London theory

The atomization energy, electron affinity and ionization potential have been calculated for 1//-azepine. and a difference in energy between the boat and chair forms of 64.8 kJ mol -1 deduced.98 The calculated dipole moment for l//-azepine is 4.67 D.98 Hiickel-London theory has been applied to calculate the ring-current octopole hypersusceptibilities of l//-azepine."... 

London theory U — I = ionization energy empirical evaluate U from El and correlate (U/I) with number of electrons in system. 

Thus, attempts to extend the London theory to distances at which (5.20) is violated must lead to unphysical (non-Hermitian) perturbation corrections, with increasingly severe mathematical and physical contradictions. These difficulties are in contrast to the corresponding NBO-based decomposition (5.8), which remains Pauli-compliant and Hermitian at all distances. 

But it was not really until 1931, when Slater and Pauling independently developed methods to explain directed chemical valence by orbital orientation that it can truly be said that a chemical quantum mechanics, rather than an application of quantum mechanics to chemistry, had been created. In a study of Slater, S. S. Schweber notes the distinction between the Heitler-London-Pauling-Slater theory and the Heitler-London theory. Heitler and London successfully explained the electron-valence pair on the basis of the Goudsmit-Uhlenbeck theory of spin. Slater and Pauling explained the carbon tetrahedron. This second explanation distinguishes quantum chemistry from quantum physics.2... 

The localized-electron model or the ligand-field approach is essentially the same as the Heitler-London theory for the hydrogen molecule. The model assumes that a crystal is composed of an assembly of independent ions fixed at their lattice sites and that overlap of atomic orbitals is small. When interatomic interactions are weak, intraatomic exchange (Hund s rule splitting) and electron-phonon interactions favour the localized behaviour of electrons. This increases the relaxation time of a charge carrier from about 10 s in an ordinary metal to 10 s, which is the order of time required for a lattice vibration in a polar crystal. 

Fig. 54. (a) Transition line Hq(T) (circles) of LuNi2B2C. Inset Hq(T) (dashed line) predicted by the GL theory without fluctuations. The solid lines in (a) show //c2(7 ). (b) Transition lines Hq(T) predicted by the nonlocal London theory including thermal fluctuations for various values of p which is a measure of nonlocality... 

To understand why the ion—ion dispersion interactions have been ignored by most theories of electrolytes, let us perform some simple calculations. The London theory provides for the ion—ion dispersion interactions 8 9... 

F. London, Theorie quantique des courants interatomiques dans les combinaisons aroma-tiques, J. Phys. Radium 8 (1937) 397. 

The Meissner effect is the exclusion of an external magnetic field from the bulk of the superconductor. By London theory the magnetic induction is... 

Fig. 7.3. The penetration depth vs. the reduced temperature By London theory...

From the experimental values of dipole moments it is possible, in a number of cases, to make a semi-quantitative evaluation of the weights of the various valence bond structures contributing to a bond (see Chapter 18). These calculations must be regarded as only approximate since the bond is described in terms of the Heider-London theory with the superposition of ionic states. The results cannot, therefore, be more precise than is permitted by the Heitler-London approximation. Nevertheless, the calculations are of significance since they permit an assessment to be made of the more important structures contributing to the bond and thus assist in predicting and explaining the reactivity of bonds. 

The theory of Born and Mayer has been extended by the work of Landshoff using the methods of quantum mechanics. Taking sodium chloride as an example, Landshoff accepts the assumption that the lattice consists of Na+ and Cl ions and calculates the ionic interaction energy on the basis of the Heitler-London theory using the known distributions of electrons in the Na+ and Cl " ions. In addition to the correction terms of Bom and Mayer, additional interactions related to the superposition of the electron clouds, the attraction between electrons and nuclei and the mutual repulsion of electrons are incorporated. The values obtained by this more exact method, however, differ from the values given in Table CXLVII by only a few kcals, the value for sodium chloride being 183 kcals. 

Using the London theory in its simplest form, and with the above reservations, one can show that the solvent shift of electronic excitation energy for attractive interactions only is (42)... 

The London theory was later modified to account for retardation effects occurring at greater separation distances ... 

The equation for the cross-energy parameter fli2 can be justified theoretically. It is based on the London theory for intermolecular forces combined with the Mie function for the intermolecular potential. We offer here a short derivation, as discussed recently. ... 

The fundamental idea of the Heitler-London theory of valency bindiug is as follows. As a model of the hydrogen molecule we imagine two nuclei a and h on the c-axis at a distance It apart, and two electrons 1 and 2 revolving about the nuclei. To the state of two widely-separated neutral atoms there corresponds a large value of It and a motion of the electrons such that each one revolves round one of the two nuclei. Let the two atoms be in the ground state and have the... 

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