Huckel’s theory

Huckel s theory initiated a wealth of experimental work, all of which fitted with the prediction of enhanced stabilization in (4n + 2)7t-systems. The measure of variegated physical and chemical quantities (such as heats of combustion and hydrogenation, magnetic susceptibilities, electronic spectra and the proclivity to react in Diels-Alder reactions) was found to correlate with the extent of aromatic character, as estimated by the theoretical methods 86). 

This non-mobile spectrum shows [18]annulene to be aromatic, a fact which follows from Huckel s theory, together with the finding 118> that this molecule is almost planar, displacement of the carbon atoms from the mean plane being less than 0.1 A. Further X-ray crystallographic analysis 114> shows that there is no bond alternation in this molecule, (but not all bonds are exactly the same length). 

More advanced developments of Huckel s theory, such as the Pariser, Parr, Pople Self-Consistent Field Theory, which takes into account the interaction of the electrons, have led to a deeper understanding of the aromatic concept and are widely applied to determine physical properties of aromatics, particularly when describing dyestuffs. 

Over 100 years have passed since Arrhenius published his Dissociation Theory of Electrolytes in 1887. Prior to this it was believed that electrolytes did not dissociate into ions in water until current was passed, and Arrhenius work was not well received. It was some decades after this that Born s theory of ionic solvation, and then, Debye and Huckel s theory of ionic activities in... 

The sixth and last part of this collection consists of Huckel s theory of the double bond. 

A Frontier Molecular Orbital Approach to Electron Transport In considering the frontier molecular orbitals the electron transport through a 7t-conjugated molecular wire can be explained easily. In case of molecular wire in the electronic circuit, when the bias voltage will be applied, one electron will be loaded from the cathode to the LUMO of the molecule. According to the Huckel s theory due to the extended 7t-conjugation present in the molecule, the LUMO will be fully delocalized all over the molecule and thus can create the channel for electrical conduction. In other words, the loaded electron in the molecule can freely travel from one part to the other with the sustained delocalized 7 -type nature of the LUMO and finally escapes to the other electrode thus... 

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... 

Huckel proposed his theory before ideas of antiaromaticity emerged We can amplify his generalization by noting that among the hydrocarbons covered by Huckel s rule those with An) tt electrons not only are not aromatic they are antiaromatic... 

The article A History of the Structural Theory of Benzene—The Aromatic Sex tet and Huckel s Rule in the February 1997 issue of the Journal of Chemical Educa tion (pp 194-201) IS a rich source of additional informa tion about this topic... 

We saw in Chapter 12 that aromaticity reveals itself in various ways. Qualitatively, aromatic compounds are more stable and less reactive than alkenes. Quantitatively, their heats of hydrogenation are smaller than expected. Theory, especially Huckel s rule, furnishes a structural basis for aromaticity. Now let s examine some novel features of their NMR spectra. 

While benzene was the first aromatic system studied, the formulation of HtickePs rule and the theory behind it created an impetus to prepare non-benzenoid species such as the tropylium cation and cyclopentadienyl anion that also obeyed Huckel s rule to see if these species were also aromatic. This required that the properties of aromatic compounds be defined. 

The Longuet-Higgins theory based on Huckel MO theory and Hund s rule was pioneering work in the area of molecular design of high-spin alternant hydrocarbons and has been appreciated for its predictability. 

It is strong evidence for Huckel s rule that 52 and 53 are not aromatic while the cyclo-propenyl cation (47) and the cyclopentadienyl anion (31) are, since simple resonance theory predicts no difference between 52 and 47 or 53 and 31 (the same number of equivalent canonical forms can be drawn for 52 as for 47 and for 53 as for 31). 

If one organic compound has dominated the historical literature of the last few years, that compound must be benzene. Most probably, this is because its structure in some respects marks a transition from the most austere form of classical organic chemistry, in which carbon was tetravalent and tetrahedral, to a continuing series of changes from oscillating molecules, through partial valencies to MO descriptions, and Huckel s rules of aromaticity. It is the case par excellence of a single substance whose history intersects all major streams of chemical theory - except perhaps the periodic law - and which also has enormous industrial and economic importance. 

This method, Huckel s 1st method or the H(eitler) L(ondon) S(later) P(auling) method or the Valence bond (V.B.) method, fits directly on to the electron pair theory of valency of Lewis, Langmuir, Ingold and others. Each configuration can be described by bond lines, each of which signifies an electron pair between the various atoms. It can be proved that these lines never intersect in the canonical structures configurations, in which this is the case, can be regarded as combinations of the former. 

Does Mayer s theory of calculating the viriai coefficients in equations such as Eq. (3.165) (which gives rise directly to the expression for the osmotic pressure of an ionic solution and less directly to those for activity coefficients) really improve on the second and third generations of the Debye-Huckel theory—those involving, respectively, an accounting for ion size and for the water removed into long-lived hydration shells ... 

Hofmann rearrangement, 118 Homotopic groups, 9 Homotropenylium, 152 Huckel array, 85 Huckel MO theory, 35, 86 Huckel 4n + 2 rule, 150, 151 Hund s rule, 114 Hunsdiecker reaction, 305-307 Hybridization, 15, 16... 

A long period elapsed before significant extensions were made to Huckel s 1931 molecular orbital theory. In the 1950s mutual repulsion energies were taken into account by Pariser, Parr, (1953) and Pople (1953) in calculating matrix elements. Hoffmann (1963) devised a scheme whereby the important off diagonal matrix elements for a bonds as well as n bonds were calculated as the arithmetic mean of the adjacent diagonal elements... 

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