Ionization of ascorbic acid in the presence of the first coordination shell of water: The effects of the conformational change, hydrogen bonding, and electrostatic interaction on the ionization energy

In this work, the effect of the first coordination shell of water, as a solvent, on the first ionization energy of the solvated ascorbic acid (AA) was investigated at different temperatures (298, 310, 330, and 350 K). The molecular dynamics (MD) simulation was employed to obtain the equilibrium configurations of the solvated AA in water. The increase of the first ionization energy of AA was observed due to the first coordination shell of the solvent. The average photoelectron spectrum of the solvated AA was calculated at each temperature, and the increase of the first ionization energy was observed with the increase in temperature. The contribution of the explicit and electrostatic effects of the water molecules to the variation of the first ionization energy of AA, compared to the gas phase, was determined. The variation of the first ionization energy of the AA versus the equilibrium snap-shots of the MD simulation, in the presence and absence of water molecules, was investigated by calculating the energy and contribution of the natural bonding orbitals (NBOs) forming the high occupied molecular orbital (HOMO) of the AA. It was found that the contribution of the pi C=O in the HOMO of AA increases with the increase of the first ionization energy of the AA in the absence of the water molecules. It was observed that the variation of the first ionization energy of the solvated AA with the change of the configuration of water molecules of the first coordination shell is mainly controlled by the pi C=C of AA.