Diclofenac
(active ingredient of Voltaren) has a significant, multifaceted
role in medicine, pharmacy, and biochemistry. Its physical properties
and impact on biomolecular structures still attract essential scientific interest. However, its interaction
with water has not been described yet at the molecular level. In the
present study, we shed light on the interaction between the steric
hindrance (the intramolecular N–H···O bond,
etc.) carboxylate group (−CO2–) with water. Aqueous solution of sodium declofenac is investigated
using attenuated total reflection-infrared (ATR-IR) and computational
approaches, i.e., classical molecular dynamics (MD) simulations and
density functional theory (DFT). Our coupled classical MD simulations,
DFT calculations, and ATR-IR spectroscopy results indicated that the
−CO2– group of the diclofenac
anion undergoes strong specific interactions with the water molecules.
The combined experimental and theoretical techniques provide significant
insights into the spectroscopic manifestation of these interactions
and the structure of the hydration shell of the −CO2– group. Moreover, the developed methodology for
the theoretical analysis of the ATR-IR spectrum could serve as a template
for the future IR/Raman studies of the strong interaction between
the steric hindrance −CO2– group
of bioactive molecules with the water molecules in dilute aqueous
solutions.