Abstract
Calorimetric studies on ice II reveal a surprising H2O/D2O isotope effect. While the ice II to ice Ic transition is endothermic for H2O, it is exothermic for D2O samples. The transition enthalpies are +40 and −140 J/mol, respectively, where such a sign change upon isotope substitution is unprecedented in ice research. To understand the observations we employ force field calculations using two water models known to perform well for H2O ice phases and their vibrational properties. These simulations reveal that the isotope effect can be traced back to zero-point energy. q-TIP4P/F fares better and is able to account for approximately three-fourths of the isotope effect, while MB-pol only catches approximately one-third. Phonon and configurational entropy contributions are necessary to predict reasonable transition enthalpies, but they do not have an impact on the isotope effect. We suggest to use these calorimetric isotope data as a benchmark for water models.
Highlights
Calorimetric studies on ice II reveal a surprising H2O/D2O isotope effect
Other high-pressure ice phases such as ice V, ice VI, or ice XII show pronounced exotherms of −926 ± 20,10 −1523 ± 16,11 and −1233 ± 23 J/mol,[12] respectively. This is explained by taking into account that the heat of transformation is composed of two contributions: The first contribution is an enthalpic term arising from the transformation from high density to low density
This change in the H atom subnetwork is responsible for the decrease in heat capacity observed in the calorimetry scans the number of degrees of freedom and heat capacity is smaller in an H-ordered ice
Summary
To compare with the measured enthalpies for the ice II → Ic transition and analyze the isotope effect, we considered the following (free) energy differences, decreasing the number of approximations in each step
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