The third new signal falls in between, in a spectral region where one would expect the TH-T self-exchange complex. Indeed, there is evidence that TH at higher concentration and longer gas recycling decomposes to form T radicals. Weak absorption in the region marked w-T also points at traces of the monohydrate of this radical, which was characterised before. To identify any monohydrate of TH in close vicinity, an expansion with high TH dilution and more water was investigated (bottom spectrum). It suppresses the w-T signals and enhances a band marked w-TH which is within 24 cm -1 of the scaled harmonic prediction for this monohydrate with a cyclic OH• • • OH• • • N arrangement. In this complex, the more downshifted OH oscillator is expected to be centred on the water molecule. A stronger band predicted at lower wavenumber due to this hydrogen bonded water OH vibration would be of interest in the context of the HyDRA challenge for monohydrates of organic molecules. It cannot be detected, perhaps due to spectral overlap or anharmonic coupling, rendering the entire w-THo assignment quite uncertain. Figure encodes these computational predictions and focuses on the TH-T complex by deliberately adding T to the expansion. Compared to the TH-only spectrum reproduced from Figure (upper trace), addition of 130 ppm T (second trace) gives rise to two strong signals marked TH-T with ≈3:1 intensity ratio, together with the already characterised T monohydrate. The higher-wavenumber signal of the TH-T pair is stronger and its relative strength increases slightly upon further dilution (third trace). This points at two isomers, with the higher-wavenumber isomer being more stable. The subtle inverse relationship between hydrogen bond shift and complex stability is another indication for the role of London dispersion interactions besides hydrogen bonding in holding the two units together. Roughly doubling the TH and w concentrations (bottom trace) doubles the TH-T and w-T signals and further increases the TH-TH and w-TH bands, as expected. TH 0.05 hPa TH 0.05 hPa T 0.10 hPa TH 0.05 hPa T 0.05 hPa TH 0.10 hPa T 0.05 hPa Fig. FTIR spectra of TH,T co-expansions in 750 hPa helium (darker red illustrates higher TH concentration). 0.96-scaled harmonic B3LYP/TZ predictions (see ESI † and Table ) are connected with the experimental positions of selected transitions.