Investigations of Substituent Effects by Nuclear Magnetic Resonance Spectroscopy and All-valence Electron Molecular Orbital Calculations. IV. 4-Substituted Phenylacetylenes

Abstract

13 C and 1 H spectra have been determined for 18 4-substituted phenylacetylenes and carbon charge densities have been estimated by CNDO/2 molecular orbital calculations. Correlations of these parameters with σ 1 , and σ R 0 indicate that the important mechanisms of transmission of substituent effects are field, resonance, and π polarization effects, as previously noted for 4-substituted styrenes. Evidence is presented that demonstrates that both 1 H and 13 C chemical shifts for these compounds reflect ground state charge densities. CNDO/2 calculations on a variety of aromatic derivatives suggest that the σ 1 , dependence of 19 F chemical shifts in these derivatives may primarily be due to π polarization.