Huckel. Erich

Hoffmaim-Ostenhoff Maria, 423 Hoffmaim-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 Huckel Erich, 392,427 Huckel Walter, 427 Hull Erika, 715 Hund Friedrich Hermann, 161,461 Hurley Andrew... 

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 m 1931 when Erich Huckel dis covered an interesting pattern m 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 Huckel found that whether a hydrocarbon of this type was aromatic depended on its number of tt electrons He set forth what we now call Huckel s rule... 

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 Huckel to explain some of the unique properties of unsaluralcd and aromatic hydrocarbons (Huckel 1931 for historical... 

We start with some biographical notes on Erich Huckel, in the context of which we also mention the merits of Otto Schmidt, the inventor of the free-electron model. The basic assumptions behind the HMO (Huckel Molecular Orbital) model are discussed, and those aspects of this model are reviewed that make it still a powerful tool in Theoretical Chemistry. We ask whether HMO should be regarded as semiempirical or parameter-free. We present closed solutions for special classes of molecules, review the important concept of alternant hydrocarbons and point out how useful perturbation theory within the HMO model is. We then come to bond alternation and the question whether the pi or the sigma bonds are responsible for bond delocalization in benzene and related molecules. Mobius hydrocarbons and diamagnetic ring currents are other topics. We come to optimistic conclusions as to the further role of the HMO model, not as an approximation for the solution of the Schrodinger equation, but as a way towards the understanding of some aspects of the Chemical Bond. 

The criteria for a compound to be aromatic were developed by Erich Huckel. 

It turns out that in addition to being cyclic, planar, and completely conjugated, a compound needs a particular number of n electrons to be aromatic. Erich Hiickel first recognized in 1931 that the following criterion, expressed in two parts and now known as Huckel s rule, had to be satisfied, as well ... 

Erich Huckel (1896-1980) was born in Stuttgart, Germany, and received his Ph.O. at the University of Gottingen with Peter Debye. He was professor of physics, first at Stuttgart and later at Marburg (1937-1961). 

German physicist Erich Hiickel used the molecular orbital theory to explain the stability of benzene and other aromatic compounds. Huckel s rule determines the number of ir electrons that give stability to an unsaturated planar ring according to the formula 4n -I- 2. Eor benzene and its analogs,... 

In 1923 Peter Debye and Erich HUckel published two remarkable papers that described an a priori method of calculating the activity coefficient of electrolytic solutes in dilute solution. Without doubt this was one of the major breakthroughs in electrolyte solution theory. 

Debye-Huckel theory A theory to explain the nonideal behaviour of electrolytes, published in 1923 by Peter Debye (1884-1966) and Erich HUckel (1896- ). It assumes that electrolytes in solution are fully dissociated and that nonideal behaviour arises because of electrostatic interactions between the ions. The theory shows how to calculate the extra free energy per ion resulting from such interactions, and consequently the activity coefficient. It gives a good description of nonideal electrolyte behaviour for very dilute solutions, but cannot be used for more concentrated electrolytes. 

Fig. 1.12 Erich Huckel (Courtesy of William B. Jensen and the Oesper Collection in the History of Chemistry, University of Qncinnati)...

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