Chemical reactivities symmetry rules

The interpretation of chemical reactivity in terms of molecular orbital symmetry. The central principle is that orbital symmetry is conserved in concerted reactions. An orbital must retain a certain symmetry element (for example, a reflection plane) during the course of a molecular reorganization in concerted reactions. It should be emphasized that orbital-symmetry rules (also referred to as Woodward-Hoffmann rules) apply only to concerted reactions. The rules are very useful in characterizing which types of reactions are likely to occur under thermal or photochemical conditions. Examples of reactions governed by orbital symmetry restrictions include cycloaddition reactions and pericyclic reactions. 

Professors Kenichi Fukui (Kyoto University) and Roald Hoffmann (Cornell University) received the 1981 Nobel Prize in Chemistry for their quantum mechanical studies of chemical reactivity. Their applied theoretical chemistry research is certainly at the core of computational chemistry by today s yardstick. Professor Fukui s name is associated with frontier electrons, which govern the transition states in reactions, while that of Hoffmann is often hyphenated to R. B. Woodward s name in regard to their orbital symmetry rules. In addition, Professor Hoffmann s name is strongly identified with the extended Hiickel molecular orbital method. Not only was he a pioneer in the development of the method, he has continued to use it in almost all of his over 300 papers. 

The period under discussion has seen intense interest in the recently discovered i7 -H2 complexes. The relevance of these to some isomerization reactions of square-planar complexes was reported in Volume 5 of this series, and is covered in another recent review. " More of these fluxional ds-dihydridoplatinum compounds have been reported, and the role of 17 -H2 derivatives in oxidative additions to d rhodium(I) and iridium(I) has been discussed. The increasing role of theoretical and bonding studies is reflected in four works relevant to 4-and 5-coordinate molecules. Electronic structure is related to chemical reactivity in the reactions of phosphine bases with d bis(l,l-dithiolato)platinum com-plexes. Huckel calculations on the reactions of bis(nitrogen donor) ligands with 16-electron platinum(II) complexes have been carried out, as has more work on symmetry selection rules for isomerization reactions, which includes pseudorotation of 5-coordinate complexes and square-planar to tetrahedral conversions of 4-coordinate molecules. ... 

The first rule is to use the easiest-to-bond type of rubber that will provide the required service performance of the part. In general, there is a hierarchy among rubber types which ranks them according to their ability to be bonded with adhesives. This hierarchy is called the bondability index [4]. What causes differences in bondability is still a matter of debate. It has been attributed to differences in polarity, chemical reactivity, solubility and molecular symmetry between the different available classes of rubbers [4]. Regardless of the cause, the bondability of general purpose rubbers is ranked as follows ... 

In 1952, Fukui published his Frontier MO theorywhich went initially unnoticed. In 1965, Woodward and Hoffmann published their principle of conservation of orbital symmetry, and applied it to all pericyclic chemical reactions. The immense success of these rules" renewed interest in Fukui s approach and together formed a new MO-based framework of thought for chemical reactivity (called, e.g., giant steps forward in chemical theory in Morrison and Boyd, pp. 934, 939, 1201, and 1203). This success of MO theory dealt a severe blow to VB theory. In this area too, despite the early calculations of the Diels-Alder and 2-1-2 cycloaddition reactions by Evans,VB theory missed making an impact, in part at least because of its blind adherence to simple resonance theory. All the subsequent VB derivations of the rules (e.g., by Oosterhoff in Ref. 90) were after the fact and failed to reestablish the status of VB theory. 

Over the years, different approaches have been developed to reveal chemical bonds. Covalent bonds are intuitively represented using conventional Lewis stractures [19]. Molecular Orbital (MO) theory has been veiy useful and successfiil for the theoretical analysis of chemical reactions and chemical reactivity. The frontier orbital theory [20] and the orbital symmetry rules of Woodward and Hoffman [21] are paradigmatic examples of the possibilities of quantum chemistry within the MO theory. 

The question about the transformations of reactants into products by the molecular reaction (direct way) and chain (complicated, multistage way) was considered in the monograph by N.N. Semenov On Some Problems of Chemical Kinetics and Reactivity. The new approach appeared because the quantum chemistry formulated the rule of conservation of orbital symmetry in chemical and photochemical reactions (Woodward-Hofinann rule). Let us consider this interesting aspect. 

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