Beginning There Was Quantum Chemistry and Spectroscopy

In the years 1945-1955 theoretical chemists as well as chemical spec-troscopists concentrated their efforts on systems containing n electrons. Such 

Sandorfy was struck by the artificial character of this approximation. His first target was the series of normal chain saturated hydrocarbons. A preliminary note on his results was published in the Comptes Rendus de VAcademie des Sciences.6 After taking up his position in Montreal, he greatly extended this work by introducing three different approximations, which he called the C CH, and H approximations.7 The first was based only on the atomic orbitals of the carbons, the second used the hybrid sp3 orbitals of the carbons and the Is orbitals of the hydrogens, and the third included all the valence atomic orbitals of the carbons and the hydrogens. 

These methods appear rather simple, yet they were the starting point of a long evolution. Gilles Klopman, whose research interests at Case-Western Reserve University later turned to modeling bioactive molecules, was the first to use Sandorfy s methods. Kenichi Fukui made extensive use of them in his well-known work on the structures and reactions of saturated hydrocarbons and their derivatives. Fukui added his frontier orbital considerations. Around 1959 the milieu of developments in quantum chemistry contributed to inspire William N. Lipscomb to conceive the extended Hiickel method, which was subsequently implemented by Lawrence L. Lohr and Roald Hoffmann.83 Soon thereafter, John Pople and his coworkers introduced self-consistent field methods based on the zero-differential overlap approximation.815 

The arrival of ab initio programs based on Gaussian-type orbitals and more powerful computers reduced all these developments to historical souvenirs. It should be emphasized, however, that Sandorfy s work constituted the first attempt to go beyond the 7t-electron approximation, and the first molecular orbital treatment of polyatomic molecules that took into account all valence electrons without using group orbitals. 

From among Sandorfy s subsequent theoretical projects, two in particular should be mentioned the first treatment of the excited states and electronic spectra of saturated hydrocarbons9 by a new rc-electron method of the Pariser-Parr type, and the first attempt to introduce Rydberg orbitals into molecular orbital calculations.10 

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