Electron-Transfer-Induced Tautomerization in Methylindanones: Electronic Control of the Tunneling Rate for Enolization

The radical cations generated from 4-methyl- and 4,7-dimethylindanone, as well as their deuterated
isotopomers, isolated in Argon matrices, were found to undergo enolization to the corresponding enol radical
cations at rates that differ by orders of magnitude. It is shown by quantum chemical calculations that the
effect of the remote methyl group in the 4-position is of purely electronic nature in that it stabilizes the unreactive
π-radical relative to the reactive σ-radical state of the 7-methylindanone radical cation. The observed kinetic
behavior of the two compounds can be reproduced satisfactorily on the basis of calculated heigth and width
of the thermal barrier for enolization, using the Bell model for quantum mechanical tunneling. High-level
calculations on the methylacrolein radical cation show that barriers for enolization in radical cations are
overestimated by B3LYP/6-31G*.