Radiation-induced Transformation of the C₂H₂∙∙∙NH₃ Complex in Cryogenic Media: Identification of C₂H₂∙∙∙NH₂^• Complex and Evidence of Cold Synthetic Routes

Acetylene and ammonia are important constituents of the interstellar medium and their coupled chemistry induced by high-energy radiation may be responsible for the formation of a variety of prebiotically important organic-nitrogen compounds. In this work, we first comprehensively characterized the vibrational spectrum of the 1:1 C2H2∙∙∙NH3 complex obtained by deposition of the C2H2/NH3/Ng (Ng = Ar, Kr, or Xe) gaseous mixtures at 5 K using FTIR spectroscopy and ab initio calculations at the CCSD(T)/L2a_3 level of theory and examined its radiation-induced transformations. The parent complex adopts a C3v symmetric top molecular structure with C2H2 acting as a proton donor. The X-ray-induced transformations of this complex result in the formation of the C2H2∙∙∙NH2• complex and various CN-containing species (CH2CNH, CH3NC, CH2NCH, CH2NC•, CCN•, and CNC•). The radical-molecule complex was identified based on comparison of experimental data with the results of the UCCSD(T)/L2a_3 computations. It is characterized by distinct features in the region of acetylene CHasym str mode, red-shifted from the corresponding absorptions of non-complexed acetylene by −72.9, −70.4, and −60.6 cm−1 for Ar, Kr, and Xe, respectively. Additionally, in krypton and xenon matrices, the blue-shifted features in the CHasym bend region of acetylene were observed, which can be also tentatively attributed to the C2H2∙∙∙NH2• complex. The UCCSD(T)/CBS binding energy of the C2H2∙∙∙NH2• complex (including zero-point vibration energy correction) is lower than that of the C2H2∙∙∙NH3 complex (1.90 and 2.51 kcal mol−1, respectively). We believe that the C2H2∙∙∙NH2• complex may be an important intermediate in cold synthetic astrochemistry.