Table 2.

Computational IR activity using the linear response approach at the PBEsol level of theory performed at $\mathrm{\Gamma }$.

Mode	Assignment	Computed frequency	IR intensity	Observed frequencies9
33	${\text{N}}_5^{+}$ ${\nu }_2$ symmetric central stretch	907.67	0.1443	872
34	${\text{N}}_5^{+}$ ${\nu }_8$ asymmetric central stretch	1304.99	7.1052	1088
35	${\text{N}}_5^{+}$ ${\nu }_7$ out-of-phase terminal stretches	2227.18	8.4356	2210
36	${\text{N}}_5^{+}$ ${\nu }_1$ in-phase terminal stretches	2286.38	1.7218	2270

Four ${\text{N}}_5^{+}$-related normal modes observed in the work of Christe et al.⁹ are presented here for comparison. Frequency wavenumbers are in cm${}^{-1}$ and IR intensities in (D/A)${}^2$/amu.