| Popis: |
FT proton spectra have been determined for all 12 methyl derivatives of benzene at 80 or 90 MHz in 2 vol% CDCl3 solutions and treated by multilinear least-squares techniques to derive an empirical set of 7 substituent parameters which allows the correlation of all 20 observed methyl group signals of these methylbenzenes with standard deviation 0.004 ppm. The parameter set derived contains one reference value (δ° = 2.349 ppm), three usual position substituent constants (Δ0 = −0.088, Δm = −0.038, and Δp = −0.044 ppm), which are numerical refinements to Lindberg and co-workers' parameter set (10–12), and three new kinds of steric constants, namely, two crowding constants, C2 = +0.057 and C3 = +0.079 ppm, for the interactive steric effect of 2 or 3 adjacent methyl substituents upon the methyl proton signal considered, and one “indirect” crowding constant I = +0.009 ppm correcting for the reduced electronic effect of two neighboring methyl substituents at 3 and 4 positions on the signal of a 1-methyl group. Comparison of C2 and C3 leads to an estimate for the steric crowding effect of two ortho methyl groups as C1 = +0.039 ppm so that as a purely electronic o-substituent constant Δ′0 = −0.127 ppm may be suggested. |
