Simulations suggest why highly unsaturated molecules are so abundant in the interstellar medium

Computer simulations of saturated organic molecules being bombarded by high-energy photons and particles have led an international team of researchers to propose fresh ideas surrounding how complex unsaturated molecules form in dense interstellar clouds.

As technologies that allow us to observe interstellar media develop, scientists are discovering that more complex organic molecules are surprisingly abundant in space. ‘These organic molecules are ubiquitous in the universe, so anywhere you point your telescope you can see organic molecules, some of which are precursors to amino acids or nuclear bases,’ says Felipe Fantuzzi of the University of Kent in the UK, who co-led the work. ‘To understand how these molecules that are essential to life are formed, and how they are delivered to planets, I think, is the most important question in astrochemistry,’ he adds.

Specifically, understanding how unsaturated molecules form in interstellar media is essential to explain the chemical diversity of molecular clouds such as Sagittarius B2, a molecular cloud complex found close to the centre of the Milky Way. ‘The carbon–carbon double bond is rife for oxidation mechanisms, so that’s where you start to see some really interesting chemistry and the formation of unique functional groups,’ says Julia Lehman an expert in interstellar spectroscopy at the University of Birmingham, UK, who wasn’t involved in the research.

Fantuzzi and their co-workers used Born–Oppenheimer molecular dynamics simulations to investigate how cosmic rays and x-rays that penetrate dense molecular clouds, ionise and fragment saturated molecules into unsaturated products. They focused on four key molecules with a high degree of saturation – ethanolamine, propanol, butanenitrile and glycolamide – which are found in molecular clouds and seen as potential precursors to more complex biomolecules. While Born–Oppenheimer molecular dynamics has been used to study molecular fragmentation in the past, this is the first time it was applied to the field of astrochemistry. ‘Essentially you are running a classical molecular dynamics simulation, but for every step of the dynamics, you are also solving the Schrodinger equation,’ explains Fantuzzi. This technique allowed the researchers to study the electronic structure of the radicals formed under high energy processes, and from that, they proposed a series of fragmentation routes that favour structures with the highest possible number of π bonds.

They identified 56 cationic fragments that could be formed from the four saturated molecules under the high energy conditions of Sagittarius B2. Most of these fragments contain at least one π bond. Of these unsaturated fragments, 21 have already been observed in interstellar media so the high energy events described in this study could explain their formation. However, a considerable proportion have not yet been detected experimentally and are proposed by the team as potential targets for future investigation by spectroscopists.

‘The researchers have done a really good job of looking into one possible route towards the formation of unsaturated molecules, but it opens up a lot of other questions,’ says Lehman. For example, she’d like to know what reactions take place following the fragmentation and what fragmentation happens under lower energy conditions. ‘If these molecules are theorised to be there, then we need to turn around and start characterising these in the lab so that the observationalists can then detect them.’