1,720,994 research outputs found
Magnetotropicity of five-membered heterocyclic molecules
No full textAb initio methods have been employed toobtainmodels for the current density field induced in theelectrons of pentatomic cyclic molecules C4H4X, withX = CH2, NH, O, S, PH, and AsH, in the presence of astatic, homogeneous magnetic field normal to the planecontaining the four ring carbon atoms. Thesemodels areexpected to provide simple and valid tools to assess themagnetotropism of these compounds and to interprettheir magnetic response
Stagnation Graphs and TopologicalModels of Magnetic-Field InducedElectron Current Density for SomeSmall Molecules in Connection withTheir Magnetic Symmetry
No full textSpatial models of magnetic-field induced electronic currents have beenconstructed for a series of small molecules of different point group symmetryvia stagnation graphs and current density maps.These tools provide fundamental help for rationalization of magnetic responseproperties such as magnetizability and nuclear magnetic shielding
On the determination of the diagonal components of the optical activity tensor in chiral molecules
No full textIt is shown that the diagonal components of the mixed electric-magnetic dipole polarizability tensor,
used to rationalize the optical rotatory power of chiral molecules, are origin independent, if they are
referred to the coordinate system defined by the eigenvectors of the dynamic electric dipole polarizability,
for a given value ω of the frequency of a monochromatic wave impinging on an ordered sample.
Within this reference frame, the individual diagonal components of the mixed electric-magnetic
dipole polarizability are separately measurable properties. The theoretical method is applied via a
test calculation to the cyclic 1,2-M enantiomer of the dioxin molecule, using a large Gaussian basis
set to estimate near Hartree-Fock values within a series of dipole length, velocity, and acceleration
representations
Chiral discrimination via nuclear magnetic resonance spectroscopy
No full textCalculations of nuclear magnetic shielding
polarisabilities of cyclic C4H2X2 molecules, for X = O, S,
Se and Te, have been carried out. Pseudoscalars of the
nuclear magnetic shielding polarisability computed for the
heavier nuclei are at least two orders of magnitude larger
than those reported so far. Calculations of the rf voltage
produced by the rotating chiral electric polarisation,
induced by the permanent magnetic dipole moment of 77Se
nucleus in 1,2-diselenin and 125Te nucleus in 1,2-ditellurin,
are predicted to be &40 nV and &80 nV, respectively,
detectable in principle with a proper experimental set-up.
Also the chiral chemical shifts are evaluated and they are
found near the experimental detection limit
Parity violation energies of C_4 H_4 X_2 molecules for X=O, S, Se, Te and Po
No full textParity-violating contributions, with the same magnitude but opposite sign, to the energies of 1,2 enantiomers of dioxin, dithiin, diselenin, ditellurin, and dipolonin have been calculated. Theoretical predictions, which can be classified among the largest reported so far, indicate that the P enantiomer is energetically more stable than M.
As expected, non-relativistic effects increase with the first power of the neutron number and the fourth power of the atomic number for X= O, S, Se, Te, Po.
The trend predicted by Laerdahl and Schwerdtfeger
[Phys. Rev. A {60}, 4439 (1999)] and by Berger
[J. Chem. Phys. {129}, 154105 (2008)] for H_2X_2 molecules, characterized by free rotation about the X-X internuclear axis, is therefore confirmed for the series
of more rigid C_4 H_4 X_2 compounds, in which limited deformations can take place
Relation between π-electron localization/delocalization and H-bond strength in derivatives of o-hydroxy-schiff bases
No full textDetailed investigations of electronic effects taking place within the molecular system of o-hydroxy Schiff bases have been performed. The analysis of geometry, local and global aromaticity, selected AIM-based parameters, and finally, ð-electron currents induced in the systems under consideration have been performedon the basis of quantum chemical calculations at the B3LYP/6-311+G** level of theory. The relation between localization/delocalization of ð-electrons within the whole system has been described. It has been shown that the character of the bond which is common to the phenylic ring and the quasi-ring formed as a result of H-bond formation has a crucial impact on the strength of H-bonding. The strongest H-bonds can be observed for the systems in which the sequence of formally single and double bonds within the H-bridged quasi-ring enable a ð-electronic coupling. These observations indicate that ð-electron effects play a fundamental role in the stabilization of the hydrogen bridge within o-hydroxy Schiff bases.Analysis of ð-ring currents induced by a magnetic field perpendicular to the molecular plane of selected analyzed systems confirms these conclusions
Magnetic field-induced alignment of molecular rotor-shaped cyclophanes
No full textMolecular pinwheels consisting of dipolar substituted cyclophanes in solution can function as free microscopic rotors in the presence of a homogeneous static magnetic field and a circularly polarized electric field E rotating on a plane containing B. Owing to the high magnetic anisotropy of [2](1,2,3,4,5,6)cyclophane and [3](1,2,3,4,5,6)cyclophane biased by strongring currents, in 10 molecules are expected to align with the symmetry axis perpendicular to a magnetic field of 21 tesla.The magnetic-field controlled alignment of rotor-shaped cyclophanes is insignificantly affected by non-polar solvent, e.g., toluene
Assessment of sigma-Diatropicity of the Cyclopropane Molecule
No full textSpatial models of the current density field induced in the cyclopropane molecule by stationary, homogeneous
magnetic fields, parallel to either the C3 or the C2 symmetry axis, have been constructed. A compact, abridged
representation of the models is given via stagnation graphs that convey essential information. Maps of
streamlines and moduli are also reported to complete current models that have proven useful to rationalize
magnetic tensor properties, that is, magnetizability, 1H and 13C nuclear shieldings, and magnetic shielding
along the C3 symmetry axis. Plots of Biot-Savart magnetic shielding density combined with current density
visualization yield an accurate, detailed account of the shielding mechanisms. The magnetropicity of the
system described by the current density model is fully consistent with the magnitude of magnetic tensors
calculated at near Hartree-Fock level. In a field perpendicular to the molecular plane, cyclopropane sustains
a diatropic ó-ring current with the following peculiar features: (i) it follows the molecular periphery rather
than the CC framework; (ii) it bifurcates in the proximity of the methylene moieties flowing along the CH
bonds, both above and below the óh plane; (iii) it has an effect on the values of response properties, although
it is not as large as expected from naive arguments (e.g., the center-of-mass value of the magnetic shielding
constant is dominated by in-plane components rather than the out-of-plane component, which is in contrast
to ð-aromatic systems such as benzene); (iv) it has a negligible effect on the strong anisotropy of carbon
magnetic shielding, which is shown to arise from local currents. No evidence for strong diatropism, and
therefore ó-aromaticity of the cyclopropane molecule, was found on the magnetic criterion
Beyond NICS: estimation of the magnetotropicity of inorganic unsaturated planar rings
No full textA simple classical model of magnetic-field induced electron flow is used to evaluate the ring current strength for a few inorganic monocyclic compounds. It is shown that, for these neutral and charged systems, sustaining delocalized electron
currents in the presence of a magnetic field Bext orthogonal to the sh plane, the out-of-plane
component of the nuclear magnetic shielding along the central axis is connected to the out-of-plane
magnetizability by a simple equation, involving the radius of an average loop of current. A novel
estimate of this effective radius is provided. Reliable ring current susceptibilities (that is, current
strengths) can be evaluated by a simple relationship, using the out-of-plane components of nuclear
shielding and magnetizability tensors. The accuracy of the current susceptibilities calculated by the
classical model is established by comparison with corresponding ab initio estimates obtained by
integrating the quantum mechanical current-density vector field. The out-of-plane components of
nuclear shielding and magnetizability are both strongly biased by the molecular geometry. Their
combined use to estimate the ring current susceptibility offers a quantifier of magnetotropicity
more reliable than (i) the out-of-plane component of magnetizability, (ii) the out-of-plane
component of the magnetic shielding at the center of mass.
The inadequacy of these commonly adopted magnetotropicity measures is demonstrated by
comparing a set of related molecules, C6H6 and Si6H6, N6 and P6
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