The synthesis of organo‐functionalized polyhedral oligomeric silsesquioxanes (POSS, (R‐SiO1.5)n, Tn) is an area of significant activity. To date, T14 is the largest such cage synthesized and isolated as a single isomer. Herein, we report an unprecedented, single‐isomer styryl‐functionalized T18 POSS. Unambiguously identified among nine possible isomers by multinuclear solution NMR (1H, 13C, and 29Si), MALDI‐MS, FTIR, and computational studies, this is the largest single‐isomer functionalized Tn compound isolated to date. A ring‐strain model was developed to correlate the 29Si 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 Tn compounds, demonstrating its general applicability for assigning 29Si resonances to Si atoms in cage silsesquioxanes and providing a useful tool for predicting Si‐atom environments.

The synthesis of organo‐functionalized polyhedral oligomeric silsesquioxanes (POSS, (R‐SiO1.5)n, Tn) is an area of significant activity. To date, T14 is the largest such cage synthesized and isolated as a single isomer. Herein, we report an unprecedented, single‐isomer styryl‐functionalized T18 POSS. Unambiguously identified among nine possible isomers by multinuclear solution NMR (1H, 13C, and 29Si), MALDI‐MS, FTIR, and computational studies, this is the largest single‐isomer functionalized Tn compound isolated to date. A ring‐strain model was developed to correlate the 29Si 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 Tn compounds, demonstrating its general applicability for assigning 29Si resonances to Si atoms in cage silsesquioxanes and providing a useful tool for predicting Si‐atom environments.