10.1021/jo060698k.s001 Kimberly J. Daoust Kimberly J. Daoust Susanna M. Hernandez Susanna M. Hernandez Kaleen M. Konrad Kaleen M. Konrad Iain D. Mackie Iain D. Mackie James Winstanley James Winstanley Richard P. Johnson Richard P. Johnson Strain Estimates for Small-Ring Cyclic Allenes and Butatrienes American Chemical Society 2006 cyclohexatriene cyclic butatrienes cyclic allene series bicyclo fragment deformation approach kcal cyclopentadiene B 3LYP level group strain estimates cyclic allenes 2006-07-21 00:00:00 Journal contribution https://acs.figshare.com/articles/journal_contribution/Strain_Estimates_for_Small_Ring_Cyclic_Allenes_and_Butatrienes/3069586 Isodesmic and homodesmic equations at the B3LYP/6-311+G(d,p)+ZPVE level of theory have been used to estimate strain for the homologous series of cyclic allenes and cyclic butatrienes. A simple fragment deformation approach also has been applied and appears to work better for the larger rings. For the cyclic allene series, estimates for allene functional group strain (kcal/mol) include:  1,2-cyclobutadiene, 65; 1,2-cyclopentadiene, 51; 1,2-cyclohexadiene, 32; 1,2-cycloheptadiene, 14; 1,2-cyclooctadiene, 5; 1,2-cyclononadiene, 2; 1,2,4-cyclohexatriene, 34; and bicyclo[3.2.1]octa-2,3-diene, 39. For cyclic butatrienes, functional group strain estimates include:  1,2,3-cyclobutatriene, >100; 1,2,3-cyclopentatriene, 80; 1,2,3-cyclohexatriene, 50; 1,2,3-cycloheptatriene, 26; 1,2,3-cyclooctatriene, 17; and 1,2,3-cyclononatriene, 4. Barriers to interconversion of enantiomers in cyclic allenes are reduced with increasing strain. Newly predicted values include:  1,2-cyclopentadiene <1 kcal/mol and bicyclo[3.2.1]octa-2,3-diene, 7.4 kcal/mol. Estimated levels of strain parallel the known reactivity of these substances.