Theory of Localized Contributions to the Chemical Shift. Application to Fluorobenzenes

Abstract

An expression for the magnetic shielding tensor is obtained by the use of single-determinant Hartree-Fock molecular wave functions. For nuclei of atoms in which the change in the second-order (paramagnetic) contribution is dominant, LCAO theory is employed to express the shielding in terms of localized bond parameters (ionic character, hybridization, and double bonding) and to compare it with the related treatments of the quadrupole coupling constant. Application of the formulation to the multifluorobenzenes provides an explanation of the available experimental chemical shift data and permits the prediction of shift values for other compounds. Of particular interest is the demonstration that double bonding in the C–F bond plays an important role in the fluorobenzenes. Also, the presence of an ``ortho'' effect in the shift is isolated by a comparison of experimental and theoretical results and tentatively explained in terms of charge repulsions.

Nuclear magnetic resonance spectra of ortho-, meta-, and para-difluorobenzenes in a nematic liquid crystal solvent

“…Also included in Table IV (5) and those of Chan and Dubin (9). The latter results, however, disagreed in sign with the predictions of the Karplus and Das theory (10). This theory, originally intended for the calculation of isotropic chemical shifts, can be used to express the shielding tensor components, and hence the anisotropy, in terms of the bond parameters; hybridization s, ionic character I, and double bond character p. the expectation value of -for the r3 2p orbital.…”

Substituent effects in fluoromethylnaphthalenes by 19F nuclear magnetic resonance

“…Theory developed to explain the screening of a fluorine nucleus relates the chemical shift to a number of terms pertaining to the electron distribution around the fluorine (38)(39)(40). It is evident that this relationship does not approximate to a relationship involving only the total electron density at fluorine.…”

Zur theoretischen Interpretation der Anisotropie der 13C-NMR-chemischen Verschiebungen des Ethylens und Benzens auf halbempirischer Grundlage

“…halbquantitativen Ab schätzung der isotropen 13C-chemischen Verschie bungen hat sich aber auch die sogenannte AENäherung bewährt. Dabei werden die Anregungs energien Ej -Ei durch eine mittlere Anregungs energie AE ersetzt [10] und der Abschirmungs beitrag kann ausschließlich durch den elektroni schen Grundzustand des Moleküls beschrieben werden [5,11] (vgl. auch [9]): e2 H2 g.AA __ 2 m2 c2AE 0 -3> 2P…”