Surface isotope segregation as a probe of temperature in water nanoclusters

P. E. VIDELA; P.J. ROSSKY; D. LARIA

Journal of Physical Chemistry Letters

American Chemical Society

Lugar: Washingto DC; Año: 2014 vol. 4 p. 2375 - 2379

1948-7185

Using Ring-Polymer-Molecular-Dynamics simulations,we examine equilibrium and dynamical characteristics of solid-like,aqueous clusters that combine isotopic mixtures of HDO dilute in H$_2$O,at temperatures intermediate between 50 K and 175 K.In particular, we focus attention on the relative thermodynamicstabilities of the two isotopes at dangling hydrogen bond sites. Thewater octamer is analyzed as a reference system.For this aggregate, decreasing temperature yields agradual stabilization of the light isotopeat dangling sites in molecules acting as single-donor-double-acceptors ofhydrogen bonds. At $Tsim 50$ K, the imbalance between the correspondingquantum kinetic energies leads to a free energy differencebetween dangling and hydrogen bonded sites of the order of $sim 2   k_BT$.Similar free energy differences were found at dangling sites in$N_w=50$ water clusters.  The extent of the H/D segregation can be adequatelymonitored by modifications in the peak intensity of the high frequencyshoulder of the stretching band of the infrared spectrum. These signals,in turn, represent a potentialexperimental signature of the elusive temperature of clusters inmolecular beams.