Marcelo Galván

A direct evaluation of regional Fukui functions in molecules

Abstract A simple formalism to obtain regional Fukui functions is presented. The model is based on an exact relationship between this local reactivity descriptor and the frontier molecular orbitals. Within this approach, it becomes possible to define an orbital Fukui function that directly yields the condensed-to-atom quantity. By this procedure, we avoid additional calculations of the anion or cation associated with the molecule, thereby maintaining the spin multiplicity of the system. It is shown that our proposed definition of the Fukui function mainly contains symmetry information about the molecular system. The proposed methodology is tested against several benchmark model reactions that are well documented in the experimental and theoretical literature.

Local softness and chemical reactivity of maleimide: nucleophilic addition

Abstract The local softness and the Fukui function of maleimide are determined through a finite differences scheme in order to show the usefulness of these concepts for rationalizing the inherent chemical reactivity of a given molecule. The local softness surface diagram of maleimide indicates that soft nucleophiles interact with the a carbon atoms, whereas hard nucleophiles interact with the carbonyl carbon atoms, in agreement with the experimental evidence.

Dynamic NMR and X-ray diffraction study of (N-B) -diphenyl(2-aminoethoxy) borane derivatives of ephedrines', and pseudoephedrines

Abstract The preparation and characterization of various (N-B)-diphenyl-(2-aminoethoxy)boranes derived from ephedrine and pseudoephedrine derivatives ( 1b–6b ) are reported: (N-B)-diphenyl(1-( R )-phenyl-2-( S )-methyl-2-aminoethoxy)borane ( 1b ), (N-B)-diphenyl(1-( R )-phenyl-2-( R )-methyl-2-aminoethoxy)borane ( 2b ), (N-B)-diphenyl[ N -( R )-methyl-(1-( R )-phenyl-2-( S )-methyl-2-aminoethoxy)]borane ( 3b - trans ), (N-B)-diphenyl[ N -( S )-methyl-(1-( R )-phenyl-2-( S )-methyl-2-aminoethoxy)]borane ( 3b - cis ), (N-B)-diphenyl[ N -( S )-methyl-(1-( R )-phenyl-2-( R )-methyl-2-aminoethoxy)]borane ( 4b - trans ). (N-B)-diphenyl[ N,N -dimethyl-(1-( R )-phenyl-2-( S )-methyl-2-aminoethoxy)]borane ( 5b ) and (N-B)-diphenyl[ N,N -dimethyl-(1-( R )-phenyl-2-( R )-methyl-2-aminoethoxy)]borane ( 6b ). The five membered N → B cyclic structures 1b–6b were assigned based on 1 H-. 13 C-, 11 B- and 15 N-NMR data and all compounds except for 3b - trans were subjected to X-ray diffraction analysis showing N → B bond lengths of 1.66(2) and 1.64(2)A for 1b , 1.657(9) and 1.664(9))A for 2b , 1.68(2) A for 3b - cis , 1.66(1)A for 4b - trans , 1.744(8)A for 5b and 1.74(1)A for 6b . The study of the intramolecular N → B coordination by means of dynamic NMR spectroscopy afforded Δ G ‡ values of 67.9, 70.9, 64.8, 68.2, 49.7 and 52.7 kJ mol −1 for the dissociation of the N → B bond in compounds 1b–6b respectively. The results show that steric interactions between the substituents at the (2-aminoethoxy)borane ring determine the stability of the N → B bond as well as the nitrogen configuration. Theoretical calculations of the electrostatic charges for the boron and nitrogen atoms in 1b , 2b , 3b - cis , 3b - trans , 4b - cis , 5b and 6b show that the increase of positive charge on the nitrogen atom causes a shift to lower frequencies in the 15 N NMR spectra.

Ab initio study of substituted 2-aminoethylborinates

Abstract A series of 2-aminoethylborinate derivatives has been studied by ab initio calculations at the HF/6–31G ∗∗ level in order to obtain information about the substituent effects around the coordinative N → B bond. Their geometries were fully optimized and the electrostatic charge, the bond order of the Mulliken population analysis and the hybridization of the different atoms were determined. Based on these data it can be shown that the N → B bond strength depends on two factors: the polarization along the N → B bond and the electrostatic repulsive interactions between the substituents around this bond. It was determined that the bond strength can be increased by the introduction of electronegative substituents at the nitrogen atom. Also, it was concluded that the conformations adopted by the five-membered heterocyclic rings depend on the electrostatic interactions around the OBN fragment. As a consequence of these interactions, the boron atom is displaced out of the molecular plane, when the nitrogen atom is substituted by at least one methyl group. Otherwise it is one of the carbon atoms of the ring.

Chemical reactivity in density functional theory: the N-differentiability problem

Abstract Making use of the definitions given by Berkowitz and Parr for hardness and softness quantities, and the properties of the static linear response function, it is shown that it is not necessary to invoke N-differentiability in order to calculate electronegativities, hardnesses and Fukui functions. This approach provides strong support for the description of chemical reactivity based on density functional theory.

Peptide sr11a from Conus spurius is a novel peptide blocker for Kv1 potassium channels.

More than a hundred conotoxins are known today and from them, only seven conopeptides have been identified to target voltage-gated potassium channels (Kv). Conotoxin sr11a belongs to the I(2)-superfamily which is characterized by four disulfide bridges and provokes muscle stiffness when injected intracranially in mice. The aim of this work was to test the biological activity of sr11a on recombinant voltage-gated Kv1 potassium channels expressed in Xenopus laevis oocytes. Peptide sr11a was purified by high-performance liquid chromatography from the venom of the vermivorous Conus spurius. We found that peptide sr11a inhibits the delayed rectifiers Kv1.2 and Kv1.6 but had not effect on the slowly inactivating Kv1.3 channel. The functional dyad composed of a basic Lys and a hydrophobic amino acid residue is a crucial structural element, regarding the binding properties and blocking activities of more than a hundred K(+) channel toxins. Peptide sr11a does not contain Lys residues and then, it lacks the functional dyad. Molecular modeling of peptide sr11a reveals the presence of exposed basic residues of Arg and suggests that Arg17 and Arg29 are important on its biological activity.

A philicity based analysis of adsorption of small molecules in zeolites

Adsorption of small molecules like CH4, CO and NH3 into the acid sites of zeolites is analysed as an interaction between an electrophile and a nucleophile. Global reactivity descriptors like softness and electrophilicity, and local reactivity descriptors like the Fukui function, local softness and local philicity are calculated within density functional as well as Hartree-Fock frameworks using both Mulliken and Hirshfeld population analysis schemes. The HSAB principle and the best electrophile-nucleophile combination suggest that the reaction between the NH3 and Brönsted acid site of the zeolite is the strongest. Interaction between the zeolite and a small probe molecule takes place through the most electrophilic atom of one with the most nucleophilic atom of the other. This result is in conformity with those provided by the frontier orbital theory and the local HSAB principle.

Nature of Hydrogen Bonds and S···S Interactions in the l-Cystine Crystal

The intermolecular interactions that govern the stability of the l-cystine crystal were studied. This task is accomplished by using density-functional theory (DFT) with the generalized-gradient approximation (GGA) and including many-body dispersion (MBD) interactions. The strengths of the different interactions within the molecular crystal were obtained by a decomposition of the total interaction energy in two-, three-, and four-body contributions. It was determined that most of the hydrogen bonds formed within the crystal are strong (13, 15, and 19 kcal/mol) and the van der Waals nature of the S···S interaction is fully confirmed. Also, the presence of strong repulsive three-body contributions is determined. The results obtained support the idea of designing crystal growth inhibitors for this system in such a way that, when inserted in the crystal, they maintain the disulfide bridge environment but its capacity of generate hydrogen-bond networks is removed.

Basis set effects on frontier molecular orbital energies and energy gaps: a comparative study between plane waves and localized basis functions in molecular systems.

In order to study the Kohn-Sham frontier molecular orbital energies in the complete basis limit, a comparative study between localized functions and plane waves, obtained with the local density approximation exchange-correlation functional is made. The analyzed systems are ethylene and butadiene, since they are theoretical and experimentally well characterized. The localized basis sets used are those developed by Dunning. For the plane-waves method, the pseudopotential approximation is employed. The results obtained by the localized basis sets suggest that it is possible to get an estimation of the orbital energies in the limit of the complete basis set, when the basis set size is large. It is shown that the frontier molecular orbital energies and the energy gaps obtained with plane waves are similar to those obtained with a large localized basis set, when the size of the supercell and the plane-wave expansion have been appropriately calibrated.

Wave function instabilities in the cis-trans isomerization and singlet-triplet energy gaps in a push-pull compound

The energy barriers for the cis–trans isomerization are obtained for a sample of push–pull compounds. The appropriate trend in the values for the barriers is obtained after an instability treatment of the restricted Kohn–Sham solution. At the instability zone, some compounds exhibit a singlet open-shell configuration. The width of the instability region around the transition state is characterized in an asymmetric push–pull compound. These results show that the instabilities in the twisted configuration of double bonds are mainly related to spin symmetry effects rather than to the molecular symmetry. For the open-shell singlet, an ensemble model is used to compute the electronic properties. The singlet–triplet energy gap along the rotation coordinate is qualitatively described by the use of spin potential and hardness.