Calorimetric Signature of Deuterated Ice II: Turning an Endotherm to an Exotherm

[Image: see text] Calorimetric studies on ice II reveal a surprising H(2)O/D(2)O isotope effect. While the ice II to ice Ic transition is endothermic for H(2)O, it is exothermic for D(2)O 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 H(2)O 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.