Three-Membered Rings from ab initio Molecular Orbital

Lathan, W. A. Radom, L., Hariharan, P. C., Hehre, W. J., and Pople, J. A. Structures and Stabilities of Three-Membered Rings from ab initio Molecular Orbital Theory. 40, 1-45... 

Structures and Stabilities of Three-Membered Rings from ab initio Molecular Orbital Theory... 

The molecular and electronic structures of cyclic disulfide cation radicals of 1,2-dithietane 6 and 1,2-dithiete 7, and radical cations of 1,2-dithiolane 2 (2a-c represent stable conformations determined in terms of the symmetry restriction of Cs, Cz, and Czv), with emphasis on the nature of a two-center three-electron (Zc-ie) sulfur-sulfur bond have been examined by ab initio molecular orbital (MO) calculations <1997JMT(418)171>. Unrestricted Hartree-Fock (UHF)/ MIDI-4(d) computations showed that this bond in organodisulfide radical cation 2 is shorter in comparison to 1,2-dithiolane 2 and possesses partial Jt-bond character (structure A), as previously implied by electron spin resonance (ESR) spectroscopy <1982JA2318>, which correlates best with the form as the most favorable conformation of the cation radical 2. Contrary to the repulsive S-S interaction in the parent 1,2-dithiolane arising from the lone pairs of electrons, the hemi-7t-bond formed by one-electron oxidation should stabilize the five-membered ring of 2, or, for example, a similar cation radical of LA 3 which is involved in diverse biochemical reactions. 

In the cyclopropenyl radical with three Ti-electrons, the third electron has to be accommodated in one of the two degenerate molecular orbitals causing the Jahn-Teller distortion, which results in a change in symmetry from Dj, to C2,.. The ab initio (STO-3G 4-31G), as well as semi-empirical (CNDO/2), molecular orbital calculations demonstrate that the odd electron is incorporated in the pyramidalized rr-type orbital giving rise to loss of resonance stabilization in the three-membered ring. Thus, the cyclopropenyl radical is considered to be a highly unstable, short-lived species. 

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