Factors governing <math xmlns="http://www.w3.org/1998/Math/MathML"> <msubsup><mrow><mi>H</mi></mrow> <mrow><mn>3</mn></mrow> <mrow><mo>+</mo></mrow> </msubsup> </math> formation from methyl halogens and pseudohalogens.
basic_science · Level V
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- Record sourced from PubMed, PMID 39762206.
- Also identified by DOI 10.1038/s41467-024-55065-5 and PMC identifier 11704215.
- Licence recorded as CC BY-NC-ND.
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Abstract
The formation of <math xmlns="http://www.w3.org/1998/Math/MathML"> <msubsup><mrow><mi>H</mi></mrow> <mrow><mn>3</mn></mrow> <mrow><mo>+</mo></mrow> </msubsup> </math> following the double ionization of small organic compounds via a roaming mechanism, which involves the generation of H<sub>2</sub> and subsequent proton abstraction, has recently garnered significant attention. Nonetheless, a cohesive model explaining trends in the yield of <math xmlns="http://www.w3.org/1998/Math/MathML"> <msubsup><mrow><mi>H</mi></mrow> <mrow><mn>3</mn></mrow> <mrow><mo>+</mo></mrow> </msubsup> </math> 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<sub>3</sub>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<sub>3</sub>X<sup>2+</sup> dications, are employed to identify the key factors governing the formation of <math xmlns="http://www.w3.org/1998/Math/MathML"> <msubsup><mrow><mi>H</mi></mrow> <mrow><mn>3</mn></mrow> <mrow><mo>+</mo></mrow> </msubsup> </math> in certain doubly ionized CH<sub>3</sub>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 <math xmlns="http://www.w3.org/1998/Math/MathML"> <msubsup><mrow><mi>H</mi></mrow> <mrow><mn>3</mn></mrow> <mrow><mo>+</mo></mrow> </msubsup> </math> production. We find that the excess relaxation energy released after double ionization of CH<sub>3</sub>X molecules combined with substantial geometrical distortion that favors H<sub>2</sub> formation prior to proton abstraction boost the generation of <math xmlns="http://www.w3.org/1998/Math/MathML"> <msubsup><mrow><mi>H</mi></mrow> <mrow><mn>3</mn></mrow> <mrow><mo>+</mo></mrow> </msubsup> </math> . Our study provides useful guidelines for examining alternative sources of <math xmlns="http://www.w3.org/1998/Math/MathML"> <msubsup><mrow><mi>H</mi></mrow> <mrow><mn>3</mn></mrow> <mrow><mo>+</mo></mrow> </msubsup> </math> in the universe.