Factors governing H3+ formation from methyl halogens and pseudohalogens

The formation of H 3 + following the double ionization of small organic compounds via a roaming mechanism, which involves the generation of H 2 and subsequent proton abstraction, has recently garnered significant attention. Nonetheless, a cohesive model explaining trends in the yield of H 3 + characterizing these unimolecular reactions is yet to be established. We report yield and femtosecond time-resolved measurements following the strong-field double ionization of CH 3 X molecules, where X = OD, Cl, NCS, CN, SCN, and I. These measurements, combined with double-ionization-potential equation-of-motion coupled-cluster ab initio calculations used to determine the geometries and energetics of CH 3 X 2+ dications, are employed to identify the key factors governing the formation of H 3 + in certain doubly ionized CH 3 X species and its absence in others. We also carry out ab initio molecular dynamics simulations to obtain detailed microscopic insights into the mechanism, yields, and timescales of H 3 + production. We find that the excess relaxation energy released after double ionization of CH 3 X molecules combined with substantial geometrical distortion that favors H 2 formation prior to proton abstraction boost the generation of H 3 + . Our study provides useful guidelines for examining alternative sources of H 3 + in the universe. The trihydrogen cation (H3+) plays a key role in the interstellar chemistry. Here the authors, using state of the art experiments and computation, identify factors that govern H3+ formation from doubly ionized small organic molecules, offering guidelines for examining alternative sources of H3+ in the universe.