Quantum Chemical Analysis of the CO-HNN⁺ Proton-Bound Complex

Proton-bound complexes produce exceptionally bright vibrational modes for stretches involving the hydrogen atom. Binding a proton between various arrangements of N₂ and carbon monoxide molecules is known to produce such behavior, and there are four distinct structures involving N₂, CO, and a proton. The problem arises in that all four have the same mass and are, consequently, extremely difficult, if not impossible, to resolve experimentally. Fortunately, quantum chemical predictions have produced accurate descriptions of this bright mode and other spectral features for OCHCO⁺, NNHNN⁺, and NN-HCO⁺. The last of this family to be analyzed is CO-HNN⁺, which is done here. Utilizing high-level coupled cluster computations and quartic force fields, the bright vibrational mode of CO-HNN⁺ is shown to shift to the red, and the C-O bond is destabilized in this arrangement as opposed to the lower-energy NN-HCO⁺ isomer studied previously. Furthermore, the 1.87 D center-of-mass dipole moment, spectroscopic constants, and other anharmonic fundamental frequencies and intensities are produced for CO-HNN⁺ to assist in definitive experimental and even astrochemical classification of this and the other three related mass-57 proton-bound complexes.