Since the 3rd century for more than a thousand years chemistry has been thought of as a complicated, hard-to-predict science. Efforts to improve even a part of its unpredictable character are said to have born fruit first of all in the success of the \" electronic theory \". This was founded mainly by organic chemists , such as Fry, Stieglitz, Lucas, Lapworth and Sidgwick, brought to a completed form by Robinson and Ingold, and developed later by many other chemists. 1 In the electronic theory, the mode of migration of electrons in molecules is noted and is considered under various judgements. For that purpose, a criterion is necessary with respect to the number of electrons which should originally exist in an atom or a bond in a molecule. Therefore, it can be said to be the concept by Lewis of the sharing of electrons that has given a firm basis to the electronic theory. 2 In the organic electronic theory, the chemical concepts such as acid and base, oxidation and reduction and so on, have been conveniently utilized from a long time ago. Furthermore, there are terms centring closer around the electron concept, such as electrophilicity and nucleophilicity, and electron donor and acceptor both being pairs of relative concepts. One may be aware that these concepts can be connected qualitatively to the scale of electron density or electric charge. In the electronic theory, the static and dynamic behaviours of molecules are explained by the electronic effects which are based on nothing but the distribution of electrons in a molecule. The mode of charge distribution in a molecule can be sketched to some extent by the use of the electronegativity concept of atoms through organic chemical experience. At the same time, it is given foundation, made quantitative , and supported by physical measurements of electron distribution and theoretical calculations based on quantum theory. The distribution of electrons or electric charge-with either use the result is unchanged-in a molecule is usually represented by the total numbers (generally not integer) of electrons in each atom and each bond, and it was a concept easily acceptable even to empirical chemists as having a tolerably realistic meaning. Therefore, chemists employed the electron density as a fundamental concept to explain or to comprehend various phenomena. In particular, for the purpose of promoting chemical investigations, researchers usually rely upon the analogy through experience, and the electron density …

https://science.sciencemag.org/content/sci/218/4574/747.full.pdf

Role of Frontier Orbitals in Chemical Reactions

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The replacement of the nondescript term ‘heavy metals’ by a biologically and chemically significant classification of metal ions

The equilibrium speciation of dissolved components in freshwater and sea water at 25°C and 1 atm pressure

The question of concerted or consecutive bond formation arises in all types of cycloaddition reactions. Stereochemical investigations and studies of regiospecificity as well as intensive kinetic investigations with regard to substituent and solvent effects, and the dependence of the reaction on temperature and pressure permit in many cases mechanistic conclusions concerning the Diels-Alder reaction. Efforts towards a theoretical interpretation, ab initio and semiempirical calculations, the application of frontier molecular orbital theory (FMO) as well as thermochemical measurements permit a description of the energy hypersurface of these [4+2]-cycloadditions. An attempt is made here to draw a line of distinction between the mechanistic alternatives—one-step reaction versus two-step reaction—considering all experimental and theoretical criteria.

Gilles. Klopman

Papers

Chemical reactivity and the concept of charge- and frontier-controlled reactions

Ground states of .sigma.-bonded molecules. I. Semiempirical S.C.F. molecular orbital treatment of hydrocarbons