New insights into the structure and chemistry of Titan’s tholins via13C and 15N solid state nuclear magnetic resonance spectroscopy

S Derenne,C Coelho,C Anquetil,C Szopa,A.S Rahman,P.F Mcmillan,F Corà,C.J Pickard,E Quirico,C Bonhomme

doi:10.1016/j.icarus.2012.03.003

Abstract

Tholins are complex C,N-containing organic compounds produced in the laboratory. They are considered to provide materials that are analogous to those responsible for the haze observed in Titan’s atmosphere. These compounds present an astrobiological interest due to their ability to release amino acids upon hydrolysis. Their chemical structure has been investigated using a large number of techniques. However, to date no detailed nuclear magnetic resonance (NMR) study has been performed on these materials despite the high potential of this technique for investigating the environment of given nuclei. Here 13C and 15N solid state NMR spectroscopy was applied to obtain new insights into the chemical structure of tholins produced through plasma discharge in gaseous N2CH4 mixtures designed to simulate the atmosphere of Titan. Due to the low natural abundance of these isotopes, a 13C and 15N-enriched tholin sample was synthesized using isotopically enriched gas precursors. Various pulse sequences including 13C and 15N single pulse, 1H13C and 1H15N cross-polarisation and 1H15N13C double cross-polarisation were used. These techniques allowed complete characterisation of the chemical and structural environments of the carbon and nitrogen atoms. The NMR assignments were supplemented and confirmed by ab initio electronic structure calculations for model structures and molecular fragments.

Highlights

Titan, the largest moon of Saturn, is characterized by a dense atmosphere, mainly composed of N2 and CH4
We investigated the effect of adding an amine function attached to the nitrile group (N„CANH2): here we expect that the chemical shift would lie at approximately 118 ppm (Table 1)
This comprehensive nuclear magnetic resonance (NMR) study has allowed the elucidation and confirmation of structural units and functional groups that have been suggested to be present within tholins from other techniques, especially the presence of nitriles

Summary

Introduction

The largest moon of Saturn, is characterized by a dense atmosphere, mainly composed of N2 (ca. 97%) and CH4 (ca. 2%). It is necessary to build on that information to refine the chemical and structural models for the Titan model tholins We investigate these complex materials using solid state nuclear magnetic resonance (NMR) techniques. Techniques using CP pulse sequences can be used to enhance the signal intensity (Schaefer and Stejskal, 1976) These experiments involve excitation of the most abundant and NMR sensitive spins within the sample (i.e., 1H nuclei in the case of the CxNyHz tholins) and magnetisation is transferred towards less abundant spins of interest (i.e., 13C or 15N). These attributions are carried out based on knowledge of functional groups within the existing NMR large data base for organic compounds These provide useful structural analysis tools but do not always provide reliable ways to interpret the potentially unusual CANAH bonding or molecular fragments present within tholins produced in the laboratory. We complemented our experimental study by carrying out ab initio predictions of 13C and 15N NMR shifts for molecular models and fragments using electronic structure methods in order to support and aid in the structural interpretation of our experimental data

Samples

NMR experiments and first principles calculations

Characterisation of carbon chemical environments using 13C NMR

NAC interactions studied via 1HA15NA13C double CP-MAS NMR

Discussion