13C chemical shifts in series of adamantane and norbornane derivatives. INDO calculations

Abstract

13C α-substituent chemical shift (α-SCS) calculations were performed with GIAO's and at the INDO-FPT and London levels of approximation for 1- and 2-substituted adamantanes, and 1-, 7-, exo 2- and endo 2-substituted norbornanes. A good overall agreement between calculated and observed α-SCS was revealed when a modified set of INDO parameters for the fluorine, oxygen and nitrogen electronegative atoms is used. α-SCS are shown to correlate with the local calculated charge, and in addition, to be determined almost entirely by the local paramagnetic term, which in turn depends on the atomic charge. 13C substituent chemical shifts as well as the corresponding Δ q atomic charges are also observed to be directly related to the group electronegativity of the α-substituent, hence emphasizing the importance of the σ inductive mechanism in the transmission of the α-substituent effect. SCS values at positions other than α in the series of substituted adamantanes and norbornanes are generally not well predicted theoretically.