1,720,968 research outputs found
Atomic size adjusted calculation of the magnetically induced current density
No full textThe continuous set of gauge transformations for calculations of magnetically induced current densities has been investigated in details. Atomic size adjustments have been inserted employing the Bragg-Slater atomic radii and considering the bond critical points determined by the electron density distribution. By means of extensive calculations on a set of selected molecules and adopting a well suited series of basis sets, it is shown that these adjustments provide a systematic improvement of the approximate current density and related properties such as the magnetic shielding constants, which are calculated by integration for different kinds of nuclei and in nucleus independent positions
Standard Response Calculation of Electric Dipole Polarizability and Specific Optical Rotation Power Densities
No full textA time-dependent method has been developed to solve the standard response equation for the calculation of dynamic molecular property densities, endowed with the characteristic of being origin-invariant, entirely in the atomic orbital basis at both HF and DFT level of theory. The method has been tuned in particular for the calculation of origin-independent electric dipole polarizability density and specific rotation power density. Some demonstrations are given for the hexabenzocoronene molecule and the Tröger′s base
Decomposition of molecular integrals into atomic contributions via becke partitioning scheme: A caveat
No full textDecomposition of molecular integrals into physically meaningful atomic contributions by means of the Becke integration scheme requires some care with respect to the choice of suitable atomic size adjustments. Using a simple illustrative example, it is shown that the adjustment of cell boundaries, as originally proposed by considering Bragg-Slater atomic radii, does not provide reliable results. Alternatively, the positions of the bond critical points of the electron density can be adopted to define heteronuclear cutoff profiles which allow for a more reasonable atomic partition of the molecular electron density
A Study of Differential Topology on the Magnetically Induced Isotropically Averaged Lorentz Force Density of a Few Simple Molecules
No full textQuantum chemical topology addresses the study of the chemical structure by applying the tools of differential topology to scalar and vector fields obtained by quantum mechanics. Here, the magnetically induced isotropically averaged Lorentz force density was computed and topologically analyzed for 11 small molecules. Critical points (attractors, repellers, and saddles) were determinedand trajectories connecting the attractors computed. It is shown that kinds and numbers of the critical points are to some extent transferable in similar molecules. CC bonds of different orders are endowed with critical points of different kinds close to their center. The sum of topological indices of the isolated critical points is influenced by the presence of repellers on the outer part of the molecules
Assessment of the performance of DFT functionals in the fulfillment of off-diagonal hypervirial relationships
No full textOff-diagonal hypervirial relationships, combined with quantum mechanical sum rules of charge-current conservation, offer a way to test electronic excited-state transition energies and moments, which does not need any external reference. A number of fundamental relationships were recast into absolute deviations from zero, which have been used to assess the performance of some popular DFT functionals. Extended TD-DFT calculations have been carried out for a pool of molecules chosen for this purpose, adopting a large basis set to ensure high quality results. A partial agreement with previous benchmarks is observed
Disentangling the Contributions to the Proton Magnetic Shielding in Carbon Nanohoops and Nanobelts: Evidence for a Paratropic Belt-Current
No full textThe proton NMR magnetic shieldings of the recently synthesized D3d isomers of methylene-bridged [6]cycloparaphenylene (MB[6]CPP) and [12]cyclophenacene hide in themselves the effect of a global paratropic current around the nanobelts, which is induced by a magnetic field parallel to the main symmetry axis of the molecules. The effect is particularly pronounced for the methylene protons of MB[6]CPP, especially for those facing inside the nanobelt. The small experimental chemical shift difference of only 0.2 ppm is incompatible with the separation of the signals caused by the belt curvature, which, by itself, is calculated to be larger than 1 ppm, with both signals shifted upfield with respect to the position detected for the nanobelt. A careful dissection of the proton magnetic shielding in terms of molecular orbital contributions, has permitted a quantitative assessment of the genuine effect on each different proton caused by a substantial paratropic belt-current, which brings all the signals in nice agreement with the experimental spectra
Electronic Current Density Induced by Uniform Magnetic Fields in Clarenes
No full textSome planar and non-planar clarenes have been studied using maps of magnetically induced quantum-mechanical current density and tools from differential topology to assess their magnetic response in connection with recent results by Du and Wang. Bond current strengths have been computed to estimate quantitative measures. Isosurfaces of the divergence of induced Lorentz force density have been shown to provide useful additional criteria, especially in the case of non-planar clarenes.Calculations of the magnetic response are reported for few selected clarenes, the most stable isomers among cycloarenes, as identified by maximization of the number of Clar sextets, and tested computationally. Only some of the rings endowed with a Clar sextet show an exaltation of the diatropic ring current, as could have been expected based on previous calculations on different polycyclics. imag
Electronic currents induced by optical fields and rotatory power density in chiral molecules
No full textThe electric dipole–magnetic dipole polarizability tensor κ′, introduced to interpret the optical activity of chiral molecules, has been expressed in terms of a series of density functions k′αβ, which can be integrated all over the three-dimensional space to evaluate components καβ′and traceκ′αα. A computational approach to k′αβ, based on frequency-dependent electronic current densities induced by monochromatic light shining on a probe molecule, has been developed. The dependence of k′αβ on the origin of the coordinate system has been investigated in connection with the corresponding change of καβ′. It is shown that only the tracek′αα of the density function defined via dynamic current density evaluated using the continuous translation of the origin of the coordinate system is invariant of the origin. Accordingly, this function is recommended as a tool that is quite useful for determining the molecular domains that determine optical activity to a major extent. A series of computations on the hydrogen peroxide molecule, for a number of different HO–OH dihedral angles, is shown to provide a pictorial documentation of the proposed method
On the Unusual Global Aromaticity of Two Cyclopenta-Ring-Fused Oligo(m-Phenylenes)
No full textSome years ago, Jishan Wu reported the synthesis of 8MC and 10MC, two homologues of the cyclopenta-ring-fused oligo(m-phenylene) macrocycles mMC, each behaving as an annulene-within-an-annulene (AWA). This was a surprising result as the AWA behavior is rare. Both molecules have a partial polyradical character, enforced by the quest for restoring some aromatic character of benzene rings. However, that restoration brings back some coupling between the two annulenes. Indeed, we found that the geometry and the magnetically induced currents indicate that, while 8MC does have an AWA character, this is not the case of the larger 10MC. Limitations of the design strategy of AWA molecules should be taken into account in future attempts to prepare novel large coronenes
Synergistic properties of arabinogalactan (Ag) and hyaluronic acid (ha) sodium salt mixtures
No full textThe properties of mixtures of two polysaccharides, arabinogalactan (AG) and hyaluronic acid (HA), were investigated in solution by the measurement of diffusion coefficients D of water protons by DOSY (Diffusion Ordered SpectroscopY), by the determination of viscosity and by the investigation of the affinity of a small molecule molecular probe versus AG/HA mixtures in the presence of bovine submaxillary mucin (BSM) by1HNMR spectroscopy. Enhanced mucoadhesive properties, decreased mobility of water and decreased viscosity were observed at the increase of AG/HA ratio and of total concentration of AG. This unusual combination of properties can lead to more effective and long-lasting hydration of certain tissues (inflamed skin, dry eye corneal surface, etc.) and can be useful in the preparation of new formulations of cosmetics and of drug release systems, with the advantage of reducing the viscosity of the solutions
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