Quantum computed vibrational normal modes of isomers of HBC⁺ ((n₁, n₂, n₃) = (18, 0, 0)) in the range of 0–4000 cm⁻¹. Panel (A_1–4) shows the IR spectrum of [C₄₂H₁₈]⁺, (n₁, n₂, n₃) = (10, 4, 4)_1–4; panel (B) shows the IR spectrum of [C₄₂H₁₈]⁺, (n₁, n₂, n₃) = (8, 5, 5); panel (C) shows the IR spectrum of [C₄₂H₁₈]⁺, (n₁, n₂, n₃) = (12, 3, 3); and panel (D) shows the IR spectrum of HBC cations, (n₁, n₂, n₃) = (18, 0, 0). The vibrational band positions are scaled by a constant factor of 0.9670, and the black line represents the spectra simulated by Gaussians with a full width at half maximum of 4 cm⁻¹ (Boersma et al. 2014).