# Origins of molecular orbital diagrams?

Does anyone remember who proposed molecular diagrams for simple molecules as taught today in most general chemistry texts? I cannot access Hund's original article, however, Mulliken's early articles on molecular orbitals do not show any diagram like this. For example, The Assignment of Quantum Numbers for Electrons in Molecules (MULLIKEN, Phys. Reviews 1928) does not have a single figure. Thanks

You mean Hund (1927)? I don’t know if he was first, but p. 756:

(Similar depictions for Zeeman splitting occur in Sommerfeld (1922, p. 553) and probably earlier.)

Added: Hall (1991) points to Lennard-Jones (1929) who “introduced the Linear Combination of Atomic Orbitals approximation for the molecular orbitals” and has (p. 679):

The possible molecular states are shown diagrammatically in Fig. 4, each being represented by a cell which can contain two electrons of opposite spins.

$$\hspace{5.5em}$$

Later in (1931, pp. 314–318) Lennard-Jones also has:

Let $$A$$ and $$B$$ be two atoms, which, when infinitely distant, are in $$S$$ states of multiplicity $$2s_a+1$$ and $$2s_b+1$$. (...) The energy levels of the system $$AB$$ depend on the distance between the atoms, and the lower levels usually have a minimum, which is the equilibrium distance of the molecule $$AB$$. An example of the relation between the energy levels of separated atoms and the same atoms when interacting with each other is shown in Fig. 8.

• Isn't there any citation of previous article? – user7532 Oct 17 '18 at 18:04
• I feel the united nuclear model (Vereinigte Kerne) was a predecessor to molecular orbital theory. – M. Farooq Oct 18 '18 at 0:34
• Sommerfeld's diagram is showing the effect of magnetic field (Zeeman effect ) on sodium atoms on a pair of doublets. The sigma and pi notation there shows the polarization of the Zeeman lines as perpendicular or parallel to the magnetic field. I think we have to go to 1927 onwards. It seems that these molecular orbitals diagrams were imaginations of chemistry teachers. I still could not find such pictures in original papers. These MO diagrams are so common in every chemistry textbook and webpage. – M. Farooq Oct 18 '18 at 5:47
• @ Francois Zeigler, Yes thanks, it seems Lennard was the first one to draw such diagrams (1929). Mulliken later published a series of notations of sigmal, pi, sigma star and pi star. – M. Farooq Oct 19 '18 at 0:56