Hoffmann classical’ methods

Molecular orbital calculations for the parent vinyl cation, Cj H3, were first reported by Hoffmann (161), who used the extended Hiickel method, and more recently by Yonezawa and co-workers (162), who used a semiempirical SCF procedure. Both treated the problem of classical, 172 (R = H), versus bridged structures, 173, but the methods suffered from their inability to account satisfactorily for bond-length changes, and neither discussed the question of linear, 172a, versus bent, 172b, structures. 

A classic story of this line of research was the discovery of a method for the manufacture of the dye now known as mauve by the English chemist Sir William Henry Perkin (1838-1907). In 1856, Perkin was an 18-year-old student of the great German chemist August Wilhelm von Hoffmann (1818-92), superintendent of the Royal College of... 

Although Otto Diels and Kurt Alder won the 1950 Nobel Prize in Chemistry for the Diels-Alder reaction, almost 20 years later R. Hoffmann and R. B. Woodward gave the explanation of this reaction. They published a classical textbook, The Conservation of Orbital Symmetry. K. Fukui (the co-recipient with R. Hoffmann of the 1981 Nobel Prize in Chemistry) gave the Frontier molecular orbital (FMO) theory, which also explains pericyclic reactions. Both theories allow us to predict the conditions under which a pericyclic reaction will occur and what the stereochemical outcome will be. Between these two fundamental approaches to pericyclic reactions, the FMO approach is simpler because it is based on a pictorial approach. Another method similar to the FMO approach of analyzing pericyclic reactions is the transition state aromaticity approach. 

The TEC model developed by Teo also has been successfully applied to rationalize the geometries of a large number of cluster compounds. The TEC model combines Lauher s rule with Euler s theorem and adds an adjustable parameter This parameter X is equal to the number of electron pairs present in excess of that predicted by the 18-electron rule. " X has also been interpreted in terms of the number of missing antibonding orbitals. Given a value for X, determined by the shape of the cluster, an equation predicts the electron count for a cluster. Theoretical justification of the parameter X is based largely upon the classical molecular orbital calculations performed by Hoffmann and Lipscomb via the extended Hiickel method on the corresponding polyhedral boron hydride clusters The values... 

It may be worthwhile mentioning that most of the arguments in the development of the Woodward-Hoffmann rules have been substantiated with the results of EH calculations 4>83). Other EH molecular orbital studies were devoted to the elucidation of the mechanism of the Wolfe rearrangement 84>, and to the investigation of the hydrolysis of acetylcholine 85>. A simplified form of the EH method has been used to determine the role of non-classical ions in 1,2 rearrangements 86>. 

Other authors subsequently revisited these conclusions at higher levels of theory, which should provide more reliable energetics. However, the results are not clear-cut. As for the simple model systems of the type Cl2TiCH3+ and H2TiCH3+. calculations based on classical ab initio.GVB, dpt hs,iso level CCSD(T) methods ° are in agreement with the conclusions of Hoffmann. As the models include the more representative Cp rings, the results obtained with different methods are contradictory. If the... 

Lipscomb [3]. The title paper published by Hoffmann in 1963 [2] was featured as a citation classic in Current Contents in 1989 [4], where Hoffmann described the background of the EH method in detail. The firm foundation of the EH method was given in the title paper [2] and in the subsequent three papers published under the general title of ""Extended Hiickel theory " in 1964 [5-7]. In the EH method the MOs y fj i — 1,2,..., w) of a system are constructed using a set of valence atomic orbitals X X2 %3 The effective one-electron Hamiltonian, determining the... 

Throughout the studies discussed above, there has been a close interplay between structural measurements and development of theoretical models of molecular electronic structure. Lipscomb s contributions have included topological description of the boron hydrides and fundamental theory. Several coworkers contributed in this area, but most notable is the work with Roald Hoffmann, which changed the way chemists approach the theory of molecules of interesting complexity (see, e.g., the comments in (2(5)). The extended Huckel method was developed in the Lipscomb s group by several people, including especially L. L. Lohr, Jr., and Roald Hoffman (see Roald s recollections in Current Contents Citation Classic, May 8, 1989). Although the method probably contributed more to chemistry than any other method imtil very recently, Lipscomb recalled decades after he introduced it that this method received intense criticism, even denouncement. More exact theory led to the first correct calculation of the rotational barrier in ethane (with R. M. Pitzer in 1963). 

R. Hoffmann, J. Chem. Phys., 39, 1397 (1963). An Extended Huckel Theory. I. Hydrocarbons. See also, R. Hoffmann, Curr. Contents, Phys., Chem., Earth Set., 29, 20 (1989). Citation Classic A Simple Quantum Chemical Method. 

---