  The nanoconfinement of water can result in dramatic differences in its
physical and chemical properties compared to bulk water. However, a detailed
molecular-level understanding of these properties is still lacking. Vibrational
spectroscopy, such as Raman and infrared, is a popular experimental tool for
studying the structure and dynamics of water, and is often complemented by
atomistic simulations to interpret experimental spectra, but there have been
few theoretical spectroscopy studies of nanoconfined water using
first-principles methods at ambient conditions, let alone under extreme
pressure-temperature conditions. Here, we computed the Raman and IR spectra of
water nanoconfined by graphene at ambient and extreme pressure-temperature
conditions using ab intio simulations. Our results revealed alterations in the
Raman stretching and low-frequency bands due to the graphene confinement. We
also found spectroscopic evidence indicating that nanoconfinement considerably
changes the tetrahedral hydrogen bond network, which is typically found in bulk
water. Furthermore, we observed an unusual bending band in the Raman spectrum
at ~10 GPa and 1000 K, which is attributed to the unique molecular structure of
confined ionic water. Additionally, we found that at ~20 GPa and 1000 K,
confined water transformed into a superionic fluid, making it challenging to
identify the IR stretching band. Finally, we computed the ionic conductivity of
confined water in the ionic and superionic phases. Our results highlight the
efficacy of Raman and IR spectroscopy in studying the structure and dynamics of
nanoconfined water in a large pressure-temperature range. Our predicted Raman
and IR spectra can serve as a valuable guide for future experiments.
