ON INFLUENCE OF H/D EXCHANGE ON SOLVENT ISOTOPE EFFECTS IN THERMODYNAMIC (ENTHALPIC) CHARACTERISTICS OF SOLVATION OF PROTON-DONATING NON-ELECTROLYTES
Some features of the solvent H/D-isotope effect method are discussed in the frame of development of ideas about the solvation mechanism including the structural and thermodynamic characteristics concept being proposed by G.A. Krestov and its followers. We have found it necessary to debate the possibility of employing the H2O-by-D2O isotope substitution for thermodynamic studying of binary aqueous systems containing a proton-donating organic non-electrolyte. In this regard, the two main aspects of the problem are discussed: (i) how the H-D exchange affects the thermodynamic (enthalpic) solvent isotope effects and (ii) how such effects dependent on the partial or complete pre-deuteration of a solute molecule. All potentially exchangeable protons (of N-H, O-H,) in heavy water are replaced by deuterons, but the fast exchange of D between D2O and the C-H (in methyl and methylen groups) does not occur. Herewith the extent to which proton-donor sites in complex molecules are stabilized by intramolecular hydrogen bonding remains uncertain, making it difficult to assess details of H-D exchange mechanisms in the hydration process. It is important to note that the H-D-isotope exchange is accompanied by changes in the molecular composition both the solute and solvent (due to forming HDO) in the nearest environment. This affects the thermodynamic (mole-additive) parameters of solvation process in heavy water as well as the corresponding isotope effects. The problem can be partly overcome by using the deuterium-substitution in a solute molecule. In this case, the molar mass of each solution component does not change but we should not forget here on the effect of secondary H/D isotope substitution in the solute. From a purely thermodynamic view, we can focus only on the analysis of H/D isotope effects in characteristics of its sublimation or vaporization. Herewith H- and D-bonded systems have the same distinctions in the (zero-point) vibration energy as in individual components. It because the specified isotope effects can be rather directly related to the condensed-phase partition functions which may be written down under the assumption of isotope-independent potential energy surface (within the precision of Born-Oppenheimer “adiabatic” approximation).