A path integral Monte Carlo study of quantum solutes in liquid ammonia and ammonia clusters

Abstract

<p>The path integral Monte Carlo method is applied to some solvation problems involving quantum solutes. Thermodynamics quantities like partial molar volume and entropy change associated with the electron solvaton process are calculate. The ionization of alkali atoms in liquid ammonia and the electron attachment to ammonia clusters are investigated.</p> <p>Path integral MC simulations at constant pressure lead to an expansion of the simulation box in satisfactorily agreement with the experimental molar volume. The structure of the solvated electron at constant pressure was found to be very similar to the one at constant volume. The electron wavefunction was only slightly expanded.</p> <p>The Debye charging trick is used to calculate the free energy of the solvated electron at constant volume. The experimental solvation entropy at constant pressure is reproduced only when a correction due to the volume expansion is introduced. The contribution to the entropy due to the ordering of the ammonia molecules induced by the electron is negative. The expansion work, performed by the light when the electron is introduced, is responsible for the positive experimental entropy.</p> <p>Spontaneous ionization is observed only for Na and Cs when a hard core (HC) pseudopotential is employed. A soft core (SC) pseudopotential leads to the formation of dipolar atoms when tested with Li and Cs. A dipolar Li atom is observed even for the HC model. These calculations predict that the minimum energy state for at least Na and Cs is indeed the separately solvated ions. This is agreement with the experimental evidence.</p> <p>The nature of electron attachment to ammonia clusters composed of 16, 36, and 54 molecules is studied. At 100 K, a negatively charged clusters of 16 molecules appears to be unstable in the sense that the electron binding energy is less than kBT. For both the 36 and 54 molecule clusters the electron binds to the cluster surface. The 54 molecule cluster also supports a (meta) stable interior solvated state. These findings are discussed in the light of experimental data on the same system.</p>