Hiickel. Erich

Housefly, sex attractant of, 255 HPLC, 432 Hiickel, Erich, 523 Htickel 4/j + 2 rule, 523... 

Hoffmann-Ostenhoff Maria, 423 Hoffmann-Ostenhoff Thomas, 423 Hogervorst Wim, 150 Hohenberg Pierre, 675 Holthausen Max C., 676, 715, 665, 676 Hooke Robert, 349 Howard Brian J., 899 Hubble Edwin Powell, 594 Hue Ivan, 971, 976 Hiickel Erich, 392, 427 Huckel Walter, 427 Hull Erika, 715 Hund Friedrich Hermann, 161, 461 Hurley Andrew... 

Hiickel, Erich Armand Arthur Joseph (1896-1980) German physical and theoretical chemist. Hiickel has two main claims to fame. His first is the theory of electrolytes which Peter Debye and he produced in 1923 and which gives a good description of the electrical and thermodynamic properties of dilue electrolytes. His second major work was the application of quantum mechanics to aromatic molecules such as benzene. He used molecular orbital theory to show that in the benzene molecule the electrons in the pi orbitals are spread out directly above and below the ring of carbon atoms, thus making the molecule more stable than it would be if one had alternating double and single bonds. Hiickel started this work in 1930 and soon extended it to predict the Hiickel rule which states that a molecule is aromatic if it has (An + 2) pi electrons. 

HMO theory is named after its developer, Erich Huckel (1896-1980), who published his theory in 1930 [9] partly in order to explain the unusual stability of benzene and other aromatic compounds. Given that digital computers had not yet been invented and that all Hiickel s calculations had to be done by hand, HMO theory necessarily includes many approximations. The first is that only the jr-molecular orbitals of the molecule are considered. This implies that the entire molecular structure is planar (because then a plane of symmetry separates the r-orbitals, which are antisymmetric with respect to this plane, from all others). It also means that only one atomic orbital must be considered for each atom in the r-system (the p-orbital that is antisymmetric with respect to the plane of the molecule) and none at all for atoms (such as hydrogen) that are not involved in the r-system. Huckel then used the technique known as linear combination of atomic orbitals (LCAO) to build these atomic orbitals up into molecular orbitals. This is illustrated in Figure 7-18 for ethylene. 

One of molecular- orbital theories early successes came in 1931 when Erich Hiickel discovered an interesting pattern in the tt orbital energy levels of benzene, cyclobutadiene, and cyclooctatetraene. By limiting his analysis to monocyclic conjugated polyenes and restricting the structures to planar- geometries, Hiickel found that whether a hydrocar bon of this type was aromatic depended on its number of tt electrons. He set forth what we now call Hiickel s rule ... 

Erich Hiickel (1896-1980) was born in Stutigart, Germany, and received his Ph D. at the University of Gottingen with Peter Debye. He was professor of physics, first at Stuttgart and later at Marburg (1937-19611. 

One important stracture in molecules are polar bonds and, as a result, polar molecules. The polarity of molecules had been first formulated by the Dutch physicist Peter Debye (1884-1966) in 1912, as he tried to build a microphysical model to explain dielectricity (the behaviour of an electric field in a substance). Later, he related the polarity of molecules to the interaction between molecules and ions. Together with Erich Hiickel he succeeded in formulating a complete theory about the behaviour of electrolytes (Hofimann, 2006). The discovery of the dipole moment caused high efforts in the research on physical chemistry. On the one hand, methods for determining the dipole momerrt were developed. On the other hand, the correlation between the shape of the molectrle and its dipole moment was investigated (Estermanrr, 1929 Errera Sherrill, 1929). 

An appreciable advance in the theory of electrostatic interaction between ions in solution was made in 1923 by Peter Debye and Erich Hiickel, who introduced the concept of ionic atmosphere to characterize the averaged distribution of the ions. In its initial form the theory was applied to fully dissociated electrolytes hence, it was named the theory of strong electrolytes. 

The beginning of the twentieth century also marked a continuation of studies of the structure and properties of electrolyte solution and of the electrode-electrolyte interface. In 1907, Gilbert Newton Lewis (1875-1946) introduced the notion of thermodynamic activity, which proved to be extremally valuable for the description of properties of solutions of strong electrolytes. In 1923, Peter Debye (1884-1966 Nobel prize, 1936) and Erich Hiickel (1896-1981) developed their theory of strong electrolyte solutions, which for the first time allowed calculation of a hitherto purely empiric parameter—the mean activity coefficients of ions in solutions. 

For all his interest in combining physics and chemistry, Lowry was not much convinced in 1925, on the eve of the breakthroughs by Heitler and London, Hund, and Erich Hiickel, that the physicists most recent mechanics had benefited chemists. No doubt, Lowry told colleagues at the second Solvay chemistry conference, the physical chemist should learn to think in terms of quanta and energy levels, but the mineral chemist and the organic chemist had not yet gained much from these latest physical theories.35 In 1931, as we saw in chapter 6, Kirrmann still was of the opinion that the time had not yet arrived for the "mathematical stage" of chemical explanation. 

An aromatic compound has a molecular structure containing cyclic clouds of delocalized tt electrons above and below the plane of the molecule, and the TT clouds contain a total of 4n -f 2) tt electrons (where n is a whole number). This is known as Hiickel s rule (introduced first by Erich Hiickel in 1931). Eor example, benzene is an aromatic compound. 

In 1923, Peter Debye and Erich Hiickel developed a classical electrostatic theory of ionic distributions in dilute electrolyte solutions [P. Debye and E. Hiickel. Phys. Z 24, 185 (1923)] that seems to account satisfactorily for the qualitative low-ra nonideality shown in Fig. 8.3. Although this theory involves some background in statistical mechanics and electrostatics that is not assumed elsewhere in this book, we briefly sketch the physical assumptions and mathematical techniques leading to the Debye-Hiickel equation (8.69) to illustrate such molecular-level description of thermodynamic relationships. 

To further illuminate the LCAO variational process, we will carry out the steps outlined above for a specific example. To keep things simple (and conceptual), we consider a flavor of molecular orbital theory developed in the 1930s by Erich Hiickel to explain some of the unique properties of unsaturated and aromatic hydrocarbons (Hiickel 1931 for historical... 

In 1931, Erich Htickel postulated that monocyclic (single ring) planar compounds that contained carbon atoms with unhybridized atomic p orbitals would possess a closed bond shell of delocalized n electrons if the number of n electrons in the molecule fit a value of 4 + 2 where n equaled any whole number. Because a closed bond shell of n electrons defines an aromatic system, you can use Hiickel s Rule to predict the aromaticity of a compound. For example, the benzene molecule, which has 3 n bonds or 6 n electrons, is aromatic. 

Chapter 4 showed how quantum mechanics was first applied to molecules of real chemical interest (pace chemical physics) by Erich Hiickel, and how the extension of the simple Hiickel method by Hoffmann gave a technique of considerable usefulness and generality, the extended Hiickel method. The simple and the extended Hiickel methods (SHM and EHM) are both based on the Schrodinger equation, and this makes them quantum mechanical methods. Both depend on... 

We have already seen examples of semiempirical methods, in Chapter 4 the simple Hiickel method (SHM, Erich Hiickel, ca. 1931) and the extended Hiickel method (EHM, Roald Hoffmann, 1963). These are semiempirical ( semi-experimental ) because they combine physical theory with experiment. Both methods start with the Schrodinger equation (theory) and derive from this a set of secular equations which may be solved for energy levels and molecular orbital coefficients (most efficiently... 

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