Steering H-atom diffusion through impurity-doped solid parahydrogen: the role of differential solvation energies

Ultraviolet irradiation of solid parahydrogen (pH$_2$) matrices doped with suitable H-atom precursor molecules generates H atoms {\it in situ\/} through a series of photoinitiated chemical reactions; these H atoms move through the pH$_2$ matrix through a quantum diffusion process that involves the tunneling-mediated process H + H$_2$ $\rightarrow$ H$_2$ + H. The mobile H atoms may react chemically with other species that are also embedded in the pH$_2$ matrix; an investigation of the kinetics of these H-atom chemical reactions provides us with information about reaction dynamics in the pH$_2$ matrix environment. A recent study of the H + N$_2$O $\rightarrow$ HNNO reaction in solid pH$_2$ [Mutunga, Follett, and Anderson, J. Chem. Phys. {\bf 139}, 151104 (2013)] demonstrates that this reaction exhibits strongly non-Arrhenius behavior, proceeding at measurable rates only when the temperature of the system drops below $T \approx 2.4$ K. A molecular-level understanding of these findings requires information about how the solid pH$_2$ matrix environment affects the long-range entrance channel of the H + N$_2$O reaction. Here, we carry out quantum Monte Carlo simulations of a simple model system (Ar-doped solid pH$_2$) to investigate matrix-induced changes to long-range interactions between H atoms and other impurities embedded in solid pH$_2$ matrices. Our results suggest that the pH$_2$ matrix creates an effective long-range repulsion between the H atom and the Ar impurity, which we explain in terms of differential solvation energies of Ar and H atoms in solid pH$_2$.Ope