This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Adding phenanthrene as a co-guest let the second-generation crystalline sponge (2G-CS) method determine X-ray structures for 24 of 30 small, low-polarity C₁₅ molecules, mostly sesquiterpenoids. Under standard conditions, none of the 30 gave guest-containing crystals.
Eleven of the 24 structures had not been reported before, including that of the liquid hydrocarbon longifolene. A chiral version of the cage gave the absolute configurations of five compounds. The 2G-CS method determines a molecule's structure without crystallizing the molecule itself.
Instead, the molecule is captured inside hollow palladium-based cages that assemble into a crystal. The method often fails for low-polarity molecules with molecular weights of 200–300. The authors attribute this to the cage cavity being too large for such small molecules.
The molecules likely have too much freedom of motion and interact only weakly with the host. Such small sesquiterpenoids are important in natural-product research and drug discovery. The research is published in the journal Angewandte Chemie International Edition.
The team used a "size-compensation strategy." A flat aromatic "modulator" is encapsulated together with the target molecule, and the pair fills the cavity. The modulator is not meant to be encapsulated alone. The ternary complex preferentially forms when both guests are present.
Among six modulators tested, phenanthrene gave the highest inclusion level for (+)-longifolene, in about 40% of cages, as measured by NMR. The team then applied the same phenanthrene-based strategy to the other 29 compounds without further optimization. Phenanthrene enabled structure determination for 24 of the 30 compounds (80%), including one analyzed with the chiral cage.
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