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

Abstract

Dissociation chemistry of the diproline anion [Pro-H] is studied using chemical dynamics simulations coupled with quantum-chemical calculations and RRKM analysis. Pro 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 [Pro-H] were followed in order to sample a larger range of possible reactivity. While different mechanisms yielding y product ions are proposed, there is only one mechanism yielding the b ion. This mechanism leads to formation of a b fragment with a diketopiperazine structure. The sole formation of a diketopiperazine b 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.

A Trajectory-Based Method to Explore Reaction Mechanisms

“…The prediction of mass spectra remains much of a challenge for the community of computational chemists. The common computational approaches employed for such endeavor include statistical rate theory calculations, MD simulations and electronic structure calculations [102][103][104][105][106][107][108][109][110][111][112][113][114][115][116]. Our automated method is very useful in this regard and can easily be coupled with MD simulations of collisions to generate theoretically-based mass spectra as described below.…”

A Trajectory-Based Method to Explore Reaction Mechanisms

“…The prediction of mass spectra remains much of a challenge for the community of computational chemists. The common computational approaches employed for such endeavor include statistical rate theory calculations, MD simulations and electronic structure calculations [99][100][101][102][103][104][105][106][107][108][109][110][111][112][113]. Our automated method is very useful in this regard and can easily be coupled with MD simulations of collisions to generate theoretically-based mass spectra as described below.…”

A Trajectory-Based Method to Explore Reactions Mechanisms

“…The prediction of mass spectra remains much of a challenge for the community of computational chemists. The common computational approaches employed for such endeavor include statistical rate theory calculations, MD simulations and electronic structure calculations [89][90][91][92][93][94][95][96][97][98]. Our automated method is very useful in this regard and can easily be coupled with MD simulations of collisions to generate theoretically-based mass spectra as described below.…”