The Reaction between Atomic Carbon and Molecular Nitrogen as a Source of Cyanamide and Carbodiimide on Interstellar Ices

TL;DR

This study investigates how atomic carbon reacting with molecular nitrogen on interstellar ice surfaces can produce cyanamide and carbodiimide, key molecules in astrochemistry, revealing a dominant formation pathway in space environments.

Contribution

It introduces a new reaction pathway involving C and N2 on ice surfaces, significantly improving astrochemical models for nitrogen-bearing molecules.

Findings

Reaction of C with N2 on ice surfaces is barrierless and forms CNN.

Hydrogenation breaks N-N bonds, leading to N-C-N molecules.

Surface reactions dominate the formation of cyanamide and carbodiimide in space.

Abstract

Reactions occurring on the ice-covered surfaces of interstellar dust grains are considered to be among the most important sources of complex species in the interstellar medium. Despite this, molecules such as cyanamide, NH2CN, are largely underpredicted by current astrochemical models suggesting that the network of reactions currently used to describe this species and its tautomer carbodiimide, HNCNH, are incomplete. Here, we performed a theoretical investigation of the reaction of ground state atomic carbon C(3P) with molecular nitrogen N2 in both the gas-phase and on the surface of amorphous solid water (ASW) clusters to examine its potential importance in the formation of NH2CN and HNCNH. We show that the reaction of gas-phase C-atoms with N2 molecules already present on the ASW surface results in the barrierless formation of CNN. Following exothermic hydrogenation reactions, the N-N bond of the C-N-N bearing intermediates is broken allowing the formation of molecules with N-C-N backbones through cyclic intermediates over low barriers. To test the importance of these processes to NH2CN and HNCNH formation, these reactions were included in a three-phase astrochemical model of low-mass protostellar evolution employing a reaction network that was updated to better describe the formation and destruction pathways of related small nitrogen bearing molecules. These simulations demonstrate that the ice surface reaction between C and N2 represents by far the dominant source of NH2CN and HNCNH in protostellar environments and in dense clouds.