Quantum Cluster Equilibrium Theory of Liquids: Isotopically substituted QCE/3-21G Model Water

The simplest Quantum Cluster Equilibrium (QCE/3-21G level) theory of water is extended to isotopically substituted water. Microstructural composition and macroscopic properties are calculated for light, heavy and superheavy water. The main goal is to study qualitative aspects of cluster populations, phase diagrams and thermodynamical properties as a function of isotopic substitution. The QCE triple point is shifted to higher temperatures about 2.0 K for deuterated and 3.0 K for tritiated water, approximating the experimental shift of 3.8 K and 4.5 K, respectively. We find that isotopic substitution leads not only to the well known lowering of zero point energies and vibrational frequencies but also to changing cluster populations and thus to slightly different population-weighted structures. This result differs from the usual assumption that the structural properties of different isotopically substituted waters are nearly identical once a zero point energy induced thermal offset is taken into account, but is in agreement with results inferred from X-ray diffraction.