The two TH-T signals are split by 31 cm -1 . This should be compared to the two lowest, almost isoenergetic computed isomers which are split by 17 cm -1 . Their absolute prediction matches experiment within 6 and 20 cm -1 , respectively, if the higher wavenumber signal is assigned to THo-Tt and the lower wavenumber signal to THo-To, as assumed in Figure . This is within the expected accuracy of the scaled harmonic estimate. An inverted assignment would introduce mismatches by 37 and 11 cm -1 in opposite directions. However, theory forecasts the higher wavenumber isomer to be slightly less stable, which contradicts experimental Table Experimentally observed OH stretching wavenumbers ν compared to harmonic B3LYP/TZ predictions ω uniformly scaled by 0.96 for TH and binary donor-acceptor complexes observed for the first time in this work. The value in parentheses refers to an unlikely alternative assignment of the strong transition to the predicted third-most stable THo-Tp isomer, see Table . intensities for these almost equally IR-active isomers and is likely due to the limited accuracy of the B3LYP/TZ approach in predicting sub-kJ/mol energy differences. To check the robustness of the energy sequence of THo-Tt and THo-To, energy calculations at B3LYP/QZ level were carried out at the TZ-optimised structures. This shrinks the electronic energy gap from 0.9 to 0.7 kJ/mol (see ESI †), moving in the right direction, but keeping the experimentally incompatible order. The third isomer remains at 1.8 kJ/mol Therefore, the B3LYP functional appears to have a small deficiency in terms of ordering the two isomers, well within its expected accuracy. Both bands have a FWHM of 2.5-3 cm -1 which is not much wider than the spectral resolution so that any tunnelling splittings due to periodic HAT are too small to be resolved. Higher spectral resolution may be limited by rovibrational structure, unless one moves to a colder and heavier helium nanodroplet environment.