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

Abstract

19 F, 13 C, and 1 H chemical shifts have been determined for β,β-difluorostyrene and eight 4-substituted derivatives. The β-fluorine chemical shift difference, Δδ F , 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 19 F chemical shifts in aryl fluorides. Substituent parameter correlations demonstrate that the primary polar effect on 19 F chemical shifts is field-induced π polarization. Abinitio molecular orbital calculations confirm that the substituent-induced 19 F chemical shifts reflect changes in fluorine π electron density.