Through-Space Transmission of 19F-13C Coupling Via an Intermediate Bond. An Experimental and Theoretical Study

Abstract

In order to obtain a deeper insight into the title effect, several compounds with an F atom very close to a C-H of a nearby functional group were synthesized and the relevant couplings measured. The most conspicuous case was that of 8-fluoro-2-hydroxynaphthalene-1-carbaldehyde where a close proximity between the F and H atoms is the result of fluorine-oxygen repulsion and the formation of an intramolecular hydrogen bond between the hydroxyl and carbonyl groups. The experimental four-bond J(F,CHO) coupling is 26.2 Hz. A compound very similar to this one, but without the OH group, was chosen on which to perform a polarization propagator analysis of the through-space (TS) coupling pathways, at the RPA-INDO level. The expression for the TS coupling in terms of the projected polarization propagator and perturbators was numerically analysed. It is found that this coupling is completely dominated by a TS component of the Fermi contact (FC) term, the main features of which are: ( i ) It decays exponentially with the F-H distance; (ii) Its main contribution comes from an electron excitation involving the F lone-pair, the C-H bond of the CHO moiety and its corresponding antibonding orbital;(iii) The π-type lone-pair does not contribute to the TS coupling pathway of the FC term.

Configurational assignments in trifluoromethylvinyl compounds using through-space carbon-fluorine coupling constants

“…This result can be ascribed to the lack of an intermediate proton for the quaternary carbons ; the proton plays an important role in transmitting the spin information. 16 Besides the conÐgurational assignments in triÑuoro-methylvinyl compounds described in this paper, the through-space carbonÈÑuorine coupling constant may be utilized for other purposes, e.g. the assignment of the stereoisomers for Ñuorine-containing cyclic compounds, the 13C NMR assignment for the carbon which is spatially proximate to a Ñuorine nucleus in a Ñourine-containing compound and even the investigation of a conformational preference in a favorable situation.…”

Proximity Effects on Nuclear Spin-Spin Coupling Constants. 1. J(CH) Couplings in the Vicinity of an Atom Bearing Lone Pairs

“…In II, the formyl C-H bond is brought in close proximity to the peri F atom owing to the intramolecular © I C=0-H-O hydrogen bond. 12 In III, an intramolecular hydrogen bond of type F-H-N is known.14 Calculations presented in this paper show that, if a C-H bond in the proximity of an atom bearing lone pairs is slightly shortened, an increase in the corresponding '/(CH) coupling is observed, and if it is slightly lengthened, a decrease in that coupling is observed. As such changes in the '/(CH) coupling are found to originate in the Fermi contact term, it can be expected that an increase in the bond strength takes place in the former, while a weakening of the bond strength is expected in the latter.…”

Indirect Nonbonded Nuclear Spin–Spin Coupling: A Guide for the Recognition and Understanding of “Through-Space” NMR J Constants in Small Organic, Organometallic, and Coordination Compounds

“…Therefore, it appears obvious that the electron pairs from the existing C–H bonds play a very important intermediate role in the nuclear spin transmission . Quite early related calculations using the INDO method supported this view for various fluorinated ketones, propenes, and hydroxynaphthalenes. − …”