  Molecules with an excess number of hydrogen-bonding partners play a crucial
role in fundamental chemical processes, ranging from the anomalous diffusion in
supercooled water to the transport of aqueous proton defects and the ordering
of water around hydrophobic solutes. Here we show that overcoordinated hydrogen
bond environments can be identified in both the ambient and supercooled regimes
of liquid water by combining experimental Raman multivariate curve resolution
measurements and machine learning accelerated quantum simulations. In
particular, we find that OH groups appearing in spectral regions usually
associated with non-hydrogen-bonded species actually correspond to hydrogen
bonds formed in overcoordinated environments. We further show that only these
species exhibit a turnover in population as a function of temperature, which is
robust and persists under both constant pressure and density conditions. This
work thus provides a new tool to identify, interpret, and elucidate the
spectral signatures of crowded hydrogen bond networks.
