The structure of coronene cluster ions inferred from H₂ uptake in the gas phase

Abstract

Mass spectra of helium nanodroplets doped with H and coronene feature anomalies in the ion abundance that reveal anomalies in the energetics of adsorption sites. The coronene monomer ion strongly adsorbs up to n = 38 H molecules indicating a commensurate solvation shell that preserves the D symmetry of the substrate. No such feature is seen in the abundance of the coronene dimer through tetramer complexed with H; this observation rules out a vertical columnar structure. Instead we see evidence for a columnar structure in which adjacent coronenes are displaced in parallel, forming terraces that offer additional strong adsorption sites. The experimental value for the number of adsorption sites per terrace, approximately six, barely depends on the number of coronene molecules. The displacement estimated from this number exceeds the value reported in several theoretical studies of the bare, neutral coronene dimer.

Solvation of coronene oligomers by para-H2 molecules: the effects of size and shape

“…Using the energetic definition, the results obtained for the single stack motifs over assemblies containing between one and four coronene molecules show approximately linear variations that are consistent with those reported experimentally by Goulart and coworkers, 31 providing further support for the stacking between successive molecules with an incremental shift. The magnitude of this shift inferred by these authors, which is of the order of the aromatic cycle, remains lower than the value in our model of coronene assembly by about 1 Å.…”

“…The approximate linear slopes that can be extracted in both cases are 11.3 and 10.0 per coronene molecule, respectively. In their mass spectrometry study, Goulart and coworkers 31 reported a slightly higher slope of 11.4 per coronene, which remains in reasonable agreement with our own data for the parallel displaced motif while being at variance with the slope obtained for the rotated stack motif along a common axis.…”

“…Our PIMD simulations confirm that the energy difference provides a more robust definition of the solvation shell, in that it is sensitive to the adhesion site of additional solvent molecules as well as the possibly already completed character of the first shell. In particular, for coronene molecules in a parallel displaced stacked form, we essentially recover the variations in the solvation shell experimentally determined in mass spectrometry experiments, 31 despite the offset between molecules being closer to that in graphite.…”

“…29 Very recently, electron diffraction was conducted for pyrene clusters under superfluid helium droplets, confirming the stacking between monomers at an interlayer distance of 3.5 Å in the dimer but indicating that molecules in the trimer were no longer completely parallel. 30 Alternatively, the structure of coronene oligomers was experimentally investigated in a more indirect way by Goulart and coworkers, 31 who identified by mass spectrometry their propensity to attach para-hydrogen molecules. More precisely, variations in ion abundances were interpreted to infer the magnitude of the displacement between two coronene molecules, which was estimated to be about twice as large as the offset between graphene layers in conventional graphite.…”

Electron Attachment and Electron Ionization of Formic Acid Clusters Embedded in Helium Nanodroplets

“…They often correlate with closure of solvation shells, subshells, or second or even third solvation shells [ 43 – 45 ]. An interesting situation arises if M ± is a planar ion, e.g., coronene; the approximate structure of coronene cluster ions has been inferred from magic numbers in the abundance distributions of He m M 2 + , He m M 3 + , and He m M 4 + [ 46 ].…”