Energetically Integrable Roaming in the Ionized CH₄ + CO System: Nonequilibrium Dynamics of Acetaldehyde Formation

Abstract

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 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 protonated acetaldehyde (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 quasi-integrable, energetically decoupled intermediate, in which the motions of CH₃ and HCO⁺ become dynamically isolated. This transient 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 protonated acetaldehyde without passing through conventional isomerization transition states. These results demonstrate that ionization can create roaming-accessible, energetically 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 represent an underrecognized source of complex organic ions in cold, low-density astrophysical environments, thereby expanding current models of interstellar chemical evolution.