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

Abstract

We report an experimental study of water clusters as guests in interactions with clusters of adamantane (Ad) as hosts that occur in doped helium droplets at extremely low temperatures. Separate experiments with pure water as dopant showed ready formation of a distribution of water clusters (H₂O)_mH⁺ that peaks at m = 11 and extends beyond m = 100 with local maxima at m = 4, 11, 21, 28 and 30 with (H₂O)₂₁H⁺ being the most anomalous and showing the greatest stability with respect to clusters immediately adjacent in water content. When adamantane is also added as a dopant, extensive hydration is seen in the formation of water/adamantane clusters, (H₂O)_mAd_n⁺; magic number clusters (H₂O)₂₁Ad_n⁺ are seen for all the adamantane clusters. Other magic numbers for water clusters attached to adamantane, (H₂O)_mAd_n⁺, are as for pristine protonated water, with m = 28 and m = 30. The icosahedral shell closure of pure adamantane at n = 13 and 19 appears to be preserved with (H₂O)₂₁ replacing one adamantane. (H₂O)₂₁Ad₁₂⁺ and (H₂O)₂₁Ad₁₈⁺ stand out in intensity and demonstrate the interplay of magic number water clusters with magic number adamantane clusters, observed perhaps for the first time in gas-phase cluster chemistry. There was no clear evidence for the formation of clathrate hydrates in which adamantane is trapped within structured water.