NMR identification of hexamethylenetetramine and its precursor in Titan tholins: Implications for Titan prebiotic chemistry

The reddish brown organic haze surrounding Titan has been investigated using methods including remote observation, direct exploration (the Cassini mission) and laboratory simulations, but its formation mechanism and the contributing chemical structures and prebiotic potential are still not well understood. We report here the structural investigation of the 13C and 15N labeled, simulated Titan haze aerosol (tholin) using solution-state NMR. These spectra demonstrate a material composed of a mixture of moderate polymer and small molecules. Hexamethylenetetramine (HMT) is identified as the major small molecule component in the Titan tholin and its precursor (1,3,5-hexahydrotriazine) is also detected. We discuss the formation mechanism of HMT and its implications for Titan and early Earth prebiotic chemistry.

► Multinuclear and multidimensional NMR investigation of Titan haze aerosol analog. ► Hexamethylenetetramine (HMT) is identified in Titan tholins for the first time. ► The HMT content in Titan tholins is determined by NMR (9.54–9.75%). ► Detection of hexahydrotriazine suggests methanimine (CH2NH) is proper precursor for Titan haze. ► We discuss the possible formation mechanism of HMT in Titan’s atmosphere.