Large Polyhedral Oligomeric Silsesquioxane Cages: The Isolation of Functionalized POSS with an Unprecedented Si ₁₈ O ₂₇ Core

The synthesis of organo‐functionalized polyhedral oligomeric silsesquioxanes (POSS, (R‐SiO 1.5 ) n , T n ) is an area of significant activity. To date, T 14 is the largest such cage synthesized and isolated as a single isomer. Herein, we report an unprecedented, single‐isomer styryl‐functionalized T 18 POSS. Unambiguously identified among nine possible isomers by multinuclear solution NMR ( 1 H, 13 C, and 29 Si), MALDI‐MS, FTIR, and computational studies, this is the largest single‐isomer functionalized T n compound isolated to date. A ring‐strain model was developed to correlate the 29 Si resonances with the number of 6‐, 5‐, and/or 4‐Si‐atom rings that each non‐equivalent Si atom is part of. The model successfully predicts the speciation of non‐equivalent Si atoms in other families of T n compounds, demonstrating its general applicability for assigning 29 Si resonances to Si atoms in cage silsesquioxanes and providing a useful tool for predicting Si‐atom environments.