Theoretical determination of molecular structure and conformation. V. Electronic effects influencing the stability of methyl substituted final ozonides

Abstract

Restricted Hartree–Fock (RHF) theory is used to explore the conformational surfaces of three methyl substituted final ozonides (FO), namely, 3-methyl-1,2,4-trioxolane (propene FO), cis-3,5-dimethyl-1,2,4-trioxolane (cis-2-butene FO) and the corresponding trans isomer (trans-2-butene FO). A split valence [3s2p/2s] basis and an augmented split valence [3s2p1d/2s] basis is employed. The interconversional processes of the FO ring are studied with a rigid pseudorotor model. Puckered ring conformations are found to be more stable than the planar forms in all three cases. Pseudorotation is hindered by barriers of 3.1 and 5.9 kcal/mole (propene FO), 1.6 and 5.4 kcal/mole (cis-2-butene FO), and 5.7 kcal/mole (trans-2-butene FO). The conformational minima are located at the twist puckered forms (OO half-chair), a result which can be rationalized in terms of lone pair delocalization or σ–π orbital mixing. In agreement with spectroscopic results, the most stable FO conformation is found to possess at least one methyl substituent in an equatorial position. Thus, a favorable interaction between ring and substituent MO’s is guaranteed which leads to a stabilization of the two highest occupied MO’s and back-donation of charge from the ring to the substituent. The difference in the conformational behavior of primary ozonides (PO) and FO’s is shown to result from the different topologies of the two trioxolane rings.