Isotopic Preferential Solvation of I^– in Low-Temperature Water Nanoclusters

Abstract

We present results from ring polymer molecular dynamics experiments that provide microscopic insights into the characteristics of the isotopic stabilizations of H and D aqueous species in the first solvation shell of a halide I(-) ion in water nanoclusters at low temperatures. The analysis of the simplest I(-)·(HOD) dimer shows a clear propensity for the light isotope to lie at the non-hydrogen-bonded dangling position. Our results predict that, at T ∼ 50 K, I(-)·(DOH) isomers are three times more abundant than I(-)·(HOD) ones. The reasons for such stabilization can be traced back to differences in the nuclear kinetic energy projected along directions perpendicular to the plane of the water molecule. Dynamical implications of these imbalances are shown to be reflected in the characteristics of the corresponding bands of the infrared spectroscopic signals. A similar analysis performed in larger aggregates containing ∼20 water molecules reveals, in contrast, a stabilization of the light isotope along I(-)···HO hydrogen bonds. Effects derived from the consideration of smaller halide anions with larger electric fields at the surface are also examined.

Isotope effects in aqueous solvation of simple halides

“…As such, the general picture that emerges from this set of data indicates that the overall differences between T X,m H and T W,m H -and consequently, the resulting values of ∆A-are dominated by contributions from directions perpendicular to the relevant O-H direction along ionic H-bonds. This direct observation is consistent with conclusions reported in previous analyses, 12,21,22,44 but contrasts with what is found in the aforementioned HX solvation scenarios, where stability control is transferred to differences in parallel projections. 11 Physical interpretations of the last features are afforded by basic quantum arguments that relate average kinetic J. Chem.…”

“…We note that a similar reasoning also explains the propensity registered in the connectivity of the simplest I − • DOH dimer, where the interplay is between ionic-bonded and dangling positions. 12,45 The previous characteristics can also be, in part, inferred from the analysis of IR 9,10 and Raman 7,46 signals. Within the spirit of the RPMD approximation, the vibrational spectrum I(ω) can be cast in terms of the Fourier transform of the second derivative of the Kubo-transformed, RPMD time correlation function of the dipole moment, namely, 15,19…”

“…[5][6][7][8] This is in large part due to limitations in the interpretation of direct spectral signals, which can stem from a variety of H-bonding scenarios. 9,10 The present analysis represents a natural continuation of two recent studies, in which we examined isotopic stabilities along H-bonds, in the vicinity of hydrogen halides and I dissolved in aqueous nanoclusters that combine HOD and H 2 O, at temperatures close to ∼50 K. 11,12 The results that emerged from these studies revealed a variety of isotopic preferences, which are the result of delicate interplays between inter and intramolecular couplings, which, in turn, are controlled by the topology of the corresponding three dimensional potential energy surfaces. As such, in what follows, we will present a comprehensive description of characteristics of H-bonding in the first solvation shells of several simple halides in solution, from a molecular perspective that focuses on preferential solvation by different isotopic species.…”

Nuclear quantum effects enter the mainstream

“…One problem where competing quantum effects have been particularly useful is in studies of the fractionation between isotopes. Fractionation of isotopes has recently been investigated in systems encompassing hydrogen/deuterium fractionation between its liquid and vapor phases [86,87], in water and ion clusters [88,89] and at the liquid-vapor interface of water [90] as well as lithium isotope fractionation between aqueous solution and phyllosilicate minerals [91]. These isotope fractionation ratios, which would be zero if NQEs were neglected, are exploited extensively in applications ranging from monitoring climatic temperature shifts [92,93] to assessing whether low barrier hydrogen bonds are present in biological systems [94,95].…”

Inclusion of nuclear quantum effects for simulations of nonlinear spectroscopy

“…65,66 Finally, the dynamics generated by the RPMD approximation preserve the initial Boltzmann distribution, which is an appealing feature for any semiclassical approximation to avoid leakage of energy between different modes. 67 The RPMD methodology has been successfully applied to a wide range of problems including rate theory, [68][69][70][71][72][73][74][75][76] simulations of linear spectroscopy in water clusters [77][78][79][80] and bulk water, 39,40,81,82 and in describing diffusion dynamics of liquid systems 21,[83][84][85] to name a few examples. In recent years, different efforts have been made to extend RPMD to describe nonadiabatic dynamics [86][87][88][89] and non-equilibrium conditions.…”