On the Electronic Structure and Electronic Spectra of Ethylene-Like Molecules

Abstract

A previously given theory of the electronic spectra and electronic structure of complex unsaturated molecules is applied to ethylene-like molecules, molecules such as ethylene itself, propylene or formaldehyde, which have two π electrons and two atomic π orbitals available for π-bond formation. Relations are derived among electronic excitation energies, intensities of electronic transitions, extra-ionic resonance energies and dipole moments of such molecules. Relative electronegativities are shown to play an important role in these relations, where the effective electro-negativity of an atom is Mulliken's electronegativity (the mean of the appropriate atomic valence-state ionization potential and electron affinity) plus the total attraction of the valence electron of the atom for all other atoms in the molecule. The analysis yields the result that the dipole moment due to a slight bond heteropolarity is proportional to the electronegativity difference of the bonded atoms, the extra-ionic resonance energy to its square. Proportionality factors are given absolutely by the theory. It is pointed out that the theory of a two-electron σ bond is formally the same, moreover, so that a derivation is provided for the familiar electronegativity relations of Pauling. Theoretical and empirical values for the proportionality factors are found to agree reasonably well for both π bonds and σ bonds. The formulas given cover multiple perturbations of any type which may be considered not to add (or subtract) π electrons or π orbitals. They thus provide a scheme for quantitative evaluation of inductive effects in ethylene-like molecules. Systematic consideration of mesomeric effects is deferred, but an outline of the procedure which may be used for their study is given, using mono-substituted ethylene as an example. To illustrate the use of the formulas, to bring out the relationships between the method and the work of other authors, and to see whether the method has promise for σ-electron systems, calculations are made of the electronic spectrum of ethylene, the twisting force constant of ethylene, the ionization potential and electron affinity of ethylene, the electronic spectrum and dipole moment of formaldehyde, and the electronic spectrum, ionization potential, and binding energy of the hydrogen molecule.