  We report an analysis of the stretching peak appearing in the IR experimental
spectra of liquid water. In the literature, ATR-IR spectroscopic measurements
were repeatedly performed in a wide range of temperature and gave rise to a
lively debate among scientists. In particular a two components model related to
H-bond complexes of different strength have been proposed in order to justify
the existence of two types of molecules as it appears from the spectroscopic
data. At the opposite, Molecular Dynamics simulations support a multistate
(continuum) system of H bond having different strength giving rise to a
(locally) tetrahedral description of liquid water. We will show that liquid
water is a quantum two-level system according to the predictions of Quantum
Electrodynamics (QED) and that several features (the asymmetric band profile,
the existence of an isosbestic point and the modifications of the vibrational
stretching band with the temperature) cannot be fully justified in the realm of
a classical picture. In particular the differences of energy and entropy
between the two phases are estimated from the experimental data and compared
with the prediction of QED showing a remarkable agreement. The behavior of
water near hydrophilic surfaces is also discussed and several feature of the so
called Exclusion Zone observed by several authors are evaluated according to
the two level system model.
