Magic Numbers for Packing Adamantane in Helium Droplets: Cluster Cations, Dications, and Trications

Abstract

We report the first observation of cations, dications and trications of large clusters of adamantane. Cluster formation was initiated near 0 K in helium droplets and ionization was achieved with one or more collisions with energetic He species (He*, He + or He* -). The occurrence of Coulomb explosion appeared to discriminate against the formation of small multiply charged clusters. High resolution mass spectrometry revealed the presence of "magic number" m/z peaks that can be attributed to the packing of adamantane molecules into cluster structures of special stability involving preferred arrangements of these molecules. These abundance anomalies were seen to be independent of charge state. While some dehydrogenation of adamantane and its clusters was seen as well, no major transformations into adamantoids or microdiamonds were observed.

The Structure of Adamantane Clusters: Atomistic vs. Coarse-Grained Predictions From Global Optimization

“…At sizes 26 and above, both models favor close-packed cubic structures, leading to the perfect truncated octahedron at N = 38 as a strong magic number. These results thus support the interpretation of experimental mass spectra from the Scheier group (Goulart et al, 2016), namely that adamantane clusters exhibit icosahedral and cubic packing as their main structural motifs, at low and large sizes, respectively. Our results indicate that icosahedral packing is the dominant motif only up to N = 14, and that orientational effects are already non-negligible at this size.…”

“…The remarkable thermodynamical and chemical stability of adamantane makes it a valuable building block of supramolecular materials, including non-covalent molecular clusters. Recent mass spectrometry measurements under the cryogenic conditions of helium droplets have found magic numbers for cationic adamantane clusters at the sizes of 13, 19, and 38 molecules, as well as others higher suggesting close packed geometries (Goulart et al, 2016). In the present work, we have modeled (neutral) adamantane clusters using a rigid body description and a site-site pairwise force field comprising the traditional Lennard-Jones potential for repulsion-dispersion forces with Coulomb interactions acting between partial charges.…”

“…Adamantane clusters were recently synthesized in the cryogenic environment of helium nanodroplets, in which they could be size-selected after ionization by an electron gun (Goulart et al, 2016). In a first approximation, adamantane is roughly spherical and interacts with other molecules via non-covalent forces of the dispersion-repulsion type, with additional Coulomb contributions arising from the partial charges carried by the hydrogen and carbon atoms having different electronegativities.…”

Cold physics and chemistry: Collisions, ionization and reactions inside helium nanodroplets close to zero K

“…In the case of adamantane clusters, (C10H16)n, from doped HND we observed pronounced magic numbers for singly, doubly and triply charged clusters that reveal a transition from icosahedral packing to fcc packing between n = 19 and n = 38. Another remarkable fact is that the magic numbers are essentially the same for all three charge states [282]. Both, magic numbers and multiply-charged dopant cluster ions suggest Coulomb explosion of initially highly-charged dopant clusters as the underlying mechanism in cluster formation.…”

Uptake and accommodation of water clusters by adamantane clusters in helium droplets: interplay between magic number clusters

“…The guest water clusters are first investigated separately in the absence of a host; our studies of the host adamantane clusters already have appeared in the literature. 45 The main focus here is on the water guest/adamantane host interactions in very cold helium droplets in which cluster formation is enhanced for both the guest and the host molecules. This provides an opportunity to study the accommodation of various sizes of water clusters by various sizes of host adamantane aggregates in the gas phase, including the accommodation of magic number water clusters by magic number adamantane clusters.…”