The spin delocalization substituent parameter σ 6. The spin-delocalizing abilities of the thiomethyl, cyclopropyl and t-butyl groups. Synthesis of three substituted trifluorostyrenes

Three substituted α, β, β-trifluorostyrenes (TFS's), i. e., p-thiomethyltrifluorostyrene (4), p-cyclopropyltrifluorostyrene (5) and p-t-butyltrifluorostyrene (6), have been synthesized. The rate constants (k₂) of the thermal cyclodimerization of these compounds have been measured in the temperature range 120–160°C. The polar parameters σ_mb of these TFS's calculated from ¹⁹F NMR chemical shifts are: for p-thiomethyl, -0.18; p-cyclopropyl, -0.31 and p-t-butyl, -0.22. The spin delocalization substituent parameters σ′_T (140°) of p-thiomethyl, cyclopropyl and t-butyl groups are 0.59, 0.27 and 0.30 respectively. Thus all of these groups act as electron-donating groups which can also effectively stabilize a spin.