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

The C-13 NMR spectra of 24 members of a series of 1-X-bicyclo[1.1.1]pentane s were measured. SCSs C-13(1) were found to linearly correlated with those on C-13(3), although the former correspond to deshielding effects on and th e latter to shielding effects. Even though the C-13(1) SCSs follow the same trend as alpha-SCSs in other types of substrates, they are significantly s maller. In order to provide an insight into the different intramolecular in teractions that define such a trend, theoretical studies that include SCSs calculated at the CIAO B3LYP/6-311G** level, electron delocalization analys is within the NBO approach at the same level and electrostatic interaction effects on chemical shifts were carried out. Important halogen heavy-atom e ffects on C-3 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, C-13 SCSs, w ere carried out with particular reference to the C-1 position. These theore tical calculations included: (a) full geometry optimizations, (b) magnetic shielding constant calculations using the GIAO (gauge-included atomic orbit als) method within the DFT (density functional theory) approach and (c) nat ural bond orbital (NBO) analysis of intramolecular charge transfer interact ions. Copyright (C) 2000 John Wiley & Sons, Ltd.