A SELF-CONSISTENT AND CROSS-CHECKED SCALE OF SPIN-DELOCALIZATION SUBSTITUENT CONSTANTS, THE SIGMA.(JJ) SCALE

On the basis of the F-19 NMR data of substituted alpha,beta,beta-trifluorostyrenes and the rate constants of their thermal cycloaddition reactions, a self-consistent and cross-checked scale of spin-delocalization substituent constants sigma(JJ). is proposed for 21 para-substituents and for 10 meta-substituents. By means of adopting different rho(mb) values, three methods for the calculation of the sigma(JJ). values at five different temperatures and their averaged values are presented. It has been shown that at each temperature and within a range of rho(mb) values from -0.20 to -0.40 the same set of sigma. constants are obtained from these three methods. There are seven advantages to the sigma(JJ). approach, including the fact that the reaction is exceptionally clean, a tailor-made polar parameter sigma(mb) is used in the dual-parameter equation, and the validity of the approach is strongly supported by the meta substituent effect which is independent of the polar effect. Some radical reactions, EPR data of substituted benzyl radicals, and bond dissociation energies are reexamined by using the sigma(JJ). in conjunction with polar sigma constants in dual-parameter equations.