An investigation into the origins of polar substituent effects upon ¹⁹F chemical shifts, using 4-substituted β,β-difluorostyrenes

Abstract

19F, 13C, and 1H chemical shifts have been determined for β,β-difluorostyrene and eight 4-substituted derivatives. The β-fluorine chemical shift difference, ΔδF, is used to evaluate the constant in the Buckingham equation. A = 3.0 × 10−11 esu for C—F bonds which is in excellent agreement with the value derived by Adcock and Khor. This allows accurate estimates of direct field effect contributions to 19F chemical shifts in aryl fluorides. Substituent parameter correlations demonstrate that the primary polar effect on 19F chemical shifts is field-induced π polarization. Abinitio molecular orbital calculations confirm that the substituent-induced 19F chemical shifts reflect changes in fluorine π electron density.

Solvent Effects on Fluorine Shielding in Fluorobenzene

“…There have been many attempts subsequently to provide an experimental value for the coefficient Ā x for fluorine by examination of shifts in conformationally rigid fluorinated molecules. For systems in which the fluorine is attached to an sp 2 carbon atom (usually in an aromatic ring) these efforts have produced values for Ā x in the range 142 to 583 ppm/au field with most near 550 ppm/au field. − A semiempirical calculation of Ā x at the INDO level suggested a value of 790 ppm/au field . The value from the theoretical work of Augspurger et al . , mentioned earlier (the one used in the calculations of δ E in our work) is 1885 ppm/au field.…”

“…For systems in which the fluorine is attached to an sp 2 carbon atom (usually in an aromatic ring) these efforts have produced values for A h x in the range 142 to 583 ppm/au field with most near 550 ppm/au field. [51][52][53][54] A semiempirical calculation of A h x at the INDO level suggested a value of 790 ppm/au field. 55 The value from the theoretical work of Augspurger et al 10,32 mentioned earlier (the one used in the calculations of δ E in our work) is 1885 ppm/au field.…”

Substituent-induced chemical shifts in 3- and 4-substituted styrenes: definition of substituent constants and determination of mechanisms of transmission of substituent effects by iterative multiple linear regression

“…We will consider this latter assumption first. While the theoretical origins of linear chemical shift -.ir electron electron correlations remain obscure (29), there is ample empirical evidence for quite precise linear relationships in the case of long-range aromatic chemical shifts (14,27,30). The facts that a, and a: are factors for short-range cheniical shifts, and that reasonable p, and p, values are obtained for ortho and n7eta chemical shifts, suggest that these correlations still exist at short range, after non-electronic effects are factored out.…”

Substituent effects in fluoromethylnaphthalenes by 19F nuclear magnetic resonance

“…More recently, a great deal of evidence has accrued from model aromatic systems to suggest that in general the main polar effect is field-induced polarization of the aromatic n electron system (21)(22)(23)(24)(25)(26)(27)(28)(29). Various analyses of the magnitude of these effects has been proposed, and the more successful of these (22,23,29) include use of the Buckingham (30) (36) and do not support the view that n-electronic effects on fluorine are responsible for the phenomenon (35).…”