Barrier Height Titration by Tunable Photoionization Combined with Chemical Monitoring: Unimolecular Keto/Enol Tautomerization of the Acetamide Cation Radical

By using a new “titration” technique, the activation barrier for the strictly unimolecular 1,3-hydrogen migration 1.+→2.+ has been determined (0.74±0.06 eV; see reaction profile). With this technique the internal energy of 1.+ is controlled by variable photoionization of acetamide and the chemistry of the resulting enol ion 2.+ is then probed by structure-specific ion–molecule reactions.