1,721,027 research outputs found
Magnetotropicity of phosphole and its arsenic analogue
No full textSpatial ring current models for the phospholemolecule and its arsenic parent have been constructed.Diatropism of thesemolecules is quite peculiarand fundamentally different from that of benzene asshown by stagnation graphs of current density field.Maps of shielding density are helpful for interpreting theeffect of electronic currents on nuclear shielding. Constrainedplanarity increases the degree of diatropicityquantitatively specified by magnetic descriptors, whichimplies that ring currents go together with π-electrondistortivity
Spatial ring current model of the [2.2]paracyclophane molecule
No full textA representation of the current density induced in the [2.2]paracyclophane molecule by a homogeneous magneticfield parallel to the line joining the centers of the phenylene rings is given in compact form by a stagnationgraph that conveys essential information. Analogous graphs were obtained for two perpendicular directions.Plots of streamlines are also reported to complete a ring current model that has been proved useful to understandthe magnetropicity of the system. Stagnation graphs, maps of streamlines and moduli of the current density,and plots of Biot-Savart magnetic shielding density provide a basic tool kit for rationalizing magnetic responseof complex systems
Spatial ring current model for the prismane molecule
No full textSpatial models of magnetic-field induced electronic ring currents have been constructed for the prismanemolecule via stagnation graphs and current density maps. These tools provide an insight into thecomplicated phenomenology resulting from competition of diatropic and paratropic regimes that determinethe magnitude of various components of magnetic susceptibility and magnetic shielding of hydrogenand carbon nuclei. Shielding density maps show that the differential Biot-Savart law, along with anatlas of the current density field, explains magnetic shielding at hydrogen and carbon nuclei and virtualshielding at ring and cage centers
Ring current models for acetylene and ethylene molecules
No full textSpatial models of the current density vector field, induced in theelectronic cloud of the acetylene and ethylene molecules by a uniform,time-independent magnetic field, are discussed in terms of topologicalstagnation graphs and three-dimensional streamline plots. The modelsare validated by documenting their ability to explain magnetic suscep-tibility and nuclear magnetic shieldings of carbon and hydrogen viarelated shielding density maps
Facoemulsificazione bimanuale: vantaggi e svantaggi
No full textValutazione dei vantaggi e svantaggi della facoemulsificazione bimanuale
Facoemulsificazione con minincisione bimanuale .
No full textDescrizione della facoemulsificazione con minincisione bimanuale
Ring-current models from the differential Biot-Savart law
No full textThe 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
Polygonal Current Model: An Effective Quantifier of Aromaticity on the Magnetic Criterion
No full textTo explain peculiar effects of electron delocalization
on the magnetic response of planar cyclic molecules, a
basic model that accounts for their actual geometrical structure
has been developed by integrating the differential Biot−Savart
law. Such a model, based on a single polygonal circuit with
ideal features, is shown to be applicable to electrically neutral
or charged monocyclic compounds, as well as linear polycyclic
condensed hydrocarbons. Two theoretical quantities, easily computed via quantum chemistry codes (the out-of-plane
components of the magnetizability, ξ∥, and the magnetic shielding σ∥(h) of points P on the symmetry axis orthogonal to the
molecular plane, at distance h from the center of mass) are shown to be linearly connected, for example, for monocyclic
structures, via the relationship σ∥(h) = ±(μ0/2π)ξ∥D(h), where D(h) is a simple function of geometrical parameters. Equations of
this type are useful to rationalize scan profiles of magnetic shielding and nucleus-independent chemical shift along the highest
symmetry axis. For a regular polygon, D(h) depends approximately on the third inverse power of the distance d of the vertices
from the center, and ξ∥ is proportional to the area of the polygon, that is, ∼d2; hence, the shielding σ∥(0) and the related nucleusindependent
chemical shift NICS∥(0) are unsafe quantifiers of magnetotropicity; they are biased by a spurious geometrical
dependence on d−1, incorrectly exhalting them in cyclic systems with smaller size. A more reliable magnetotropicity measure for a
cyclic compound, in the presence of a magnetic field Bext applied at right angles to the molecular plane, is defined within the
polygonal current model by the current susceptibility or current strength, ∂I/∂Bext = −ξ∥/Aeff, expressed in nanoampe re per tesla,
where Aeff is a properly defined area enclosed with the polygonal circuit. An extended numerical test on a wide series of monoand
polycyclic compounds and a comparison with corresponding ab initio current susceptibilities prove the superior quality of
this indicator over other commonly employed aromaticity/antiaromaticity benchmarks on the magnetic criterion
Correlation between the out-of-Plane Components of Magnetizability and Central Magnetic Shielding in Unsaturated Cyclic Molecules
No full textA simple classical model of magnetic-field induced pi-electron flow is discussed, showing that the contribution to the sigma(parallel to) out-of-plane component of the virtual magnetic shielding provided by pi-ring currents, at points P along the C axis of cyclic planar unsaturated hydrocarbons C H with D-nh symmetry, in the presence of a magnetic field B-ext at right angles to the a plane, is, with good approximation, connected with the pi-electron contribution to the out-of-plane component of the magnetizability, xi(parallel to). The relationship is sigma(parallel to) (h) = (mu(0)/2 pi) (s(2) + h(2))(-3/2) xi(parallel to), where s is the distance of a C nucleus from the center of the carbon ring, and h is the distance of P from a. The ring current susceptibility, that is, the strength of the pi currents, expressed in nA/T (nano ampere per tesla) within the SI system of units, is given by partial derivative I/partial derivative B-ext = xi(parallel to)(pi s(2)), which can be used as a reliable virtual measure of magnetotropicity and relative pi-electron mobility in isoelectronic systems. Criteria for the practicality of the proposed ring current model are discussed
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