Hybrid Orbitals and Directed Valence
The following quotation is taken from the recent monograph
Jan C. A. Boeyens, "Chemistry from First Principles"
Springer, page 62, 2008
In contrast to the four tetrahedrally oriented elliptic orbits of the Sommerfeld model, the new theory leads to only three, mutually orthogonal orbitals, at variance with the known structure of methane. A further new theory that developed to overcome this problem is known as the theory of orbital hybridization. In order to simulate the carbon atom's basicity of four an additional orbital is clearly required. The only possible candidate is the 2s orbital, but because it lies at a much lower energy and has no angular momentum to match, it cannot possibly mix with the p-eigenfunctions on an equal footing. The precise manoeuvre to overcome this dilemma is never fully disclosed and appears to rely on the process of chemical resonance, invented by Pauling to address this, and other, problems. With resonance, it is as sumed that, linear combinations of an s and three p eigenfunctions produce a set of hybrid orbitals with the required tetrahedral properties.
This contrived solution of the problem was gratefully embraced by the chemical community. It has never been challenged successfully for almost eighty years, despite many glaring defects.
A significant attempt to challenge these ideas was made in 1955 by B. F. Gray, M.Sc. Thesis, Manchester (1955); the thesis was divided into two parts, one experimental dealing with the thermal decomposition of octafluorocyclobutane, the other examining misunderstandings about the relationship of orbital hybridisation with bond angles and dipole moments. In accepting the thesis, after 5 pages of blunt criticism, the External Examiner concluded:
". . . . . . . that the author obviously possesses a critical and lively mind and that he has put his finger on one or two real difficulties which beset the idea of hybridisation as it is discussed in the current literature. His thesis, though decidedly "brash" represents a challenge to theoretical chemists to clarify their concepts . . . . . . , and it is a healthy sign to find a research student who is so critical of present ideas in theoretical chemistry."
The challenge never succeeded; the work was written up for journal publication, but encountered stiff resistance, and the triply-rejected manuscript (1.6 Mb) is reproduced here, "warts and all" (in those days, rejected manuscripts were often salvaged with, as far as possible, only hand-written corrections to avoid delays at the typing pool)! A sanitised rump was published as "Electrostatic Interpretation of Directed Valence" in J. Chem. Phys., 25, 779-780 (1956) (115 Kb).
Also reproduced here is the programme for the Manchester Mark I computer (1.0 Mb), complete with running notes and copy of the programme tape (233 Kb), used to create the diagrams (168 Kb) in this long-forgotten manuscript.
It is the most completely-documented scientific programme for this computer that is known to exist.