Radiation-induced transformation of the CH⋯NH complex in cryogenic media: Identification of CH⋯NH 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 CH⋯NH complex obtained by deposition of the CH/NH/Ng (Ng = Ar, Kr, or Xe) gaseous mixtures at 5 K using Fourier transform infrared 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 C symmetric top molecular structure with CH acting as a proton donor. The x-ray-induced transformations of this complex result in the formation of the CH⋯NH complex and various CN-containing species (CHCNH, CHNC, CHNCH, CHNC, 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 CH mode, red-shifted from the corresponding absorptions of non-complexed acetylene by -72.9, -70.4, and -60.6 cm for Ar, Kr, and Xe, respectively. Additionally, in krypton and xenon matrices, the blue-shifted features in the CH region of acetylene were observed, which can be also tentatively attributed to the CH⋯NH complex. The extrapolated to the complete basis set limit unrestricted coupled cluster method with single and double, and perturbative triple excitations binding energy of the CH⋯NH complex (including zero-point vibration energy correction) is lower than that of the CH⋯NH complex (1.90 and 2.51 kcal mol, respectively). We believe that the CH⋯NH complex may be an important intermediate in cold synthetic astrochemistry.