  The supra-molecular structure of a liquid is strongly connected to its
dynamics which in turn controls macroscopic properties such as viscosity.
Consequently, detailed knowledge about how this structure changes with
temperature is essential to understand the thermal evolution of the dynamics
ranging from the liquid to the glass. Here we combine infrared spectroscopy
(IR) measurements of the hydrogen (H) bond stretching vibration of water with
molecular dynamics simulations and employ a quantitative analysis to extract
the inter-molecular H-bond length in a wide temperature range of the liquid.
The extracted expansivity of this H-bond differs strongly from that of the
average nearest neighbor distance of oxygen atoms obtained through a common
conversion of mass density. However, both properties can be connected through a
simple model based on a random loose packing of spheres with a variable
coordination number which demonstrates the relevance of supra-molecular
arrangement. Further, the exclusion of the expansivity of the inter-molecular
H-bonds reveals that the most compact molecular arrangement is formed in the
range $\sim316-331\,\text{K}$ (i.e. above the density maximum) close to the
temperature of several pressure-related anomalies which indicates a
characteristic point in the supra-molecular arrangement. These results confirm
our earlier approach to deduce inter-molecular H-bond lengths via IR in
polyalcohols [Gabriel et al. 2021] quantitatively and open a new alley to
investigate the role of inter-molecular expansion as a precursor of molecular
fluctuations on a bond-specific level.
