Roaming-controlled formation of dimethyl ether and methanol: role of nonequilibrium dynamics and integrability

We investigated dimethyl ether formation via ionization reactions between carbene and methanol using quantum mechanical calculations and ab initio molecular dynamics (AIMD) simulations. Monovalent ionization formed a carbene-methanol bond but did not induce isomerization to dimethyl ether. In contrast, electron recombination of the ionized complex efficiently triggered isomerization. A comparative study with the carbene-water system showed that recombination also produced methanol; however, dimethyl ether yield in the methanol system was markedly higher. This enhancement arose from roaming enabled by nonequilibrium energy partitioning. In the carbene-methanol complex, excess energy failed to redistribute via intramolecular vibrational energy redistribution (IVR) and instead weakened interfragment interactions. At dissociation onset, kinetic energy was asymmetrically partitioned, strongly biased toward methanol. Despite this bias, methanol remained nearly stationary due to its larger mass and internal energy localization, while carbene roamed around it. This motion delayed dissociation and promoted isomerization. By contrast, the carbene-water system showed more balanced energy distribution, leading to prompt dissociation and lower product yield. During roaming, energy exchange between fragments was strongly suppressed, generating a transient quasi-integrable regime with dynamically decoupled motions. The roaming mechanism thus emerges as a dynamical sequence: kinetic energy bias → weakened interactions → suppressed energy exchange → quasi-integrable regime → roaming. These findings demonstrate that product selectivity in such clusters arises from nonstatistical dynamics beyond conventional transition state theory, providing a new framework for controlling ion-molecule reactivity relevant to astrochemical and nonthermal environments.