The metastable structures of anthracene-argon clusters inside helium nanodroplets

Abstract

Recent experiments have revealed that anthracene forms rather diverse complexes with multiple argon atoms upon embedding in helium nanodroplets. In this work we have modeled such complexes and investigated the relative stability of various isomeric forms using classical force fields reparametrized on electronic structure data at the level of spin-component scaled MP2, nuclear quantum effects being accounted for by means of path-integral molecular dynamics simulations. We generally find that the structures obtained are very sensitive to the details of the potential, but also to the inclusion of zero- point motion, for which the harmonic approximation is found to be generally accurate. Inside He-1000 clusters, the complexes also exhibit different relative stabilities, the quantum solvent usually favoring more asymmetric complexes. No evidence for fluxionality at 1 K in a given cluster is found. Using ring-polymer molecular dynamics trajectories, the stability of an argon dimer initially distant from the anthracene molecule was also investigated. Such metastable configurations are found to have a lifetime of the order of nanoseconds, confirming the possibility of trapping them in experiments.