Charge nucleus independent

The aromatic nature of the presently discussed carbocations and dications have been further established by subjecting their NMR parameters to charge density chemical shift relationship449,475 77 originally developed by Spiesecke and Schneider.478,479 Furthermore, NICS (nucleus-independent chemical shift) developed by Schleyer et al.480 offers a simple and efficient probe for aromaticity. 

Calculations of the structure of the mesoionic thiopyrylium-3-olate 74 suggest that the C-S bond lengths are similar to those in the pyrylium cation at ca. 168 pm, perhaps supporting the fully charge-separated betaine structure. However, the charge at oxygen is closer to 0.5 than the 1.0 expected for such a structure. Furthermore, the nucleus-independent chemical shift value is appreciably lower than that for the thiopyrylium cation. These data point toward an ylidic structure with an acceptor moiety rather than an aromatic cation and an exocyclic oxyanion <2002IJQ(90)1055>. 

Magnitudes of BLA in A, Stabilization Energies in kcal/mol (Af), Nucleus-Independent Chemical Shift (NICS) in ppm. Charge Density at the Ring Critical Point (Prcp) in e/A Units, and Laplacian of the Charge Density (V Prcp) in e/A Units for the Systems Considered... 

Remarkably, only one nuclear constant, Q, is needed in (4.17) to describe the quadrupole moment of the nucleus, whereas the full quadrupole tensor Q has five independent invariants. The simplification is possible because the nucleus has a definite angular momentum (7) which, in classical terms, imposes cylindrical symmetry of the charge distribution. Choosing x, = z as symmetry axis, the off-diagonal elements Qij are zero and the energy change caused by nuclear... 

It is important to understand that the atomic charges refer to atoms that are not spherical. Consequently the centroid of electronic charge of an atom does not in general coincide with the nucleus, and each atom therefore has an electric dipole moment—or, more generally, an electric dipolar polarization (since only the dipole moment of electrically neutral atoms is origin independent). 

At this point one needs to remember that electrons are not completely independent particles, moving randomly in a positively charged background. They are a charged species and as such, they also interact with the positive ions (nucleus)... 

A considerable amount of evidence indicates that nuclear forces are charge-independent, i.e, the neutron-neutron, neutron-proton, and proton-proton forces are identical. The meson theory of nuclear forces, originated by Yukawa, postulates the atomic nucleus being held together by an exchange force in which particles, now called mesons, are exchanged between individual nucleons within the nucleus. 

An example of the energy level matching in the mirror pair 17F, 170 is shown in Figure 6.7. The agreement of the levels is quite remarkable and can be taken as strong evidence for the charge independence of the nuclear force, that is, the protons and neutrons move in essentially identical but separate orbitals in the nucleus. 

Which of the following statements are true If false, why are they false (a) The effective nuclear charge Zeff is independent of the number of electrons present in an atom, (b) Electrons in an s-orbital are more effective at shielding electrons in other orbitals from the nuclear charge because an electron in an s-orbital can penetrate to the nucleus of the atom. 

One solution for this problem, the most optimistic, suggested the existence of three independent sets of o--constants. The first set, the Hammett constants, would be applicable to side-chain reactions in which resonance interactions between the substituent and the side-chain were either small or insignificant. The second set, the w-constants, would apply to side-chain reactions of phenols and anilines and nucleophilic aromatic substitution reactions in which a negative charge was introduced in the aromatic nucleus (Miller, 1956). A third set, the c7+-constants, would apply to electrophilic substitution and electrophilic side-chain reactions for which resonance interactions between the reaction site and the substituent were important. 

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