Infrared spectra and anharmonic coupling of proton-bound nitrogen dimers N₂-H⁺-N₂, N₂-D⁺-N₂, and ¹⁵N₂-H⁺-¹⁵N₂ in solid Para -hydrogen

The proton-bound nitrogen dimer, N₂-H⁺-N₂, and its isotopologues were investigated by means of vibrational spectroscopy. These species were produced upon electron bombardment of mixtures of N₂ (or ¹⁵N₂) and para-hydrogen (p-H₂) or normal-D₂ (n-D₂) during deposition at 3.2 K. Reduced-dimension anharmonic vibrational Schrödinger equations were constructed to account for the strong anharmonic effects in these protonated species. The fundamental lines of proton motions in N₂-H⁺-N₂ were observed at 715.0 (NH⁺N antisymmetric stretch, ν₄), 1129.6 (NH⁺N bend, ν₆), and 2352.7 (antisymmetric NN/NN stretch, ν₃) cm^-1, in agreement with values of 763, 1144, and 2423 cm^-1 predicted with anharmonic calculations using the discrete-variable representation (DVR) method at the CCSD/aug-cc-pVDZ level. The lines at 1030.2 and 1395.5 cm^-1 were assigned to combination bands involving nν₂ + ν₄ (n = 1 and 2) according to theoretical calculations; ν₂ is the N₂⋯N₂ stretching mode. For ¹⁵N₂-H⁺-¹⁵N₂ in solid p-H₂, the corresponding major lines were observed at 710.0 (ν₄), 1016.7 (ν₂ + ν₄), 1124.3 (ν₆), 1384.8 (2ν₂ + ν₄), and 2274.9 (ν₃) cm^-1. For N₂-D⁺-N₂ in solid n-D₂, the corresponding major lines were observed at 494.0 (ν₄), 840.7 (ν₂ + ν₄), 825.5 (ν₆), and 2356.2 (ν₃) cm^-1. In addition, two lines at 762.0 (weak) and 808.3 cm^-1 were tentatively assigned to be some modes of N₂-H⁺-N₂ perturbed or activated by a third N₂ near the proton.