Molecular orbital calculations of transition state geometries

1 Molecular orbital calculations of transition state geometries 

The semi-empirical molecular orbital method MINDO/3 has been used to calculate transition state geometries for the following decompositions. 

The MINDO/3 calculations [215] on (C4H9)+ considered the 2-butyl cation decomposing to the 1-methylallyl cation and hydrogen. The transition state contained a pentacoordinated carbon atom and the loss of H2 from the transition state formally resembled a 1,1 elimination. The large proportion of the reverse critical energy observed to be released as translation was attributed to repulsive forces and tightness of the transition state. 

The study of (C3H7)+ losing H2 found the transition state to be cyclic, (CH2CH2CH3)+, with both of the hydrogen atoms to be eliminated attached to the same (penta-coordinated) carbon atom [744]. The loss of H2 from the transition state resembled a 1, 1 elimination this decomposition has been observed to release most of its reverse critical energy as translational energy. This paper also contains a careful examination of the isotope effects /Hj //Hd/ d2 on metastable ion abundances. 

A feature held in common by all of the above MINDO/3 calculations of transition state geometries for 1, 2 (and 1, 3) eliminations of H2 (as these decompositions would be designated conventionally) is that the actual loss of H2 from the transition state resembles a 1,1 elimination. That is to say, in all cases the overall decomposition involves initial rearrangement so that, in the transition state, both hydrogens to be eliminated appear to be associated with the same carbon atom. 

On the basis of the translational energy releases in the collision-induced decompositions relative to those in the unimolecular decompositions, it appeared [216] that, in the losses of H2 from (CH3OH2), (CHsOH) and (CH20H), the energy releases became smaller at the higher excess energies. This somewhat surprising conclusion appears not to be supported by the results of a PIPECO study of methanol [18, 658] however, the PIPE CO results are not sufficiently precise to constitute definite proof that the conclusion is incorrect. 

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