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.Thu, 03 Dec 2020 11:38:21 GMT2020-12-03T11:38:21Z10181Ring-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/11380/3055112004-01-01T00:00:00ZOn 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/11380/3070302002-01-01T00:00:00ZThe ring-current model of the paratropic pentalene moleculehttp://hdl.handle.net/11380/306773Titolo: The ring-current model of the paratropic pentalene molecule
Abstract: The ring-current model for the pentalene molecule has been constructed by ab initio techniques. The current density vector field has been used to obtain magnetic shielding density maps for the different protons. It is shown that the paramagnetic flow of the pi electrons causes an increase of the out-plane component of proton shielding tensors, that is overall shielding. Distant portions of the it flow also shield carbon nuclei.
Sat, 01 Jan 2005 00:00:00 GMThttp://hdl.handle.net/11380/3067732005-01-01T00:00:00ZRing current effects on nuclear magnetic shielding of carbon in the benzene moleculehttp://hdl.handle.net/11380/307584Titolo: Ring current effects on nuclear magnetic shielding of carbon in the benzene molecule
Abstract: The differential Biot-Savart law of classical electrodynamics was applied to develop a ring current model for the magnetic shielding of the carbon nucleus in benzene. It is shown that the local effect of the pi currents, induced by a magnetic field normal to the molecular plane, on the sigma(C) out-of-plane shielding tensor component vanishes. However, approximately 10% of sigma(C) is due to the shielding contributions from pi current density in the region of the other carbon atoms. Magnetic shielding density maps obtained via quantum mechanical procedures confirm the predictions of the classical model.
Sat, 01 Jan 2005 00:00:00 GMThttp://hdl.handle.net/11380/3075842005-01-01T00:00:00ZCorrelated-gauge invariant calculations of nuclear magnetic shielding constants using the continuous transformation of the origin of the current density approach. II. Coupled cluster and density-functional theory results for eight small moleculeshttp://hdl.handle.net/11380/454649Titolo: Correlated-gauge invariant calculations of nuclear magnetic shielding constants using the continuous transformation of the origin of the current density approach. II. Coupled cluster and density-functional theory results for eight small molecules
Abstract: The quantum mechanical current density induced in a molecule by an external magnetic field isinvariant to translations of the coordinate system. This fundamental symmetry is exploited toformally annihilate the diamagnetic contribution to the current density via the approach of“continuous transformation of the origin of the current density-diamagnetic zero” CTOCD-DZ.The relationships obtained by this method for the magnetic shielding at the nuclei are intrinsicallyindependent of the origin of the coordinate system for any approximate computational schemerelying on the algebraic approximation. The authors report for the first time an extended series oforigin-independent estimates of nuclear magnetic shielding constants using the CTOCD-DZapproach at the level of density functional theory DFT with four different types of functionals andunrelaxed coupled cluster singles and doubles linear response CCSD-LR theory. The resultsobtained indicate that in the case of DFT the procedure employed is competitive with currentlyadopted computational methods allowing for basis sets of gauge-including atomic orbitals, whereaslarger differences between CTOCD-DZ and common origin CCSD-LR results are observed due tothe incomplete fulfillment of hypervirial relations in standard CCSD-LR theory. It was foundfurthermore that the unrelaxed CCSD-LR calculations predict larger correlation corrections for theshielding constants of almost all nonhydrogen atoms in their set of molecules than the usual relaxedenergy derivative CCSD calculations. Finally the results confirm the excellent performance of Kealand Tozer’s third functional, in particular, for the multiply bonded systems with a lot of electroncorrelation, but find also that the simple local density functional gives even better results for the fewsingly bonded molecules in their study where correlation effects are small.
Mon, 01 Jan 2007 00:00:00 GMThttp://hdl.handle.net/11380/4546492007-01-01T00:00:00ZThe Leap-frog Effect of Ring Currents in Benzenehttp://hdl.handle.net/11380/454439Titolo: The Leap-frog Effect of Ring Currents in Benzene
Abstract: Symmetry arguments show that the ring-current model proposed by Pauling, Lonsdale, andLondon to explain the enhanced diamagnetism of benzene is flawed by an intrinsic drawback. The minimalbasis set of six atomic 2p orbitals taken into account to develop such a model is inherently insufficient topredict a paramagnetic contribution to the perpendicular component of magnetic susceptibility in planarring systems such as benzene. Analogous considerations can be made for the hypothetical H6 cyclicmolecule. A model allowing for extended basis sets is necessary to rationalize the magnetism of aromatics.According to high-quality coupled Hartree-Fock calculations, the trajectories of the current density vectorfield induced by a magnetic field perpendicular to the skeletal plane of benzene in the ð electrons arenoticeably different from those typical of a Larmor diamagnetic circulation, in that (i) significant deformationof the orbits from circular to hexagonal symmetry occurs, which is responsible for a paramagnetic contributionof ð electrons to the out-of-plane component of susceptibility, and (ii) a sizable component of the ð currentdensity vector parallel to the inducing field is predicted. This causes a waving motion of ð electrons;streamlines are characterized by a “leap-frog effect”.
Tue, 01 Jan 2002 00:00:00 GMThttp://hdl.handle.net/11380/4544392002-01-01T00:00:00ZCurrent density maps, magnetizability and nuclear magnetic shielding tensors for anthracene, phenanthrene and triphenylenehttp://hdl.handle.net/11380/454414Titolo: Current density maps, magnetizability and nuclear magnetic shielding tensors for anthracene, phenanthrene and triphenylene
Abstract: Ab initio magnetically induced ð-electron (first-order) current density maps and second-order magneticproperties, i.e., magnetizabilies, proton, and 13 C magnetic shielding tensors, calculated at coupled Hartree-Fock level of theory by means of the continuous transformation of origin of the current density method (CTOCD),are presented for a series of polycyclic aromatic hydrocarbons. The reliability of the current density maps isdocumented by the nice agreement between theoretical values for principal values of magnetizabilities andproton magnetic shieldings and corresponding experimental data presently available. For all three of themolecules, the ð-electron current flows mainly on the external circuit of carbon atoms. Other intense circulationslocalized on a single ring take place over the central hexagon of anthracene and, on the contrary, on theexternal hexagons of phenanthrene and triphenylene. In light of the results obtained in the present work, e.g.,taking advantage of high quality current density maps, important details of the ring current model for themolecules under study can be reexamined. Eventually, it is shown that the fundamental information which canbe gained from knowledge of the principal components of the magnetic response tensors is easily accessibleby means of the CTOCD computational method adopted here.
Fri, 01 Jan 1999 00:00:00 GMThttp://hdl.handle.net/11380/4544141999-01-01T00:00:00ZTheoretical Determination of Parity-Violating Vibrational Frequency Differences between the Enantiomers of the CHFClBr Moleculehttp://hdl.handle.net/11380/454422Titolo: Theoretical Determination of Parity-Violating Vibrational Frequency Differences between the Enantiomers of the CHFClBr Molecule
Abstract: A coupled Hartree-Fock procedure has been employed to estimate the frequency shift in the infrared spectraof the S and R enantiomers of the CHFClBr molecule due to the parity-violating electroweak interaction. Thecalculations indicate that a resolving power n /Dn'131016, i.e., three orders of magnitude larger than thatobtained in recent experiments, would be necessary to detect the parity-violating effects. The largest frequencyshift was found for C-Cl stretching.
Sat, 01 Jan 2000 00:00:00 GMThttp://hdl.handle.net/11380/4544222000-01-01T00:00:00ZTheoretical results which strengthen the hypothesis of electroweak bioenantioselectionhttp://hdl.handle.net/11380/454416Titolo: Theoretical results which strengthen the hypothesis of electroweak bioenantioselection
Abstract: It is shown via a large series of numerical tests on two fundamental organic molecules, the L-a-amino acidL-valine and the sugar precursor hydrated D-glyceraldheyde, that the ab initio calculation of the parityviolatingenergy shift, at the random-phase approximation level of accuracy, provides results that are about oneorder of magnitude larger than those obtained by means of less accurate methods employed previously. Thesefindings would make more plausible the hypothesis of electroweak selection of natural enantiomers via theKondepudi-Nelson scenario, or could imply that Salam phase-transition temperature is higher than previouslyinferred: accordingly, the hypothesis of terrestrial origin of life would become more realistic.
Fri, 01 Jan 1999 00:00:00 GMThttp://hdl.handle.net/11380/4544161999-01-01T00:00:00ZOn the stabilization of natural L-alpha-amino acids and D-sugars via parity-violating effects.http://hdl.handle.net/11380/462014Titolo: On the stabilization of natural L-alpha-amino acids and D-sugars via parity-violating effects.
Abstract: Classically a chiral molecule and its enantiomer have been considered energetically equivalent:<ΨDHpcΨD>=<ΨLP−1HpcPΨL>=<ΨLHpcΨL>as the parity-conserving hamiltonian Hpc does not change under the transformation P−1 HpcP, where P is the parity operator which transforms the wavefunction of one enantiomer into the other. However the parity-violating weak interactions ensure that this equivalence is no longer exact.
Fri, 01 Jan 1999 00:00:00 GMThttp://hdl.handle.net/11380/4620141999-01-01T00:00:00Z