AB-INITIO MOLECULAR-ORBITAL STUDIES ON C2H5O+ AND C2H4FO+ - OXONIUM ION, CARBOCATION, PROTONATED ALDEHYDE, AND RELATED TRANSITION-STATE STRUCTURES

We have investigated the effect of including electron correlation in calculations of the geometries and energies of the oxonium ion, carbocation, and protonated aldehyde species of oxirane and monofluorooxirane by comparing data obtained at the RHF/6-31G*//RHF/6-31G*, MP2/6-31G*//RHF/6-31G*, and MP2/6-31G*//MP2/6-31G* levels. As well as characterizing the oxonium ions and carbocations as stable intermediates or as transition states, many of the transition states interlinking the stable intermediates have also been identified. The inclusion of electron correlation in the energy calculation is found to favor the ring structures, whereas its inclusion in the geometry optimization scarcely alters the reaction energies and activation energies. Stereospecific pathways stem from the syn and anti invertomers of the fluoro oxonium ion, the former giving the trans/anti rotamer of the fluorocarbocation, and the latter the cis/anti rotamer. 1,2-Hydrogen shifts then result in the formation of the trans/anti and cis/anti rotamers of protonated fluoro acetaldehyde, respectively. The total atomic charge on the shifting hydrogen in the transition states is appreciably less positive than that on the other H-C bonded hydrogens, in accord with the formal ''hydride ion'' transfer mechanism. The carbon-oxygen bond length and the total atomic charge on the oxygen in the protonated acetaldehyde and fluoro acetaldehyde structures show the bond to have significant double-bond character. Likewise, the carbon-fluorine bond length and the total atomic charge on the fluorine in the fluorocarbocations show that this bond also possesses considerable double-bond character.