Trends in Methylidyne Radical Reactions at Temperatures Relevant to the Interstellar Medium.

The methylidyne radical, CH, can react rapidly with an unusually broad range of molecules under temperatures relevant to cold molecular clouds, making CH an important driver of molecular complexity in the interstellar medium. This review focuses on reactions of CH with polyatomic partners where low-temperature kinetic data are available (down to 23 K), drawing together information from theoretical studies, CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme), and crossed molecular beam experiments. Trends in reaction rate coefficients as a function of temperature are surprisingly similar despite the differences in molecular reaction partner, with k(T ) increasing with decreasing temperature before a small turnover in behavior near 50-70 K. This review evaluates k(T ) and mechanistic trends in the context of a capture-controlled framework, highlighting where it breaks down and how observed trends in fitted rate parameters point toward predictive expressions for CH reactivity, while emphasizing the hydrocarbon and heteroatom-containing systems most in need of further study.