Inverted Carbon Geometries: Challenges to Experiment and Theory

Abstract

Disproving a long C–C-bond textbook example: The reported 1.643 Å C–C bond in 5-cyano-1,3-dehydroadamantane was redetermined and “only” amounts to 1.584 Å. While this value is well reproduced with <i>ab initio</i> methods, some common DFT approaches perform poorly and are only consistent with CCSD­(T)/cc-pVTZ optimizations for noninverted carbons. Large deviations from experiment were also found for other molecules with atypical electron density distributions, e.g., cubane, bicyclo[2.2.0]­hexane, and bicyclo[2.1.0]- and bicyclo[1.1.1]­pentane, thereby presenting challenging structures for some DFT implementations