Low temperature rate coefficients for the reaction CN + HC3N

Area of Science:

• Chemical Kinetics
• Astrochemistry
• Atmospheric Chemistry

Background:

• The reaction of CN radicals with HC3N is vital for cyanopolyyne synthesis in interstellar and circumstellar environments.
• Understanding this reaction is also important for modeling the atmospheric chemistry of Titan.

Purpose of the Study:

• To experimentally measure the rate coefficient for the CN + HC3N reaction at low temperatures (down to 22 K).
• To compare experimental results with theoretical calculations and previous room-temperature data.

Main Methods:

• Utilized the CRESU (Reaction Kinetics in Uniform Supersonic Flow) technique coupled with pulsed laser photolysis-laser-induced fluorescence.
• Employed a novel pulsed CRESU setup with a spinning disk valve for some measurements.
• Performed high-accuracy ab initio quantum chemical calculations and variational two-transition state theory.

Main Results:

• The rate coefficient shows a marked increase as temperature decreases, deviating from room-temperature behavior.
• Experimental data were well-represented by the equation k(T) = 1.79 × 10⁻¹¹(T/300 K)⁻⁰.⁶⁷ cm³ molecule⁻¹ s⁻¹.
• Theoretical calculations showed excellent agreement with experimental findings within combined uncertainties.

Conclusions:

• This study provides the first low-temperature rate coefficients for the CN + HC3N reaction.
• The findings are crucial for astrochemical and planetary atmospheric modelers, particularly for Titan's atmosphere.
• The agreement between experimental and theoretical results validates the employed methodologies.