Scaled harmonic IR spectra of HMT⁺ calculated at the UCAM-B3LYP/aug-cc-pVTZ level for both predicted optimized ground-state structures (C_3v, C_2v). The spectra are convoluted using a Lorentzian line profile with an FWHM of 4 cm^–1. The employed scaling factors are 0.9450 (2500–3000 cm^–1, CH₂ stretch), 0.9700 (1300–1600 cm^–1, CH₂ wag), 0.9664 (900–1300 cm^–1, CN stretch), and 0.9618 (50–900 cm^–1, cage bending). These are determined by comparing the calculated frequencies of neutral HMT with those determined from the rotationally resolved IR spectrum of HMT measured by O. Pirali et al. (2014).