About the detection of urea in the interstellar medium: the energetic aspect

Abstract

Context. The results of an observational search for gas phase urea, (NH 2 ) 2 CO toward the Sgr B2-LMH region, have been reported recently. In spite of strong presumptions, whether it is urea or another species (for example an isomer) seems to remain a pending question. Aims. In this note, we consider the energetic aspect of this would-be detection by addressing the relative stabilities of the 22 isomers that can possibly be formed with the CH 4 N 2 O set of atoms. By extension, we also consider the 22 sulphur analogues of CH 4 N 2 S chemical formula. Methods. The question was first addressed by means of quantum density functional theory (DFT) simulations. The hybrid B3LYP functional was used throughout. The geometries of the 44 molecules part of this survey were fully optimized and verified to be real minima by vibrational analysis. The lowest isomers found this way were then reconsidered in higher level post Hartree-Fock MP2 and coupled cluster CCSD and CCSD(T) calculations to derive more accurate energy differences and dipole moments, whose knowledge is crucial for interpreting micro-and millimetre-wave spectra. Results. We found that urea and thio-urea are the most stable compounds in their respective families. The closest isomers on the energy scale are the iminol tautomeric forms, HN=COH-NH 2 and HN=CSH-NH 2 , whose rotational constants and dipole moments have also been determined. Conclusions. That urea is the lowest energy isomer possibly formed is a strong argument making the detection of this species more than probable. After formamide and acetamide, this result confirms the greatest stability of the -[NH-C=O]-linkage, underlining the interest of the minimum energy criterion as a tool for the primary search of target molecules. Additionally, thio-urea should the analogue to search for.

The formation of urea in space

“…In this work we have investigated, by highly correlated quantum chemistry calculations, the thermodynamic and kinetic possibility of forming urea in ISM conditions through ion-molecule, neutral-neutral, and radical reactions. Recently a tentative detection of urea in the ISM was reported by Remijan et al (2014) and this is the first quantum chemistry study devoted to understanding possible gas-phase reactions concerning the formation of this molecule, while a recent theoretical chemistry study relevant to the astrophysical presence of urea considered the stability of different urea isomers, concluding that the (NH 2 ) 2 CO form is the one likely to be observed in the ISM (Fourré et al 2016).…”

“…Spontaneous emission rate constants were calculated using frequencies and Einstein coefficients obtained by ab initio calculations and using the method reported by Herbst (1982) with our in-house code. All quantum chemistry and thermochemistry calculations were performed using Gaussian 09 software (Frisch et al 2009).…”

High‐Energy Proton‐Beam‐Induced Polymerization/Oxygenation of Hydroxynaphthalenes on Meteorites and Nitrogen Transfer from Urea: Modeling Insoluble Organic Matter?

“…Data for the irradiation of 1‐HN 1 in the presence of meteorites and urea are reported in Scheme 1 and Table 1. Urea is a ubiquitous nitrogen‐containing compound that has been observed in the interstellar medium [44] . This compound is easily produced by sparking mixtures of gases [45] and irradiation of hydrogen cyanide (HCN) in both liquid and solid state [46] .…”

Quantum Chemical Study of the Formation of Urea in Interstellar Medium

“…Over the past decade, various essential biomolecules have been detected in interstellar medium (ISM) 3,5 . In 2014, the first tentative detection of urea (NH 2 CONH 2 ) was observed, which affirmed the possibility of the formation of peptide bonds that constitute essential proteins in ISM 1 . These discoveries suggest the origins of life began in space and were brought to Earth.…”

“…Linglan Zhu * ,1 , Akash Kothari * ,1 , Jon Babi 1 , Natalie J. Galant 2 , Anita Rágyanszki 1,3 and Imre G. Csizmadia 1,4…”