An improved cluster pair correlation method for obtaining the absolute proton hydration energy and enthalpy evaluated...

Area of Science:

• Computational chemistry
• Physical chemistry
• Thermodynamics

Background:

• The Tuttle et al. cluster pair correlation method provides a framework for calculating proton hydration free energy.
• Previous applications of this method were limited by smaller datasets, potentially introducing uncertainty.
• Proton hydration is a fundamental process in chemistry and biology.

Purpose of the Study:

• To develop and evaluate an improved cluster pair correlation method for calculating proton hydration thermodynamics.
• To assess the impact of dataset size and ion selection on the accuracy of hydration free energy calculations.
• To determine a more reliable value for the standard absolute proton hydration free energy.

Main Methods:

• An improved cluster pair correlation method was developed based on the Tuttle et al. approach.
• A significantly larger dataset than previously used was employed for evaluation.
• Data from ions with extreme pK(a) values, deemed unreliable, were excluded.

Main Results:

• The improved method yielded a proton hydration free energy of -265.0 kcal/mol, compared to -259.3 kcal/mol with the original method on the larger dataset.
• The "best" value for the standard absolute proton hydration free energy was determined to be -263.4 kcal/mol.
• Absolute proton hydration enthalpy decreased from -273.1 to -275.3 kcal/mol with increasing cluster size (1-6 water molecules).

Conclusions:

• The improved method offers higher precision and better evaluation of cluster size effects on hydration thermodynamics.
• The dependence of results on dataset size highlights potential uncertainties in the original method.
• The calculated enthalpy may not have fully converged for small clusters, suggesting further investigation with larger clusters is warranted.