(a) Equilibrium C–N bond lengths (in angstroms) of HMT⁺ in the calculated stable conformers with C_3v (blue) and C_2v (black) symmetry. The C_3v structure is slightly more stable than the C_2v structure, with a small barrier of 14.6 kJ mol^–1 before zero-point energy correction. (b) NTO pairs representing the D₃←D₀ transition for both optimized D₀ structures (C_3v, C_2v).