NMR (3)J(C-1, H-3) couplings in 1-X-bicyclo[1.1.1] pentanes. FPT-DFT and NBO studies of hyperconjugative interactions and heavy atom substituent effects

In this work a rationalization of the very large substituent effects on (3) J(C-1,H-3) couplings in 1-X-bicyclo[1.1.1]pentanes is presented. The Fermi contact contribution to such couplings was calculated in a series of 13 X-d erivatives within the DFT-B3LYP framework using the finite perturbation the ory. Core electrons for atoms beyond the Second Row were taken into account using effective core potentials. Calculated couplings are in very good agr eement with experimental values. The role played by hyperconjugative intera ctions involving bonds or antibonds belonging to the coupling pathway are s tudied using the NBO approach. Heavy atom contribution to substituent effec ts on (3)J(C-1,H-3) couplings was estimated as small. This contrasts notabl y with trends observed in the corresponding C-13 substituent chemical shift s, SCSs. The latter were estimated comparing for X = Cl, Br, I, SnMe3, calc ulated SCSs with their experimental values. Such estimations are in line wi th explicit calculations of the spin-orbit contribution reported in the lit erature for smaller compounds. (C) 2001 John Wiley & Sons, Inc.