Solvent-induced changes in 2J(H, F) for fluoroform via van der Waals interactions. Non-contact contributions to spin–spin coupling constants involving a proton?

Abstract

The spin–spin coupling between the proton and the fluorine nuclei, 2J, in fluoroform varies by 1% in a range of solvents. It is argued that 2J decreases algebraically as the van der Waals solute–solvent interactions increase in magnitude. Such a decrease is also observed for coupling constants which likely contain a substantial positive orbital contribution. If the van der Waals interactions perturb the spin–orbital term in J, then 2J in fluoroform may well contain orbital contributions, as recently calculated for 2J in methyl fluoride. In that event, the large discrepancies between observed 2J(H,F) values and those calculated by semiempirical theories of the contact term may be partially attributed to the neglect of orbital terms.