THE NATURE OF THE CHEMICAL BOND 1993

J. F. Ogilvie

Academia Sinica Institute of Atomic and Molecular Sciences,

P. 0. Box 23-166, Taipei 10764, Taiwan

There are no such things as orbitals

....The important conclusion from this brief outline of a computational procedure is that, although one may start the calculation with a basic set of orbitals, the simple solutions of Schrodinger's equation for the one-electron atom, by the time that one attains the Hartree-Fock limit, or beyond, the nature of the initially chosen oneelectron functions is irrelevant. Thus only at the beginning of the calculation, and even then only in a mathematical sense (within the context of a particular computational method), do the orbitals have any meaning.

A novel approach to the equations of Dirac, Hartree, and Fock with the use of a finite. basis set was claimed [21] to be suitable for both atomic and molecular calculations with no problems of spurious roots, variational collapse or continuum that h.- plagued the conventional Dirac equation for applications to systems with many electrons; this development would permit in principle the calculation of atomic and molecular properties that suffer from the (self-imposed) tyranny of Schrodinger's equation, but during the several years this claim was announced little or no further progress has been reported. Thus the philosopher's stone for calculation of atomic and molecular structure, is, so far, as elusive as its literal precursor to make gold from base metal.

Therefore the structure of methane ... is a regular tetrahedron"; in a later edition [29], the question "Why is CH4 tetrahedral?" once again evokes an answer by reference to orbitals and hybridization, although the causal relationship is less succinctly stated. Gillespie [30) quoted an instance of a textbook of general chemistry in which the author wrote that the structure of methane is tetrahedral because of sp3 hybridization, and a few pages later that the hybridization is known to be sp3 because the structure is tetrahedral a completely and explicitly circular argument! Is the argument of Atkins [291 less circular because it is implicit?

We quote again from Coulson's Valence [28b]: "It would be quite wrong to say that, for example, CH4 was tetrahedral because the carbon atom was sp3 hybridised. The equilibrium geometry of a molecule depends on energy and energy only..

Coulson's Valence [28c], "... orbitals do not exist! They are artifacts of a particular theory, based on a model of independent particles. i. e. based on non-repelling electrons, For this reason also we refrain from interpreting photoelectron spectra as involving the ionization of electrons from (or even associated with) particular molecular orbitals, despite the widespread practice of this fallacy (for instance 5, 34). The classification of electrons as bonding, nonbonding or antibonding is similarly erroneous because electrons are fundamentally indistinguishable.

Why has CH4 a tetrahedral structure? Why does our solar system contain about nine planets? These are theological questions, thus extra-scientific. In the middle ages in Europe, learned philosophers (or theologians) are alleged to have debated how many angels could dance on the head of a pin; at a conference I have heard famous chemists disputing whether a certain effect in a transition-metal compound was due more to "pi donation" or to "back donation into d orbitals". In 1723 Jonathan Swift chronicled a voyage of one Lemuel Gulliver to Balnibarbi in which he observed speculative research on diverse topics; in the past sixty years, innumerable chemists have attributed chemical and physical phenomena of all kinds to [nonexistent] orbitals. Is the progress of man's thinking an illusion?

Chemistry is not only a science of molecules, but also a science of materials. Chemistry remains the only basic science to constitute the foundation of a major industry. Chemistry owes its importance in the modern community to its materials, not to its i-molecules. All the space devoted to orbitals, the aufbou principle, hybridization, resonance, sigma and pi bonds, electro-negativity, hyper-conjugation, HOMO, LUMO, inductive and mesomeric effects and the like excess baggage that burdens the textbooks of general, inorganic, organic and (even, if to a lesser extent) physical chemistry, and the corresponding proportion of the curriculum and duration of lecture and tutorial classes, detracts from more instructive and accurate content about chemical reactions, chemical substances, and mixtures as materials. The conspiracy interpretation' of quantum mechanics to which Condon [9b] referred has its analogue currently in the infatuation 0f many academic chemists with orbitals. The authors of textbooks clearly perpetrate myths such as that the structure of methane is tetrahedral because of sp3 hybridization, and similar fallacies not because they understand quantum mechanics. The readers of these textbooks, be they professors or students, duly perpetuate the same fictions because they apparently constitute the current paradigm in chemistry.

*"Perhaps the mood was best stunned up by Bergen Davis (1869-1958) ..... who commented on quantum mechanics in the spring of 1928 that, 'I don't think you young [physicists] understand it any better than I do, but you all stick together and say the same thing.' This has been called the conspiracy interpretation of quantum mechanics. " [9b]

A correspondent has stated that he "prefers a universe [in which] science can attempt to answer the big question 'WHY"'? For many chemists the answer to the question "why does some phenomenon occur?" is "because of orbitals", which is equivalent to "because of Schrodinger's equation". According to this approach the further question. "Why Schrodinger's equation?" although logical, is ignored because this problem lies clearly outside the province of chemical competence. If Schrodinger had devoted all his energies to his other pursuits, then the Schrodinger's equation might never have appeared. Would chemistry or physics have been the poorer? We should still have matrix mechanics that preceded the discovery of wave mechanics; because in principle the two calculation methods are entirely equivalent, algorithms to implement calculations of electronic structure would presumably have been developed in terms of matrices, in which case they might have been readily adaptable for efficient execution on current computers with vector processors. One might imagine the content of textbooks of general chemistry under these hypothetical circumstances. Whether an alternative explanation [85] of the chemical bond in terms of entropy of the electrons is useful or valid remains to be proved.

[85] Gankin, V. V. and Gankin, Y. V. (1991) "The New Theory of Chemical Bonding and Chemical Kinetics", ASTA, St. Petersburg, Russia.

Pauling wrote in The Consequent Implications for Chemical Education:

"The concept of the chemical bond is the most valuable concept in chemistry. Its development over the past 150 years has been one of the greatest triumphs of the human intellect. I doubt that there is a chemist in the world who does not use it in his/her thinking. Much of modern science and technology has developed because of the existence of this concept ".

"The truly great discoveries about the chemical bond were made by great chemists of the 19th century: Berzelius, Butlerov, Frankland, Couper, Kekule, Van't Hoff, and LeBel. G.N Lewis and Langmuir made significant contributions during the period of 1916 to 1920..."

"I agree with Ogilvie that some things, especially molecular orbitals, should be left out, but in my opinion, the chemical concept of the chemical bond, together with its recent refinements, must be included in the course, together with a good bit of descriptive chemistry... During recent years, much more information about molecules and crystals and their reactions, both experimental and theoretical, has become available, but this information has not decreased the value of the concept of the chemical bond. I am pleased and gratified that in 1992 the chemical bond is alive and well."