A DFT/GIAO/NBO and experimental study of ¹³C SCSs in 1-X-bicyclo[1.1.1]pentanes

The ¹³C NMR spectra of 24 members of a series of 1-X-bicyclo[1.1.1]pentanes were measured. SCSs on ¹³C₁ were found to linearly correlated with those on ¹³C₃, although the former correspond to deshielding effects and the latter to shielding effects. Even though the ¹³C₁ SCSs follow the same trend as α-SCSs in other types of substrates, they are significantly smaller. In order to provide an insight into the different intramolecular interactions that define such a trend, theoretical studies that include SCSs calculated at the GIAO B3LYP/6-311G** level, electron delocalization analysis within the NBO approach at the same level and electrostatic interaction effects on chemical shifts were carried out. Important halogen heavy-atom effects on C₃ were observed, suggesting that the spin-orbit interaction is transmitted through space between the bridgehead carbon atoms. Additionally, in a few members of the series X = H, F, CH₃, NH₂, OCH₃, CN, COOH, COCH₃), theoretical calculations of the substituent chemical shifts, ¹³C SCSs, were carried out with particular reference to the C₁ position. These theoretical calculations included: (a) full geometry optimizations, (b) magnetic shielding constant calculations using the GIAO (gauge-included atomic orbitals) method within the DFT (density functional theory) approach and (c) natural bond orbital (NBO) analysis of intramolecular charge transfer interactions.