Electronic Structures of Polyatomic Molecules and Valence. II. Quantum Theory of the Double Bond

The electronic structure of CH₂ in its probable normal and first excited states is described in terms of molecular orbitals. The formation of C₂H₄ from two (excited) CH₂ radicals is discussed, and it is shown, with the help of the Slater-Pauling overlapping criterion, to be theoretically obvious, in agreement with experiment, that for the normal state of C₂H₄ the energy is surely considerably lower if the two CH₂ are arranged symmetrically in one plane than if their planes make an angle of 90^ (Figs. 1, 2). Similar statements apply to the derivatives of C₂H₄. The C=N and N=N double bonds can be treated similarly. The C=C and O=O double bonds are compared. The (BH₃) = (BH₃) bond in B₂H₆ probably resembles the O=O more than the C=C bond. For certain predicted excited states of C₂H₄ and its derivatives, which are probably the upper states of ultraviolet absorption bands of these compounds, it is shown that the energy should be higher for the plane form than for the perp. form (one CR'R" plane rotated through 90^). Hence the plane form should tend to go over spontaneously by rotation into the perp. form (90^) and on to the other plane form (180^) after absorption of suitable ultraviolet light. In this way the observed transformations of cis into trans isomers or vice versa by ultraviolet light may be explained.