Analysis of Substituent Effects on C-13 NMR Parameters of Substituted Arylacetylene Derivatives. Linear Free Energy Relationships and PM3 Semiempirical Calculations

The substituent-induced chemical shifts (SCS) of C(α,β) on the 13C NMR spectra of arylalkynes (i.e. containing H, CH3, Cl and Br) were studied. The correlation between SCS and Hammett constants shows that the tendency of the effect by the substituents on the phenyl ring is Br (ρ=8.15)>Cl (ρ=7.27)>CH3 (ρ=6.79)>H (ρ=5.78). This order can be rationalized as due to the ability of the group on the alkyne to stabilize the partial positive charge on C(β) resulting from polarization with π electron transfer from C(β) to the phenyl ring. The SCS values are also well correlated with the electron densities obtained from PM3 calculations. The solvent effect on the 13C chemical shifts of phenylalkynyl bromide demonstrates a strong dependence on the relative permittivity as well as the shielding character of the solvents.