TGA–FT-IR study of pyrolysis of poly(hydrogen cyanide) synthesized from thermal decomposition of formamide. Implications in cometary emissions

Abstract

The thermal treatment of formamide (HCONH 2) causes its decomposition yielding a black polymer. It is shown by FT-IR and 13C NMR spectroscopy that the structure of the polymer from formamide is poly(hydrogen cyanide) (PHC). The TGA–FT-IR of PHC causes its depolymerization with a continuous release of HCN from 300 °C to 850 °C. Ammonia is also released from 200 °C and isocyanic acid (H–N C O) is released together with HCN and NH 3 from 350 °C. It is thought that PHC or its simple derivatives are responsible for the emission of HCN and CN radicals from comets. PHC is present on the surface of the comets or mixed with the ices and then is released copiously by depolymerization reaction caused by heat or by energetic photons and particles when the comets approach the sun. The thermal decomposition reaction of formamide is a quite complex reaction. In addition to PHC it produces CO and NH 3 and at higher temperatures also HCN. The formation of PHC is accompanied with the formation of ammonium carbamate (H 2N–COONH 4) and ammonium carbonates. The heterocyclic compound isocyanuric acid and its tautomer have been thought to be formed as secondary products from formamide decomposition to explain the liberation of isocyanic acid during the pyrolysis experiment at the TGA–FT-IR. Additionally, purine has been isolated and characterized from PHC. At the temperature of the carbon arc formamide decomposes into CO, CO 2, HCN and HC CH.