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In a radical reaction, the concentration of starting materials governs the selectivity of a radical. For example, the reaction between an alkyl halide and an alkene, in the presence of tin hydride and AIBN, begins with the generation of a tin radical. The generated radical then abstracts halogen from the alkyl halide, producing an alkyl radical. This alkyl radical can either react with tin hydride, yielding an alkane, or add to an alkene, generating a nitrile-stabilized radical, eventually...
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Enolate ions are formed by the acid–base reaction of a carbonyl compound with a base. This leads to deprotonation of the α hydrogen atom, leading to a resonance-stabilized enolate ion where one of the contributing structures is an oxyanion, which imparts additional stability. Therefore, the proton on the α carbon is more acidic in nature than that of other sp3-hybridized C–H bonds but less acidic than those in O–H bonds where the negative charge in the conjugate...
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Colligative Properties of Electrolytes
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If a set of reactants can yield multiple constitutional isomers, but one of the isomers is obtained as the major product, the reaction is said to be regioselective. In such reactions, bond formation or breaking is favored at one reaction site over others.
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Reaction Kinetics and Combustion Dynamics of I4O9 and Aluminum Mixtures
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Ionic Reactant Orientation Inhibits Ion-Neutral Reactions at Low Temperatures.

Yongxu Peng1, Junlong Li2, Zongao Song1

  • 1State Key Laboratory of Quantum Functional Materials, Department of Chemistry, and Center for Advanced Light Source, Southern University of Science and Technology, Shenzhen, Guangdong 518055, China.

JACS Au
|September 26, 2025
PubMed
Summary

Barrierless ion-molecule reactions in interstellar chemistry are slower than predicted. Steric effects, or molecular orientation, significantly impact reaction rates at low temperatures, requiring improved models.

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ion−molecule reactionreactant orientationreaction rate constant

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Area of Science:

  • Astrochemistry
  • Chemical Physics
  • Quantum Chemistry

Background:

  • Barrierless ion-molecule reactions are crucial for interstellar chemistry, especially at low temperatures.
  • Classical capture theory often overestimates reaction rates by neglecting ion orientation.

Purpose of the Study:

  • To investigate the rate coefficient of the BeD+ and O2 reaction at low temperatures.
  • To determine the influence of steric effects on ion-molecule reaction dynamics.

Main Methods:

  • Utilized a linear quadrupole ion trap and time-of-flight mass spectrometry.
  • Employed sympathetically cooled BeD+ ions and O2.
  • Performed isotopic substitution to confirm reaction products.
  • Conducted master equation modeling with high-level electronic structure calculations (CCSDT-(Q)).

Main Results:

  • Measured rate coefficient k = (5.4 ± 1.2) × 10^-11 cm^3/s at 97 K.
  • Observed rate is ~15 times lower than predicted by classical capture theory.
  • Identified BeOD+ and O as the sole products.
  • Master equation modeling accurately reproduced experimental results.

Conclusions:

  • Steric constraints significantly reduce reactivity in barrierless ion-molecule reactions at low temperatures.
  • Classical capture theory overestimates reaction rates due to its neglect of steric effects.
  • Accurate interstellar chemistry models must incorporate steric considerations for ion-molecule reactions.