Archivio della ricerca dell'Università di Modena e Reggio Emiliahttps://iris.unimore.itIl sistema di repository digitale IRIS acquisisce, archivia, indicizza, conserva e rende accessibili prodotti digitali della ricerca.Sat, 28 Nov 2020 20:25:29 GMT2020-11-28T20:25:29Z102991Ring-current models from the differential Biot-Savart lawhttp://hdl.handle.net/11380/305511Titolo: Ring-current models from the differential Biot-Savart law
Abstract: The differential Biot-Savart law provides simple models for the pi ring currents induced in diatropic and paratropic planar conjugated molecules by a perpendicular magnetic field. The model predictions are confirmed by ab initio maps of nuclear magnetic shielding density. The effects on the protons and on the ring carbon atoms from the closest and furthest segments of the current loop are easily interpreted.
Thu, 01 Jan 2004 00:00:00 GMThttp://hdl.handle.net/11380/3055112004-01-01T00:00:00ZDelocalized Currents without a Ring of Bonded Atoms: Strong Delocalized Electron Currents Induced by Magnetic Fields in Noncyclic Moleculeshttp://hdl.handle.net/11380/1013315Titolo: Delocalized Currents without a Ring of Bonded Atoms: Strong Delocalized Electron Currents Induced by Magnetic Fields in Noncyclic Molecules
Abstract: Some noncyclic small molecules, electrically neutral or charged, sustain
interatomic electronic currents in the presence of a stationary, spatially uniform magnetic
field. The existence of fairly large delocalized electron flow is demonstrated in H2O, BH3,
NH3, CH4, CH3−CH3, H3O+, CH3+, and NH4
+, by plots of quantum mechanical current
density. Convincing quantitative evidence is arrived at by current strengths, defined via a flux
integral of the ab initio current density. Application of a simple ring current model shows
that the delocalized current strengths account for the out-of-plane component of the
magnetic shielding tensor along the symmetry axis. A definition of delocalized electron
current as a current flowing along a closed loop containing three or more atoms is discussed.
Wed, 01 Jan 2014 00:00:00 GMThttp://hdl.handle.net/11380/10133152014-01-01T00:00:00ZAb initio and experimental study of NMR coupling constants in bicyclo [1.1.1]pentanehttp://hdl.handle.net/11380/305553Titolo: Ab initio and experimental study of NMR coupling constants in bicyclo [1.1.1]pentane
Abstract: The different dependences of NMR spin-spin coupling constants of a medium-sized molecule on the quality of the ground-state wavefunction is studied using three different basis sets of increasing size, namely, 6-31G, 6-31G* and 6-31G**. Calculations of the Fermi contact terms were also carried out using partially uncontracted s and p atomic orbitals with the same exponents as those of the 6-31G basis set. Bicyclo[1.1.1]pentane was chosen as a model compound; it was synthesized for the measurement of several couplings as part of this work in order to compare theoretical results with experimental values. The through-space transmission of couplings via the interaction of rear lobes of orbitals inside the bicyclic cage were studied with the IPPP approach using the above basis sets.
Sun, 01 Jan 1995 00:00:00 GMThttp://hdl.handle.net/11380/3055531995-01-01T00:00:00ZDiatropicity of tetraazanaphthaleneshttp://hdl.handle.net/11380/306355Titolo: Diatropicity of tetraazanaphthalenes
Abstract: Tetraazanaphthalenes are diatropic molecules, whose magnetic response to a magnetic field perpendicular to the molecular plane closely resembles that of naphthalene. The out-of-plane component of the magnetic susceptibility tensor and its strong anisotropy can be used as quantifiers of magnetic aromaticity. Maps showing streamlines and modulus of the current density field provide clear evidence for diatropicity of these systems. They also explain the strong anisotropy of carbon and nitrogen magnetic shielding, which is determined by the big out-of-plane component of the nuclear shielding tensor. The electronic ring currents observed in the map deshield the nuclei of ring hydrogens by enforcing the local magnetic field and diminishing the out-of-plane component of proton shielding. (C) 2006 Wiley Periodicals, Inc.
Sun, 01 Jan 2006 00:00:00 GMThttp://hdl.handle.net/11380/3063552006-01-01T00:00:00ZStructure, magnetizability, and nuclear magnetic shielding tensors of bis-heteropentalenes. IV. Dihydrophospholophosphole isomershttp://hdl.handle.net/11380/306774Titolo: Structure, magnetizability, and nuclear magnetic shielding tensors of bis-heteropentalenes. IV. Dihydrophospholophosphole isomers
Abstract: 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.
Sun, 01 Jan 2006 00:00:00 GMThttp://hdl.handle.net/11380/3067742006-01-01T00:00:00ZCalculation of third-rank molecular hypermagnetizabilities by continuous transformation of the origin of the current densityhttp://hdl.handle.net/11380/306783Titolo: Calculation of third-rank molecular hypermagnetizabilities by continuous transformation of the origin of the current density
Abstract: The third-rank hypermagnetizabilities of three small molecules have been evaluated at the Hartree-Fock level of accuracy within the conventional common-origin approach as well as alternative procedures formally based on continuous transformation of the origin of the electronic current density induced by a perturbing electromagnetic field. Gaugeless basis sets of increasing size and flexibility have been employed in a numerical test (i) to assess the practicality of distributed origin methods whereby the diamagnetic contribution to the current density is formally annihilated, and (ii) to estimate the degree of convergence of diamagnetic and paramagnetic contributions to tensor components. It is shown that two nice features characterize the computational scheme adopted in this study: (i) the results depend only linearly on a shift of origin; (ii) a permutational symmetry condition of tensor indices yields a natural criterion for the near-Hartree-Fock quality of computed values.
Tue, 01 Jan 2002 00:00:00 GMThttp://hdl.handle.net/11380/3067832002-01-01T00:00:00ZOn the resolution of the optical rotatory power of chiral molecules into atomic terms. A study of hydrogen peroxidehttp://hdl.handle.net/11380/307030Titolo: On the resolution of the optical rotatory power of chiral molecules into atomic terms. A study of hydrogen peroxide
Abstract: An additive scheme for resolving average optical rotatory power of a molecule into atomic contributions, based on the acceleration gauge for the electric dipole, and/or the torque formalism, has been applied to hydrogen peroxide. Extended calculations have been carried out to test the reliability of the partition method. Gross atomic isotropic contributions to the average molecular property from oxygen and hydrogen atoms have been evaluated. The force and torque gauges provide different numerical values for atomic contributions.
Tue, 01 Jan 2002 00:00:00 GMThttp://hdl.handle.net/11380/3070302002-01-01T00:00:00ZTheoretical determination of paramagnetic susceptibilities from nuclear electromagnetic shieldingshttp://hdl.handle.net/11380/306803Titolo: Theoretical determination of paramagnetic susceptibilities from nuclear electromagnetic shieldings
Abstract: A new method for calculating paramagnetic susceptibilities from nuclear electromagnetic shieldings is outlined. A distributed origin gauge with origins at nuclei has been used. The quantum mechanical constraints for gauge invariance and charge-current conservation have been written in terms of nuclear shieldings. An extended numerical test on first- and second-row hydrides has been carried out. For a given basis set larger than double zeta the computed paramagnetic susceptibilities are more accurate than those obtained via conventional direct calculations. The results for the largest basis sets are in excellent agreement with near Hartree-Fock paramagnetic susceptibilities from other sources and with available experimental values. It has also been found that polarised 6-31G basis sets provide near Hartree-Fock paramagnetic susceptibilities for the second-row hydrides and satisfy the conservation conditions to high extent.
Tue, 01 Jan 1991 00:00:00 GMThttp://hdl.handle.net/11380/3068031991-01-01T00:00:00ZSum rules for invariance of the fourth-rank hypermagnetisability in a gauge translationhttp://hdl.handle.net/11380/306807Titolo: Sum rules for invariance of the fourth-rank hypermagnetisability in a gauge translation
Abstract: The conditions for invariance in a gauge translation of the fourth-rank molecular hypermagnetisability tensor, introduced to rationalize the cubic response of a molecule in the presence of an external magnetic field, are discussed in terms of quantum mechanical sum rules. Eight relationships, connecting electric dipole polarisability, polarisability of magnetisability, and other third- and fourth-rank tensors that can be regarded as intrinsic molecular properties tout court, have been obtained. Numerical tests have been carried out to determine the Hartree-Fock limit for the sum-rules in a set of small molecules.
Sat, 01 Jan 2005 00:00:00 GMThttp://hdl.handle.net/11380/3068072005-01-01T00:00:00ZEffects of strong magnetic fields on the electron distribution and magnetisability of rare gas atomshttp://hdl.handle.net/11380/306818Titolo: Effects of strong magnetic fields on the electron distribution and magnetisability of rare gas atoms
Abstract: Strong uniform static magnetic fields compress the electronic distribution of rare gas atoms and cause a ´spindle effect´, which can be illustrated by plotting charge-density functions which depend quadratically on the flux density of the applied field. The fourth rank hypermagnetisabilities of He, Ne, Ar and Kr are predicted to have small positive values. Accordingly, the diamagnetism of rare gas atoms diminishes by a very little amount in the presence of intense magnetic field.
Thu, 01 Jan 2004 00:00:00 GMThttp://hdl.handle.net/11380/3068182004-01-01T00:00:00Z