussian basis set presented here is generated taking into account the shell constraint (the sharing of exponential functions between all s, p, d, and f atomic orbitals), and can be used as starting basis set in ab initio relativistic Hartree–Fock–Roothaan calculations. © 1997 John Wiley & Sons, Inc. J Comput Chem 18: 1565–1569, 1997

A structure–activity relationship (SAR) study of synthetic neolignans and related compounds with biological activity against Escherichia coli

Abstract Structure–activity relationship techniques were employed to classify neolignan compounds and related analogues against Escherichia coli . The AM1 (Austin Model) method was used to calculate a set of molecular descriptors (properties) for 16 synthetic neolignan compounds. The descriptors were further analyzed using the principal component analysis (PCA), hierarchical cluster analysis (HCA) and K-nearest neighbor (KNN) chemometric methods. The PCA and HCA methods showed quite efficient to classify the 16 compounds in two groups (active and inactive) and three descriptors were found to be important in the classification. By using the chemometric results, 14 new neolignan molecules were analyzed through the PCA, HCA and KNN methods and 11 of them are proposed as molecules potentially active against E. coli .