Isotopic Effects on Intermolecular and Intramolecular Structure and Dynamics in Hydrogen, Deuterium, and Tritium Liquids: Normal Liquid and Weakly and Strongly Cooled Liquids

Abstract

Differences in properties such as phase-transition temperature and transport coefficients among liquids of different isotopic compositions, hydrogen, deuterium, and tritium, should originate from their differently pronounced nuclear quantum effects (NQEs) rather than from any subtle difference in the electronic interaction potentials. Accurate and efficient determination of structural and dynamical isotopic effects in the quantum liquids still remains as one of the challenging problems in condensed-phase physics. With a recently developed nonempirical real-time molecular dynamics method which describes nonspherical molecules with the NQEs, we computationally realized and investigated dynamical and quantum isotopic effects of not only traditionally studied isotopes, hydrogen, and deuterium but also a lesser known radioisotope, tritium, in broad thermodynamic conditions from normal liquid to weakly and strongly cooled liquids, which have been hindered by rapid crystallization in spite of numerous experimental attempts at supercooling. Reproducing the previously reported experimental isotope dependence on the bond length and vibrational frequencies of hydrogen, deuterium, and tritium liquids, we further demonstrate that distinctive isotope effects appear in their intermolecular and intramolecular structure and dynamics not only at lower temperature but also at higher temperature, which none has so far been able to obtain quantitative results for realistic systems. Rationalization of their physical origins and the obtained physical insights will help future experimental searching and monitoring intermolecular and intramolecular dynamics and structures of these isotopes not only in normal liquid but also in supercooled liquid.

Decelerated Liquid Dynamics Induced by Component-Dependent Supercooling in Hydrogen and Deuterium Quantum Mixtures

“…The RDFs exhibit little difference regardless of the temperature and the D 2 ratio. The electronic potentials created by the EWPs are identical for H 2 and D 2 , leading to the similar RDFs under the same thermodynamic conditions, although a much lower temperature could induce a difference . The maximum difference of 0.0149 Å between the H–H and D–D bond length does not induce any significant difference in the present RDFs (see Figure S4).…”

“…In contrast, the power spectra at 18 K possess the broad low-frequency peaks around 20 cm −1 which can be assigned as the so-called boson peak that is known as a direct evidence of the supercooled and glassy states. 72,73 The broad peak around 20 cm −1 also appeared when the pure H 2 and D 2 liquids became supercooled below the freezing temperature, 60,62 supporting that the mixtures at 18 K are actually supercooled now. It is interesting that the intensity of the extremely lowfrequency power spectra below 10 cm −1 reflecting the low-energy translational dynamics 60,62 increases in the same order as the self-diffusion coefficients at any temperature, e.g.…”

“…While the semiquantum methods based on the imaginary-time path-integral theory were proposed, , we recently developed the nonempirical ab initio simulation method of nuclear and electron wave packet molecular dynamics (NEWPMD) where the important NQEs such as nuclear delocalization and zero-point energy were nonperturbatively taken into account. − The NEWPMD method has successfully given the structural and dynamical properties which are close to the experimental observations in the condensed p -H 2 and o -D 2 systems reflecting the proper isotope-dependent NQEs. Specifically, the NEWPMD method has reproduced the radial distribution functions and self-diffusion coefficients in the p -H 2 and o -D 2 vapor-pressure liquids including their anomalous temperature dependence, ,, the hexagonal close-packed structures, librational frequency and collective phonon modes in the p -H 2 vapor-pressure solids, the freezing temperature indicated by the stable p -H 2 vapor-pressure solid phase at 13 K, the stable p -H 2 vapor-pressure liquid phase at 14 K in addition to the metastable supercooled phase with the lower density even below the freezing temperature, , the boson peak around 20 cm –1 in the supercooled p -H 2 liquids analogous to the low-frequency boson peaks observed in various glasses and supercooled liquids, the intramolecular bond length and vibrational frequencies of p -H 2 and o -D 2 molecules including the discrete frequency jump due to the phase transition from the p -H 2 vapor-pressure liquid to solid, , and the thermal conductivity directly obtained through Fourier’s law in the nonequilibrium p -H 2 liquids under heat conduction . The above facts ensure that the NEWPMD simulations reasonably sit in the experimental thermodynamic region.…”

“…(Table S1) The averages of the nuclear kinetic energy and remaining potential energies are also shown in A mean square displacement(MSD) characterizes liquid dynamics including kinetic motion inside a solvation shell and long-range diffusion out of a shell. 60,62,66,67 The MSDs of the H 2 -D 2 mixtures at the different temperatures are shown in Figs. 1(a)-(c).…”

“…2 D 2 liquid cases. 57,60,62 Although the θ-TCFs at 25 K and 18 K exhibit the clear bidecay process, 58,60,62 the decay profile of the θ-TCFs at 30 K seems more likely a single decay process, indicating that a solvation cage is more fragile in the mixtures at 30 K. We note that there is no qualitative difference between the TCFs of θ and the self-orientation φ defined in Fig. S8; their angular dynamics is similar and the choice of θ or φ does not affect the current conclusions.…”