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In the series ethylene episulfide, episulfoxide, and episulfone the sulfone stands out by its very long CC bond and short CS and SO bonds. We find that an analysis of the bond length trends in these molecules leads naturally to a bonding picture in which the molecules are described as complexes of an ethylene with S, SO, or S02. We trace the long CC bond in the episulfone to two factors. First, and invoking no 3d orbitals, when the molecule is viewed as an ethylene-S02 complex, the S02 provides a low-lying orbital which populates the ethylene w* level better than in the analogous sulfur case. Second, and directly invoking 3d participation, the S02 ligand provides 3d orbitals which more efficiently withdraw bonding electron density from the ethylene fragment level. Our analysis predicts how to strengthen further the CC bond in episulfides and how to weaken still more the long CC bond in episulfones. The mode of that bond opening should be conrotatory in the episulfide and disrotatory in the episulfone. Finally the ethylene-complex viewpoint is connected to the general theory of substituent effects on cyclopropane bond strengths.In the heterocyclic series of thiirane (1, ethylene episulfide), thiirane 1-oxide (2, ethylene episulfoxide), and thiirane 1,1-dioxide (3, ethylene episulfone) we are presented by Nature with a regular modification of a three-membered ring by oxygen addition, a progressive change in the coordination of sulfur. The equilibrium structure of the heterocycle responds in a significant and apparently irregular way to the pattern of sulfur coordination. From the several consistent structural studies on these molecules1-4 we have chosen the set of microwave spectroscopic structures illustrated below.1-3

Zusammenfassung. In Dicarbonylen wie Glyoxal oder Benzochinon bilden die nichtbindenden Sauerstofforbitale delokalisierte Molekelorbitale mit verschiedenen Energien. Wir berichten iiber EHT-und CNDO/Z-Berechnungen einiger Dione und Trione. Die berechnete Aufspaltung zwischen den Nnichtbindenden 0 Kombinationen wird auf der Grundlage der u through-space 1) und H throughbond )) Wechselwirkung analysiert. Bei den tiefliegenden nichtbindenden Orbitalen der Carbonylgruppen scheint der zweite Faktor zu dominieren. Gerust-cr-Orbitale und nichtbindende Sauerstofforbitale sind stark miteinander gemischt.An isolated carbonyl group is characterized by two 'lone-pair' orbitals, described as equivalent in the valence structure 1. In any molecular orbital description of a carbonyl group, symmetry adapted combinations of these two equivalent lone-pairs must be taken, resulting in the s and fi type lone-pairs of 2. The latter orbitals are D 1 2 nonequivalent and possess widely differing energies. There is little doubt that the highest occupied u molecular orbital of simple aldehydes and ketones is p type. It is this orbital which is most directly involved in (n, n*) electronic transitions and in the mass spectral reactions of such compounds.In molecules containing several equivalent carbonyl groups the 'nonbonding ' p orbitals must be combined into more delocalized wave functions. These molecular orbitals, degenerate in the absence of any interaction, may as a result of interaction between the individual nonbonding orbitals be differentiated in energy. The energy splitting between these molecular orbitals is the prime operational measure of the extent of orbital interaction. Experimentally, this splitting may be obtained by ascertaining the ionization potentials of the appropriate orbitals. Through the technique of photoelectron spectroscopy [l] such measurements have become feasible. Whenever nonbonding orbital interaction was considered in the past, that interaction was judged small1). This is now not in accord with either theoretical or experimental results. Lone-pairs, double bonds and other isolated subunits of a molecule interact significantly with other such units by direct through-space and indirect through-bond mechanisms. These interactions are easily analyzed See for example the considerations for glyoxal and biacetyl in [Zj.

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Potential energy surfaces for the isomerization of diimide and azomethane in the ground, 1(n,π*),3 (n,π*), and 3(π,π*) states have been calculated by ab initio molecular orbital methods. Two mechanisms are considered in detail, the first involving in-plane motion of the substituent group and the second involving twisting about the N—N bond. The first mechanism is preferred for the ground and 3(π,π*) states, whereas the second mechanism is preferred for the (n,π*) states. A significant barrier to rotation is predicted for the 3(π,π*) state. The vibrationally-relaxed (twisted) lowest triplet of diimide is predicted to lie 36 kcal mol−1 above the ground state.