Importance of thermal reactivity for hexamethylenetetramine formation from simulated interstellar ices

¹ Université Aix-Marseille, PIIM: Laboratoire de Physique des Interactions Ioniques et Moléculaires, UMR CNRS 7345, Centre de St-Jérôme, Avenue Escadrille Normandie-Niemen, 13397 Marseille, France
e-mail: vassilissa.vinogradoff@etu.univ-amu.fr
² Université Paris Est Créteil (UPEC), Université Paris Diderot (UPD), LISA: Laboratoire Interuniversitaire des Systèmes Atmosphériques, UMR CNRS 7583, 61 Avenue du Général de Gaule, 94010 Créteil Cedex, France
e-mail: nicolas.fray@lisa.u-pec.fr
³ Université Paris Sud, CSNSM: Centre de Spectrométrie Nucléaire et de Spectrométrie de Masse, UMR 8609 CNRS/IN2P3, bâtiment 104, 91405 Orsay Campus, France

Received: 7 December 2012
Accepted: 23 January 2013

Abstract

Context. Complex organic molecules are observed in a broad variety of astrophysical objects, but little is known about their formation mechanism. Laboratory simulations on interstellar ice analogues are therefore crucial for understanding the origin of these complex organic molecules. In this context, we focus on the thermal reactivity for the formation of the organic residue obtained after photolysis at 25 K of the interstellar ice analogue (H₂O:CH₃OH:NH₃) warmed to 300 K.

Aims. We determine the formation mechanism of one major product detected in the organic residue: hexamethylenetetramine (HMT). We compare the warming of the photolysed interstellar ice analogue with the warming of the two non-photolysed specific ice mixtures H₂CO:NH₃:HCOOH and CH₂NH:HCOOH, which are used as references. Using both general and specific approaches, we show the precise role of the UV photons and the thermal processing in the HMT formation.

Methods. We used Fourier transform infrared spectroscopy (FTIR) to monitor the chemical changes induced by the heating of the photolysed ice analogue and characterize some important species that will subsequently evolve in the formation of HMT in the residue.

Results. We show that the thermal processes play a key role in the HMT formation in photolysed ice analogues heated at 300 K. We identify the stable intermediates in the HMT formation that are formed during the warming: the aminomethanol (NH₂CH₂OH) and the protonated ion trimethyletriamine (TMTH⁺, C₃H₁₀N₃⁺). We also identify for the first time a new product in the organic residue, the polymethylenimine PMI (-(CH₂ -NH)_n). Results from this study will be interesting for the analysis of the forthcoming Rosetta mission.