Microhydration of phenylboronic acid and its hydration free energy

Understanding the interactions of phenylboronic acid with its surrounding water molecules is essential for several applications in solvated systems. In the present work, we investigated the microhydration of the phenylboronic acid (PBA) and calculated its hydration free energy using the cluster continuum solvation model. Microhydration of PBA has not been investigated previously in the literature. It requires the structures of PBA to be surrounded by n explicit water molecules (PBA-W $_n^{}$ ). The results show that the B(OH) $_2^{}$ unit of phenylboronic acid forms clusters with water molecules that are similar to those of neutral water clusters. The QTAIM analysis shows that the structures of phenylboronic acid-water clusters are stabilized by strong OH $\cdots$ O and weak CH $\cdots$ O hydrogen bonds. In addition to QTAIM analysis, NBO analysis was also performed on the most stable configurations to better understand the delocalization of electron density from donor to proper acceptor within the compound. In addition, we found that the most stable structures dominate the population of the clusters for temperatures from 20 to 400 K. Finally, using the structures of the microhydrated phenylboronic acid, we estimated the free energy of hydration and the enthalpy of hydration of PBA. At room temperature, the phenylboronic acid's free energy and enthalpy of hydration are respectively evaluated to be $-$ 72.1 kcal/mol and $-$ 85.5k cal/mol. Assessment of temperature effects on the free energy and the enthalpy of hydration shows that the enthalpy is temperature-independent, while the free energy increases linearly with temperature.

Initial configurations of PBA-W $_n^{}$ have been generated using classical molecular dynamics and subsequently optimized using the level of theory, $\omega$ B97X-D/def2-TZVP. Optimizations, frequency calculations, and NBO analysis are performed using the Gaussian 16 suite of programs. On the most stable configurations, we have performed the quantum theory of atoms in molecules (QTAIM) analysis to get insights into the hydrogen bond network of PBA-W $_n^{}$ . QTAIM is performed using AIMAll software. Thermodynamic properties as a function of temperature are evaluated using a homemade FORTRAN code-named TEMPO.