A New Cluster Pair Method of Determining Absolute Single Ion Solvation Energies Demonstrated in Water and Applied to Ammonia

Abstract

A new method of determining standard absolute solvation energies (enthalpies and Gibbs free energies) for individual ions is presented. This method originated from the cluster pair based approximation used in earlier work [Tissandier et al., J. Phys. Chem. A 1998, 102, 7787] and is called the cluster pair correlation scheme. Unlike the earlier approximation, the new scheme makes analysis possible in solvent systems for which bulk ion-solvation data are complemented by cluster ion data, either experimental or theoretical, for clusters containing only a single solvent molecule. The correlation scheme features linear plots of half the difference between standard conventional cluster solvation energies for anions and cations containing equal numbers of solvent molecules in the cluster vs their bulk counterparts. All of the correlation lines should intersect at a point whose ordinate defines the unknown shift between the conventional and absolute energy scales, the standard absolute energy of formation of the proton. The results obtained using this correlation scheme to analyze the available cluster ion data for water are in substantial agreement with the results reported earlier using the cluster pair based approximation. Preliminary values for the standard absolute enthalpy and Gibbs free energy of ammonation of the proton are obtained when the cluster pair correlation scheme is applied to ammonia, and a clear indication is obtained of what additional data are needed to improve these preliminary estimates.

TD-DFT study of the for coumarins

“…The free energy of the proton in gas phase (G H þ gas ) and its solvation free energy (DG H þ solv ) are derived from experiments and are taken to be À6.28 kcal/mol and À263.98 kcal/mol, respectively [35,36]. An additional energy contribution, DG add (-1.9 kcal/mol), should be included to account for the transfer of a solute molecule from a 1 atm gas phase into a 1 M solvent standard state.…”

Single-ion hydration thermodynamics from clusters to bulk solutions: Recent insights from molecular modeling

“…Sometimes the term 'absolute' is used to characterize hydration properties, typically invoked as a synonymous for 'intrinsic' [20,21], but it is also used to describe the results of cluster experiments [7,22,23], whose outcome has been extensively considered to measure 'real' properties [21,[24][25][26], which naturally include the cluster surface potential [20]. To avoid confusion, here we will avoid the term 'absolute' in favor of 'real' and 'intrinsic'.…”

“…Tuttle et al [22] developed a cluster-pair correlation method in which the values of proton hydration thermodynamics are determined from the common intersection of plots of 0:5 D aq X ; con B À H 2 O [23]. The authors also noticed a non-negligible dependence of the calculated hydration enthalpy on the size of the clusters.…”

“…The higher multipolar terms in Eq. (22) were truncated. Cendagorta and Ichiye [105], equated the electrochemical surface potential to the one felt by an H + test probe sampling regions outside the van der Walls radii of water molecule atoms and shown that deep in the bulk aqueous phase it is approximately equal to the dipolar contribution f sp;D G to the full surface potential.…”

“…The same substitution of intrinsic for conventional quantities can be applied to the original CPA expressions, given by Eqs. (19) and (20) [7], as well as to the related cluster-pair correlation expressions [12,22,23].…”