The spin delocalization substituent parameter s̀ 8. The spin-delocalizing abilities of the nitro, cyano, carbomethoxy, carboxy and carbamyl groups

The rate constants (k) of the homo-cycloaddition reactions of five substituted α,β,β-trifluorostyrenes (TFS's), i.e. p-nitrotrifluorostyrene (4), p-cyanotrifluorostyrene (5), p-carbomethoxytri-fluorostyrene (6), p-carboxytrifluorostyrene (7) and p-carbamyltrifluorostyrene (8), have been measured in the temperature range of 110–160°C. The s̀_mb polar substituent parameters of these TFS's calculated from ¹⁰F NMR chemical shifts are: NO₂, 0.86; CN, 0.86; CO₃CH₃, 0.40; CO₂H, 0.31; CONH₂, 0.10. The spin delocalization substituent parameters σ_T^˙ of NO₂, CN, CO₂CH₃, CO₂H and CONH₂ are 0.32, 0.38, 0.31, 0.37 and 0.37 respectively. Thus all these electron-pair attracting groups are also very effective spin-stabilizers.