Unimolecular Fragmentation Induced By Low-Energy Collision: Statistically or Dynamically Driven?

By combining chemical dynamics simulations and RRKM statistical theory we have characterized collision induced dissociation (CID) mechanisms of [M­(formamide)]²⁺ ions (M = Ca, Sr) at different timescales, from few femtoseconds to microseconds. Chemical dynamics simulations account for the short-time and dynamically driven reactivity, such as impulsive collision mechanism for formamide neutral loss. From the simulations, we also got the amounts of energy transferred during the collision and, especially important, the vibrational and rotational energy distributions of the ions that did not react during the simulation time length of 2.5 ps. These internal energy distributions were in turn used in combination with RRKM theory to estimate the rate constants of the possible reactive pathways. Hence, we performed a statistical analysis of the CID dynamics accounting for the long-time and statistical reactivity (i.e., through an IVR mechanism). This multiscale approach allowed us to account for all the products observed in the CID experimental spectra of [Ca­(formamide)]²⁺ and [Sr­(formamide)]²⁺ doubly charged cations, as well as the differences between them.