Atsushi Shigihara

Studies on structures and fragmentations of molecular protonated ions by semi-empirical molecular orbital theory.

The equilibrium structures and fragmentations of the protonated butane, aminopropanol, formates, and acetates are studied by semi-empirical MINDO/3 MO method. The results obtained were discussed in relation with the role of the proton. Comparison in the energy barriers leads the following conclusions. 1) The H2 elimination easily occurs in the protonated hydrocarbons, while as for the protonated aminopropanol, H2O loss is favorable. 2) The fragmentations do not always occur from the most stable molecular protonated ion. 3) The olefin elimination from the protonated esters favors the stepwise process via intermediate ion A (HCOOH…H+…CH2=CHR) more than any other processes via ring transition states, resulting in good accord with the observed ratios of the hydrogen migrations in protonated propyl ester.

Olefin elimination mechanism of protonated formates by MINDO/3.

The potential energies of olefin elimination from the protonated formates have been determined by MINDO/3. The stepwise process via the intermediate A is energetically more favorable than any other processes via ring transition states, resulting in a good agreement with the observed ratios of the hydrogen migrations in protonated n-propyl ester.

An MO Theoretical Study on Approximate Potential Energies of Proton Migration and Fragmentation in Protonated n-Propanolamine and n-Butanolamine

Potential energies of proton migration and the H2, H2O, and NH3 elimination reactions in the protonated n-propanolamine were determined by MINDO/3. The elimination reactions in the protonated n-butanolamine were also examined and discussed in an attempt to explain the difference of the ratio of loss of water to loss of ammonia between these alcoholamines. Proton migration between carbon atoms in the protonated alkyl group was found to involve a low activation energy less than 84 kJ/mol but unlikely to take place because of other channels. Elimination of H2 was shown to take place from the C-protonated MH+ ion. There were several reaction paths for water and ammonia losses from the MH+ ion and the paths with hydrogen rearrangements were more likely to occur than the simple cyclization mechanism with the neighboring assistance of the hetero atom of the other end.