Isotope-Substitution Effects on the Thermodynamic, Dynamic, and Structural Properties of Water: H2O, HDO, D2O, and T2O.

We study the isotope-substitution effects on the thermodynamic, dynamical, and structural properties of liquid water at (i) constant molar volume (v = 18.0 cm³/mol, corresponding to a density for H₂O of ρ = 1.0 g/cm³) and (ii) constant pressure (P = 0.1 MPa) over a wide temperature range, 200 ≤ T ≤ 400 K. Our results are based on path-integral and classical computer simulations of H₂O, HDO, D₂O, and T₂O using the q-TIP4P/F water model. We find that some properties, such as the pressure P(T) (at constant v) and molar volume v(T) (at constant P) are weakly sensitive to isotope substitution effects, while others, including the isochoric/isobaric heat capacity, self-diffusion coefficient, vibrational density of states, and infrared (IR) spectra, are considerably affected by nuclear quantum effects (NQE). The IR spectra and diffusion coefficients obtained from ring-polymer molecular dynamics (RPMD) simulations are in very good agreement with available experimental data. Our path integral computer simulations, particularly at low temperatures, show that the (H → D → T)-substitution in water leads to a slightly more structured liquid with shorter (smaller OO distance) and more linear (smaller HOO angle) hydrogen bonds (HB). This is rationalized in terms of the very small decrease in the atom delocalization (NQE) along the sequence (H → D → T). In all three cases, the H/D/T atoms are preferentially delocalized along the direction perpendicular to the O-(H/D/T) covalent bond. The different delocalization of H/D/T leads to a slightly more energetic HB (<4%) and hence, to a slightly stronger HB-network, along the sequence H₂O → HDO → D₂O → T₂O (as NQE becomes less pronounced). Interestingly, some properties of HDO, such as the IR spectra, radial distribution functions, and HB geometry, suggest that the OD and OH covalent bonds of HDO behave, respectively, as the OD covalent bond of D₂O and the OH covalent bond of H₂O.