Multifrequency Electron Spin Resonance Detection of Solid-State Organic Free Radicals in HCN Polymer and a Titan Tholin

Abstract

Macromolecules derived from hydrogen cyanide (HCN) may be major components of the dark matter observed in bodies in the outer Solar System, which include comets and asteroids. HCN oligomers and polymers are readily formed at room temperature and react with water to produce polypeptides and alpha-amino acids or undergo pyrolysis to produce nitrogen heterocycles. Electron spin resonance (ESR) spectroscopy shows that HCN polymer mixtures contain a significant amount of long-lived organic free radicals that are primarily carbon-based. For comparison, we have also examined samples of tholins produced from experimental analogs of Titan aerosols, which has been shown by trace organic analysis to consist partly of HCN polymer. The "Titan tholin" exhibits at least two ESR signals that can be assigned to nitrogen- and carbon-centered radicals, although heating the sample eliminates the nitrogen centers and increases the signal from the carbon centers. This result suggests that the nitrogen-centered radicals may be thermodynamically less stable, but are kinetically trapped during the spark-discharge reactions that produce tholins from mixtures of gases such as methane and nitrogen. The results strongly support previous proposals of free radical mechanisms for HCN polymerization.

HCN polymers characterized by solid state NMR: Chains and sheets formed in the neat liquid

“…The latter seems more likely for the dark polymer because ͑1͒ Direct observation of free radicals has recently been reported in dark HCN polymers. 26 ͑2͒ Since the dimer of HCN is expected to form a biradical, 32-34 one does not have to look far for a radical initiator. ͑3͒ Seeding a toluene solution of HCN with dark polymer promotes the formation of new dark polymer.…”

Prebiotic synthesis of triazines from urea: a theoretical study of free radical routes to melamine, ammeline, ammelide and cyanuric acid

“…The use of spark discharges for the formation of triazines suggests that free radicals may be involved in the processes. Electron spin resonance spectroscopy analysis of tholins produced by spark discharges showed both carbon-centered and nitrogen-centered free radicals 12 and carbon-carbon bond forming free radical reactions were previously suggested to take place at low temperature. Bernstein et al proposed that these carbon-carbon bonds may be formed by the reaction of either "hot" radicals generated by photolysis at 12 K or "cold" radicals mobilized by an increase in temperature.…”

Genetics first or metabolism first? The formamide clue

“…Radical species can be also detected in HCN oligomers under spatial conditions. 96 HCN oligomers are precursors of nucleic acid bases and important biomolecules. As an example, boiling HCN oligomers in 6.0 N HCl yield adenine, hypoxanthine, AICN, 3,4-dihydroxypyrimidine 1 (an uracil isomer) and 5-hydroxyuracil 2 (Scheme 5, pathway A), while under basic conditions adenine, AICN and orotic acid 3 were recovered in appreciable yields.…”