Effect of Electronic Interactions on NMR ¹J_CF and ²J_CF Couplings in cis- and trans-4-t-Butyl-2-fluorocyclohexanones and Their Alcohol Derivatives

Abstract

In order to study the influence of hyperconjugative, inductive, steric, and hydrogen-bond interactions on (1)J(CF) and (2)J(CF) NMR spin-spin coupling constants (SSCCs), they were measured in cis- and trans-4-t-butyl-2-fluorocyclohexanones and their alcohol derivatives. The four isotropic terms of those SSCCs, Fermi contact (FC), spin dipolar (SD), paramagnetic spin-orbit (PSO), and diamagnetic spin-orbit (DSO), were calculated at the SOPPA(CCSD)/EPR-III level. Significant changes in FC and PSO terms along that series of compounds were rationalized in terms of their transmission mechanisms by employing a qualitative analysis of their expressions in terms of the polarization propagator formalism. The PSO term is found to be sensitive to proximate interactions like steric compression and hydrogen bonding; we describe how it could be used to gauge such interactions. The FC term of (2)J(CF) SSCC in cis-4-t-butyl-2-fluorocyclohexanone is rationalized as transmitted in part by the superposition of the F and O electronic clouds.

1hJFHcoupling in 2‐fluorophenol revisited: Is intramolecular hydrogen bond responsible for this long‐range coupling?

“…Recently, a new approach to study coupling pathways for the FC term of SSCCs, was presented and it was dubbed FCCP-CMO [24,25] , which stands for FC coupling pathways studied using canonical molecular orbitals. The corresponding analysis is based on the polarization propagator (PP) expression for the FC term, Eqns (1-3) [26][27][28][29] Figure 1.…”

F···HO intramolecular hydrogen bond as the main transmission mechanism for 1hJF,H(O) coupling constant in 2′‐fluoroflavonol

“…The FC term, which dominates the 1h J F,H(O) coupling in 2′‐fluoroflavonol, is transmitted mainly by core electrons, that is, those with higher s % character; orbitals involved in hydrogen bond exhibit large s % character . The fluorine lone pairs (LP F ) are involved in charge transfer toward the σ* OH orbital, and hence, the s % character of these lone pairs should indicate the establishment of F···H‐O hydrogen bond and, consequently, the mechanism for 1h J F,H(O) in 2′‐fluoroflavonol.…”

DFT study of the 1JCF coupling constant in XCHFX fragments (X=O and S)

“…For the 1 J C-F coupling constant, it is appropriate to search for the CMO containing the C-F bonding orbital [21]. NBOs contributing to important electronic densities at the sites of the coupling nuclei (C 2 and F) are shown in Table 3.…”