DFT study of the 1JC-F coupling constant in X-CHF-X fragments (X = O and S)

Abstract

The fluorine (reverse) Perlin-like effect is a coupling constant-related phenomenon that can be useful to investigate the stereochemistry and molecular properties of organofluorine compounds. While 1,3-dioxane is known to exhibit the Perlin effect (1JC-Hax < 1JC-Heq) and 1,3-dithiane experiences the reverse Perlin effect (1JC-Hax > 1JC-Heq), both their fluorinated analogues 2-fluoro-1,3-dioxane (1) and 2-fluoro-1,3-dithiane (2) showed the reverse fluorine Perlin-like effect (|1JC-Fax| > |1JC-Feq|). There has been some recent discussion in the literature about the origin of these effects, either as being due to hyperconjugation or dipolar interactions, but our findings indicate that while the angular dependence of 1JC-Fax in 2-fluorotetrahydropyran is consistent with the molecular dipole moment (ruled by the orientation of local dipole vectors from CF bond and oxygen electron lone pairs), the reverse fluorine Perlin-like effect in 1 and 2 is due to a combination of hyperconjugative, steric and electrostatic interactions. The role of CX and nX orbitals on the Fermi contact term for the 1JCF coupling constant has also been analyzed on the basis of canonical molecular orbitals.