As illustrated in Fig. and proven by rotational spectroscopy , the water molecule inserts into the intramolecular hydrogen bond between the α-hydroxy group and the carbonyl group of the ester, leading to a O-H• • • O-H• • • O= chain of two coupled oscillators. The two top Raman spectra show the effect of adding water to a highly diluted MLA expansion. A strong band of the 1:1 complex emerges at 3474 cm -1 . Minor satellite contributions near 3479 and 3491 cm -1 may be due to traces of larger complexes or due to dark overtone and combination states stealing some intensity from the O-H stretching fundamental by perturbation. If the latter is true, the deperturbed (zeroth-order) state which should be compared to a harmonic or low-order anharmonic prediction is slightly higher than the peak wavenumber, perhaps around 3475-3478 cm -1 . However, this is not so relevant, because the Raman spectrum is most sensitive for the in-phase stretching of the two O-H oscillators. The out-of-phase stretching mode is better seen in the Raman spectra of the more concentrated expansions (central traces) at 3524 cm -1 and it is very prominent in the IR spectra (lowest spectrum in Fig. ), because it involves a larger change of the dipole moment. As shown in the ESI †, it is the only strong transition attributable to the monohydrate in this spectral window and therefore corresponds to the requested wavenumber for the challenge. However, O-substitution in the water molecule demonstrates that both the out-of-phase and the in-phase O-H stretching signals involve some water character, because they both downshift (see ESI †). As the downshift is more pronounced for the 3524 cm -1 band (8 vs. 5 cm -1 ), this is the band which should be compared to single theoretical predictions of the water O-H stretch. Evidently, a comparison of both wavenumbers (listed in Fig. for MLA) to their theoretical counterparts could be valuable as well in this particular case, but will be left for the future and the individual theory groups.