• Energy Research

Disproportionation of superoxide to peroxide and O2 generates the highly reactive singlet O2, which needs to be avoided for highly reversible metal–O2 batteries.

Unimolecular dissociation of tripeptidesviachemical dynamics simulations with different activation modes.

Chemical dynamics simulations are performed to study the collision induced gas phase unimolecular fragmentation of a model peptide with the sequence acetyl-His1-Cys2-Gly3-Pro4-Tyr5-His6-Cys7 (analogue methanobactin peptide-5, amb5) and in particular to explore the role of zinc binding on reactivity. Fragmentation pathways, their mechanisms, and collision energy transfer are discussed. The probability distributions of the pathways are compared with the results of the experimental IM-MS, MS/MS spectrum and previous thermal simulations. Collisional activation gives both statistical and non-statistical fragmentation pathways with non-statistical shattering mechanisms accounting for a relevant percentage of reactive trajectories, becoming dominant at higher energies. The tetra-coordination of zinc changes qualitative and quantitative fragmentation, in particular the shattering. The collision energy threshold for the shattering mechanism was found to be 118.9 kcal/mol which is substantially higher than the statistical Arrhenius activation barrier of 35.8 kcal/mol identified previously during thermal simulations. This difference can be attributed to the tetra-coordinated zinc complex that hinders the availability of the sidechains to undergo direct collision with the Ar projectile.

Beyond the established use of thermodynamic vs kinetic control to explain chemical reaction selectivity, the concept of bifurcations on a potential energy surface (PES) is proving to be of pivotal importance with regard to selectivity. In this article, we studied by means of post-transition state (TS) direct dynamics simulations the effect that vibrational and rotational excitation at the TS may have on selectivity on a bifurcating PES. With this aim, we studied the post-TS unimolecular reactivity of the [Ca(formamide)](2+) ion, for which Coulomb explosion and neutral loss reactions compete. The PES exhibits different kinds of nonintrinsic reaction coordinate (IRC) dynamics, among them PES bifurcations, which direct the trajectories to multiple reaction paths after passing the TS. Direct dynamics simulations were used to distinguish between the bifurcation non-IRC dynamics and non-IRC dynamics arising from atomistic motions directing the trajectories away from the IRC. Overall, we corroborated the idea that kinetic selectivity often does not reduce to a simple choice between paths with different barrier heights and instead dynamical behavior after passing the TS may be crucial. Importantly, rotational excitation may play a pivotal role on the reaction selectivity favoring nonthermodynamic products.

Abstract In the present work we have investigated the gas phase reactivity of protonated urea after collision with the diatomic inert gas N 2 , by studying the energy transfer and fragmentation induced by collisions. We first developed an analytical pair potential to describe the interaction between the projectile and the ion, and then performed QM/MM direct chemical dynamics simulations of the collision between the projectile and protonated urea in its two most stable isomers. In particular, the effect of the diatomic projectile, and the role of its initial rotational state, were compared with the fragmentation and energy transfer obtained previously ( J. Phys. Chem. A 2009, 113 , 13853) for the monoatomic projectile Ar. The diatomic projectile was found to be less efficient in energy transfer compared to the monoatomic projectile. In addition, rotational activation of UreaH + is dependent on the initial rotational quantum number of N 2 . Finally, we investigated the UreaH + gas phase reactivity as a function of its rovibrational activation by means of chemical dynamics simulations where the initial structure for the simulations is the transition state (TS) that the system can reach after collisional activation of the most stable isomer. The simulation time-length is not able to directly access this TS from the most stable isomer since its lifetime is notably longer, of about two order of magnitude in time.