π-Ring currents in doped coronenes with nitrogen and boron: diatropic-paratropic duality.

The change in the electronic structure of coronene upon doping with nitrogen or boron has been theoretically studied by means of its magnetic properties and magnetic field induced current density maps. The addition of two atoms of nitrogen or boron to the central ring of coronene causes a drastic variation in the delocalization of π-electrons, which does not depend on its nature but instead on its position. Then, doping in the para position makes coronene more aromatic while doping in the meta position makes it to become antiaromatic. The magnetic behavior of the pristine molecule is characterized by two concentric currents flowing in opposite senses that are converted into hemi-perimetric currents in the ortho and meta isomers, so dividing the molecule into aromatic and antiaromatic regions. The paratropic and diatropic ring currents of the coronene moiety may, therefore, be modulated through the position of the heteroatom and, consequently, also the localized/delocalized behavior.

CCSD-CTOCD static dipole shielding polarizability for quantification of the chiral NMR effects in oxaziridine derivatives.

Chiral discrimination by nuclear magnetic resonance (NMR) spectroscopy might be achieved through the pseudo-scalar derived from the dipole shielding polarizability tensor. Coupled Cluster Singles and Doubles-Quadratic Response (CCSD-QR) calculations inside the continuous translation of the origin of the current density formalism have been carried out to determine the effects of basis set, electron correlation, and gauge translation on the determination of this magnitude in oxaziridine derivatives. Inclusion of electronic correlation is needed for adequately describing the pseudo-scalar for the heavier nuclei, making CCSD a rigorous and affordable method to compute these high order properties in medium-sized molecules. The observable magnitudes for chiral discrimination (produced RF voltage and required electric field) are calculated. Half of the considered molecules show values of the observable magnitudes near the lower limit for experimental detection. Nuclei (19)F, (31)P, and (79)Br produce the largest values of RF voltage (50-80 nV). Moreover, (31)P and (79)Br are the nuclei requiring smallest electric fields (3 MVm(-1)) to separate the NMR signals, being then suitable for both the techniques.

Energy interactions in amyloid-like fibrils from NNQQNY.

We use large-scale MP2 calculations to analyze the interactions appearing in amyloid fibers, which are difficult to determine experimentally. To this end, dimers and trimers of the hexapeptide NNQQNY from the yeast prion-like protein Sup35 were considered as model systems. We studied the energy interactions present in the three levels of organization in which the formation of amyloid fibrils is structured. The structural changes in the hydrogen bonds were studied too. It was found that the most energetic process is the formation of the ß-sheet, which is equally due to both hydrogen bonds and van der Waals interactions. The aromatic rings help stabilize these aggregates through stacking of the aromatic rings of tyrosine, the stability produced by the aromatics residues increasing with their aromaticity. The formation of the basic unit of the assembled proto-fiber, the steric zipper, is less energetic and is associated to both dispersion forces and hydrogen bonds. The interactions between pair of ß-sheets across the peptide-to-peptide contact through the tyrosine rings are cooperative and due to dispersion effects. Moreover, the strength of this interaction can rationalize the variation of mobility of the aromatic ring in the tyrosine units found in solid NMR experiments.

Parity-violation energy of biomolecules-IV: protein secondary structure.

The parity-violation energy difference between enantiomeric forms of the same amino acid sequence, from the amyloid ß-peptide involved in Alzheimer's disease, in both α-helix and ß-sheet configurations, is investigated with ab-initio techniques. To this end, we develop an extension of the N2 computational scheme that selectively includes neighboring amino acids to preserve the relevant H-bonds. In agreement with previous speculations, it is found that the helical α structure is associated with larger parity-violation energy differences than the corresponding ß form. Implications for the evolution of biological homochirality are discussed as well as the relative importance of various effects in determining the parity-violation energy.

Cholesky decomposition-based definition of atomic subsystems in electronic structure calculations.

Decomposing the Hartree-Fock one-electron density matrix and a virtual pseudodensity matrix, we obtain an orthogonal set of normalized molecular orbitals with local character to be used in post-Hartree-Fock calculations. The applicability of the procedure is illustrated by calculating CCSD(T) energies and CCSD molecular properties in reduced active spaces.

Aromaticity of α-Oligothiophenes and Equivalent Oligothienoacenes.

The aromaticity and the degree of π-electronic delocalization have been theoretically investigated for α,α'-linked oligothiophenes containing three and five rings and for their fused analogs oligothienoacenes. By computing magnetic susceptibilities and (1)H NMR shieldings as well as current density maps, it is found that the fused oligomers are more aromatic than the corresponding nonfused partners. The increase of aromaticity with the size of the oligomer-even in the case of quinoidal forms-is also proven. The π-currents induced by an external magnetic field show that oligothienoacenes behave as single cycles since they present an intense diamagnetic current flowing around the whole molecular perimeter. In contrast, nonfused α-oligothiophenes exhibit diamagnetic currents localized over each thiophene ring. For the quinoidal oligomers, local diamagnetic π vortices appear around CC double bonds, indicating that the π electrons are rather localized as in conjugated, nonaromatic polyenes. For quinoidal nonathienoacene, it is however found that the electronic circulation around the ethylenic bonds tends to delocalize all over the carbon skeleton, indicating a more effective π-conjugation and some aromatic character.

Theoretical study on the structures and electronic spectra of TCNE2-.

Investigations into the charge-separated states and electron-transfer transitions in tetracyanoethylene (TCNE) complexes have recently generated much interest. In this work we present theoretical calculations showing that the most stable structure of the dianion TCNE2- has D2d symmetry in vacuum as well as in the solvents dichloromethane and acetonitrile. By means of the coupled cluster linear response, we compute the vertical electronic spectrum in both the gas phase and solution. The theoretical results are compared to the experimental data and good agreement is achieved.

Structure, magnetizability, and nuclear magnetic shielding tensors of bis-heteropentalenes. IV. Dihydrophospholophosphole isomers.

The geometry of the heteropentalenes formed by two phosphole units has been determined at the DFT level. The magnetic susceptibility and the nuclear magnetic shielding at the nuclei of these systems have also been calculated using gauge-including atomic orbitals and a large Gaussian basis set to achieve near Hartree-Fock estimates. A comparative study of the various isomers, of their flattened analogs, and of the parent phosphole molecule, shows that the [3,4-c] isomer is the most aromatic system in the set considered, assuming diatropicity and degree of planarity as indicators, even if it is the less stable in terms of total molecular energy. Plots of magnetic field-induced current densities confirm diatropicity of P-containing bis-heteropentalenenes, showing, however, significant differences from the analogous systems with distinct heteroatoms. The maps give evidence of spiral flow nearby C-C bonds, compatible with prevalent distortive behavior of pi electrons exalted by pyramidalization at P, and competing against the sigma electron compression, which would favor planar structure.