The PYR monohydrate is a model system for photoinduced homolytic water splitting . Its structure is quite flexible, as evidenced by rotational spectroscopy . It includes a tunneling motion with an inverted H/D isotope effect, but this is not directly relevant for the vibrational transition at low resolution. There is a room temperature gas phase value for the hydrogenbonded OH stretching mode in PYR monohydrate , at 3480 cm -1 . However, for such a soft and flexible hydrogen bond, it is likely that the cold transition obtainable in a jet expansion occurs at significantly lower wavenumber. While ultracold helium droplets should exhibit much smaller, but still significant environmental shifts than a room temperature gas phase measurement, a corresponding recent study does not address the relevant spectral range and isotope composition.