Nuclear Spin−Spin Coupling via Nonbonded Interactions. 8.¹ The Distance Dependence of Through-Space Fluorine−Fluorine Coupling

Abstract

The 19 F 19 F nuclear spin-spin coupling constants J FF for a set of eighteen compounds related structurally to 1,8-difluoronaphthalene were measured by 19 F NMR spectroscopy. The FF distances d FF in these compounds were determined by ab initio 3-21G* molecular orbital calculations. Consistent with the lone-pair overlap theory of the origins of through-space 19 F 19 F coupling, an exponential relationship is found between J FF and d FF (regression coefficient r 2 ) 0.991), and a linear relationship is found between J FF and the extent of the overlap interaction between the in-plane fluorine 2p lone-pair orbitals (regression coefficient r 2 ) 0.993). The magnitudes of these lone-pair interactions were estimated from molecular orbital energies obtained by ab initio 6-31G* calculations for a model consisting of a pair of HF molecules separated by various distances.

Conformational Dependence of Through‐Space Tellurium–Tellurium Spin–Spin Coupling in Peri‐Substituted Bis(Tellurides)

“…The deviation observed for these derivatives compared to the remaining members of the series was attributed to the difference in aromaticity of the acenaphthylene backbone and a greater interaction between the fluorine lone‐pair orbitals and the π‐system. Confirmed by a later DFT study, the elevated J FF values resulted from a greater through‐bond contribution to the overall coupling due to the increased π‐interactions, subsequently causing the observed deviation from the d FF / J FF exponential curve 6…”

“…Whilst the efficiency of lone‐pair orbital overlap, and the magnitude of through‐space coupling, is greatly influenced by the non‐bonded internuclear distance, it is also sensitive to the angular orientation of the overlapping lone‐pairs. For instance, 4,5‐difluorophenanthrene derivatives of type C (Figure 1), in which the exocyclic CF bonds are no longer coplanar, nor parallel, produce much larger J ( 19 F, 19 F) SSCCs than their naphthalene equivalents, with J values of 165–175 Hz truly massive for formally five‐bond ( 5 J FF ) couplings 6…”

Computational 19F NMR. 1. General features

“…Indeed, several such long-range couplings have been ascribed to a through-space overlap of non-bonding orbitals, thereby giving rise to ''through-space couplings'', which have been known for a long time [10][11][12][13].…”

Technical and practical aspects of 19F NMR‐based screening: toward sensitive high‐throughput screening with rapid deconvolution

“…In addition, it should be pointed out that two fluorines that are separated by many bonds can display large coupling constants if they are close in space. Many examples with large through space n J FF have been reported in the literature (see, for example, Refs and ). The generated library is then tested with the pulse sequence of Fig.…”