A Kinetic Study of the Gas-Phase O( 1 D) + CH3OH and O( 1 D) + CH3CN Reactions. Low Temperature Rate Constants and Atomic Hydrogen Product Yields

TL;DR

This study investigates the kinetics and product yields of the reactions between excited atomic oxygen O(1 D) and methanol and acetonitrile at low temperatures, revealing barrierless reactions and significant hydrogen atom production relevant to planetary and interstellar chemistry.

Contribution

First kinetic measurements of O(1 D) reactions with CH3OH and CH3CN at low temperatures, including new insights into hydrogen atom formation channels.

Findings

Reactions are barrierless and fast (> 2 x 10^-10 cm^3 s^-1).

Both reactions produce atomic hydrogen in measurable quantities.

H-atom yields in CH3CN reaction are supported by electronic structure calculations.

Abstract

Atomic oxygen in its first excited singlet state, O(1 D), is an important species in the photochemistry of several planetary atmospheres and has been predicted to be a potentially important reactive species on interstellar ices. Here, we report the results of a kinetic study of the reactions of O(1 D) with methanol, CH3OH, and acetonitrile, CH3CN, over the 50-296 K temperature range. A continuous supersonic flow reactor was used to attain these low temperatures coupled with pulsed laser photolysis and pulsed laser induced fluorescence to generate and monitor O(1 D) atoms respectively. Secondary experiments examining the atomic hydrogen product channels of these reactions were also performed, through laser induced fluorescence measurements of H(2 S) atom formation. On the kinetics side, the rate constants for these reactions were seen to be large (> 2 x 10-10 cm 3 s-1) and consistent with barrierless reactions, although they display contrasting dependences as a function of temperature. On the product formation side, both reactions are seen to yield non-negligible quantities of atomic hydrogen. For the O(1 D) + CH3OH reaction, the derived yields are in good agreement with the conclusions of previous experimental and theoretical work. For the O(1 D) + CH3CN reaction, whose H-atom formation channels had not previously been investigated, electronic structure calculations of several new product formation channels were performed to explain the observed H-atom yields. These calculations demonstrate the barrierless and exothermic nature of the relevant exit channels, confirming that atomic hydrogen is also an important product of the O(1 D) + CH3CN reaction.