Energetically quasi-integrable roaming in the ionized CH₄ + CO system: nonequilibrium dynamics of acetaldehyde...

Roaming reactions-nonstatistical pathways that bypass conventional transition states-were first discovered in photochemistry and were later shown to influence decomposition mechanisms under high-energy conditions relevant to combustion and atmospheric chemistry. Recent work has further revealed that roaming is often enabled by transient, energetically quasi-integrable states emerging within nonequilibrium energy distributions, where fragment motions become dynamically decoupled and intramolecular energy redistribution is strongly suppressed. Building on this perspective, we investigate an ionization-triggered roaming process in the CH₄ + CO system that leads to the formation of an acetaldehyde cation (CH₃CHO⁺). Using high-level electronic structure calculations and ab initio direct molecular dynamics (AIMD) simulations, we show that ionization generates a strongly perturbed complex in which the internal energy is partitioned unevenly among its fragments. Rather than following the minimum-energy reaction coordinate, the system enters a dynamically decoupled intermediate, in which the motions of CH₃ and HCO⁺ become dynamically isolated. This transient quasi-integrable state suppresses intramolecular energy redistribution, stabilizing a long-lived roaming configuration characterized by large-amplitude relative fragment motion. This dynamical regime enables C-C bond formation to yield an acetaldehyde cation without passing through conventional isomerization transition states. These results demonstrate that ionization can create roaming-accessible, energetically quasi-integrable regimes even in small molecular systems that are often assumed to follow statistical behavior. The findings further suggest that ionization-driven, roaming-mediated reactions may provide a possible pathway to complex organic ions in cold, low-density astrophysical environments. However, the present simulations do not include radiative cooling or collisional stabilization, and the astrophysical relevance of this mechanism therefore remains speculative.