New ab initio study of the spectroscopy of HCNH+

Highly correlated ab initio computations have been performed to investigate the rovibrational energy levels and the rotational transitions of the HCNH⁺ cation and its isotopologues. Using explicitly correlated coupled-cluster methodology and core-valence basis sets, the six-dimensional potential energy surface of the electronic ground state of this ion has been generated. The resulting analytical representation has been used in the full-dimensional variational calculations of the rovibrational energy levels. For all of the isotopologues, the fundamental modes are found to be in close agreement with the available experimental data, and reliable predictions are given for those modes that are not observed experimentally. The rotational transitions, up to the J = 10 value of the total angular momentum quantum number, have also been calculated. The rotational transitions obtained computationally are compared with the values deduced from laboratory experiments or interstellar media detections. Some of these results are predictive, especially those for DCNH⁺ and H¹³CNH⁺. Finally, we provide a theoretical simulation of the rotational spectra for these species at 5 K, 7 K, and 10 K.