Shielding Effect

4 Key excerpts on "Shielding Effect"

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  eBook - ePub

  Biophysical Basis of Physiology and Calcium Signaling Mechanism in Cardiac and Smooth Muscle

  • Tetsuya Watanabe(Author)
  • 2018(Publication Date)
  • Academic Press

    (Publisher)

  Chapter 3

  Shielding Effect and Chemical Bonding

  Abstract

  Because of the Shielding Effect of valence electrons, a different atom has the different ability to attract electrons toward it and the resultant bond has a partial ionic character. The polar molecule like water is important because it will create the electric field and induce the instantaneous localization of electron on the neighbor atom. A hydrogen bond is an electrostatic interaction between a partially positive hydrogen atom in a polar bond and a strong hydrogen acceptor of the other. The oxidation-reduction reactions occur between atoms with different ability to accept electrons as seen in electron transport. An oxidizing agent such as oxygen accepts electrons from another element that has a lower electronegativity. The strong reactivity of hydroxyl radical and superoxide causes sometimes harmful biological effects on DNA. Another radical, nitric oxide has a physiological effect, which diffuses freely across the cell membranes and stimulates the production of cyclic GMP that signals for the smooth muscle in blood vessels to relax.

  Keywords

  Shielding Effect; Electron affinity; Electronegativity; Polar molecule; Free radicals; Oxidation-reduction reaction; Derivation of Nernst equation; Electron transport and ATP production

  3.1 Introduction

  The number of the electrons in an atomic orbital is the same as the number of the protons if it is not ionic. The properties of elements vary periodically with proton number. The elements with similar chemical properties fall in the same column of the periodic table. Lithium, sodium, and potassium have one valence electron in s -orbital. They are so reactive, willing to give off the electron in s -orbital since the outermost electron feels the week net attraction by + 1e from the nucleus due to the Shielding Effect. Fluorine, chlorine, bromine, and iodine have five electrons in p -orbital, tending to attract one more electron from another atom and then drop to a lower energy state. Atoms in the middle of the periodic table cannot form ionic bond because it takes too much energy for the atom to gain or lose some electrons to achieve a noble-gas configuration. In fact, carbon bonds to other atoms not by accepting or giving off electrons, but sharing them. A pure covalent bond occurs in symmetrical molecules such as H2 , N2 , and O2 . Usually shared electrons are more likely near one atom than the other if bond forms from different kinds of atoms. A different atom has the different ability to attract electrons toward it, and the resultant bond has a partial ionic character. A hydrogen bond is an electrostatic interaction between a partially positive hydrogen atom in a polar bond and a strong hydrogen acceptor such as nitrogen, oxygen, and fluorine atoms. Hydrogen bond is a stronger type of intermolecular interaction, but much weaker than covalent, or ionic bonds. The complementary strands of DNA are held together by hydrogen bonds which are easily separated during DNA replication. Chemical reactions often occur between a molecule with high ability to accept electrons and a molecule with high ability to donate electrons. The oxidation-reduction reactions occur between atoms with different electronegativity. An oxidizing agent such as oxygen has a higher electronegativity and accepts electrons from another element that has a lower electronegativity, and the oxidizing agent itself is reduced. A reducing agent such as hydrogen has a lower electronegativity and donates electrons to another one that has a higher electronegativity, and the reducing agent itself is oxidized. Radicals are molecules with an unpaired electron. The strong reactivity of hydroxyl radical (OH) and superoxide (

  O 2

  − 1

  ) causes sometimes harmful biological effects on DNA. Another radical, nitric oxide has a physiological effect, which diffuses freely across the cell membranes and stimulates the production of cyclic GMP that signals for the smooth muscle in blood vessels to relax. (See Table 3.1

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  Reviews in Computational Chemistry, Volume 8

  • Kenny B. Lipkowitz, Donald B. Boyd(Authors)
  • 2009(Publication Date)
  • Wiley-VCH

    (Publisher)

  In the presence of an external magnetic field there is a net electronic current that induces additional magnetic fields at all points of the molecular systems, in particular at the sites of the nuclear moments, and it is these internal fields that give rise to chemical shielding. Consider a molecule with a single nuclear moment, 6, in the presence of an external magnetic field, 8. The energy levels of the nuclear moment are characterized by an effective or “spin” Hamiltonian given by = - l ; . B + l ; . G , B where the - (i B term is the classical moment-field interaction, and the l; . G * B term characterizes the dominant interaction of the nuclear moment with the Theory 251 field induced by the electrons' motion, this term being found to be B(ind) = "a h (vide infra). The chemical shielding tensor ii is a second-order, asymmetrical tensor. The nuclear spin Hamiltonian is an effective Hamiltonian for the nu- clear moment in that it contains no explicit reference to the other particles in the system; coupling between the nuclear moment and the electrons is present, of course, and their effect on the nuclear moment is contained (to first order in B ) in the shielding tensor term. Often one sees the shielding tensor defined as where E is taken as the electronic energy of the system. The subscript B = I; = 0 signifies that the components of the shielding tensor are, by definition, the terms in the energy expansion in terms of the external magnetic field 6 and the nuclear moment @ that are bilinear in the various components of these two quantities. This can be confusing because whereas the external field B is indeed a parameter in which the electronic energy (and wavefunction) can ,be expanded, the nuclear moment @ should really be treated as an opera- tor, and it is not immediately clear how one can take a derivative with respect to it. The definition in Eq. [3] does not refer to Eq. [2] and is clear only after a more formal derivation, which we now outline.

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  Spectroscopy With Coherent Radiation: Selected Papers Of Norman F Ramsey (With Commentary)

  Selected Papers of Norman F Ramsey (With Commentary)

  • Norman F Ramsey(Author)
  • 1998(Publication Date)
  • World Scientific

    (Publisher)

  341 Section 4 THEORIES OF NUCLEAR MAGNETIC SHIELDING AND NMR CHEMICAL SHIFTS I became interested in this problem before its importance for NMR chemical analysis was recognized. In 1947, as I was planning the high resolution molecular beam apparatus described in Paper 2.4,1 was confident the apparatus would give the expected high precision in measuring the nuclear precession frequencies, but I realized our derived nuclear magnetic moments would be much less precise due to the unknown corrections for the magnetic shieldings of the nuclei by the molecular electrons. The application of the external magnetic field induces a circulation of the electrons which makes the magnetic field at the nucleus slightly different from the one applied. W. Lamb [Phys. Rev. 60, 817-820 (1941)] had developed a theory for such shielding in atoms, for which there is a single central force and the applied magnetic field induces a simple circular rotation of the electron distribution at the well-known Larmor precession frequency. I realized that in a molecule, with no single center, the electron motion and hence the calculation of the magnetic shielding would be more complicated. Furthermore, the shielding for the same nucleus would be different in different molecules. I developed a general theory of magnetic shielding in molecules analogous to J. H. Van Vleck's successful theory of diamagnetic susceptibility (Electric and Magnetic Susceptibilities, Oxford University Press, 1932). I published my theory in Papers 4.1 and 4.4, where I averaged the shielding over all orientation states of the molecules, since frequent collisions produce such averaging for NMR experiments with gases and liquids. However, that averaging does not occur in molecular beam experiments or in NMR experiments with crystals, so I later wrote Paper 4.3, on the dependence of the shielding upon molecular orientation.

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  Nuclear Magnetic Resonance

  Volume 1

  • R K Harris(Author)
  • 2007(Publication Date)
  • Royal Society of Chemistry

    (Publisher)

  This empirical equation is essentially the same as equation (38), with the addition of the final term to represent intramolecular dispersion forces; the quantity (E2 ) is identical with F o f equation (41). For the compounds with hydrogen as substituent fairly close agreement was obtained between calculated and observed quantities, whatever the choice of sign for the C-H bond moment. The work of Homer and Callaghan indicates the importance of electric-field effects and dispersion forces on nuclear shielding. G. Steric Effects.-The introduction of a bulky substituent group into a molecule may often lead to a change in the molecular conformation or the molecular geometry. Such changes will inevitably cause magnetic nuclei elsewhere in the molecule to be differently shielded from those in the un- substituted compound. Effects such as this are clearly steric in origin but in the present subsection such effects are excluded since the shielding changes actually occur through the various other mechanisms described in the present section. Here we are dealing with the situation when two nuclei in the same molecule, nuclei not bonded to each other, are so close together that their electron clouds overlap. The consequent overlap forces will produce shielding changes for both nuclei. This may take place either indirectly, because of small alterations of molecular geometry, or directly, because of a mutual distortion of the electron clouds, or both. Some experimental proton reson- ance results for some condensed, benzenoid hydrocarbons that are probably influenced by steric effectshave been given earlier in Section 5D. Although, as mentioned there, a deshielding is usually observed, it is not as yet firmly established whether steric effects always cause a deshielding.

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