Surface Isotope Segregation as a Probe of Temperature in Water Nanoclusters.
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 H2O, at temperatures intermediate between 50 and 175 K. In particular, we focus attention on the relative thermodynamic stabilities of the two isotopes at dangling hydrogen bond sites. The water octamer is analyzed as a reference system. For this aggregate, decreasing temperature yields a gradual stabilization of the light isotope at dangling sites in molecules acting as single-donor-double-acceptors of hydrogen bonds. At T ∼ 50 K, the imbalance between the corresponding quantum kinetic energies leads to a free energy difference between dangling and hydrogen bonded sites of the order of ∼2kBT. Similar free energy differences were found at dangling sites in Nw = 50 water clusters. The extent of the H/D segregation can be adequately monitored by modifications in the peak intensity of the high frequency shoulder of the stretching band of the infrared spectrum. These signals, in turn, represent a potential experimental signature of the elusive temperature of clusters in molecular beams.
†Departamento de Química Inorgánica Analítica y Química-Física e INQUIMAE, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Pabellón II, 1428 Buenos Aires, Argentina.
Pablo E
PE
Videla
‡Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712-0165, United States.
Peter J
PJ
Rossky
†Departamento de Química Inorgánica Analítica y Química-Física e INQUIMAE, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Pabellón II, 1428 Buenos Aires, Argentina.|§Departamento de Física de la Materia Condensada, Comisión Nacional de Energía Atómica, Avenida Libertador 8250, 1429 Buenos Aires, Argentina.
Daniel
D
Laria
isotope effects; nuclear quantum fluctuations; path integrals; ring polymer molecular dynamics