Unimolecular Fragmentation of Deprotonated Diproline [Pro₂-H]⁻ Studied by Chemical Dynamics Simulations and IRMPD Spectroscopy

Dissociation chemistry of the diproline anion [Pro₂-H]⁻ is studied using chemical dynamics simulations coupled with quantum-chemical calculations and RRKM analysis. Pro₂^– is chosen due to its reduced size and the small number of sites where deprotonation can take place. The mechanisms leading to the two dominant collision-induced dissociation (CID) product ions are elucidated. Trajectories from a variety of isomers of [Pro₂-H]⁻ were followed in order to sample a larger range of possible reactivity. While different mechanisms yielding y₁^– product ions are proposed, there is only one mechanism yielding the b₂^– ion. This mechanism leads to formation of a b₂^– fragment with a diketopiperazine structure. The sole formation of a diketopiperazine b₂ sequence ion is experimentally confirmed by infrared ion spectroscopy of the fragment anion. Furthermore, collisional and internal energy activation simulations are used in parallel to identify the different dynamical aspects of the observed reactivity.