Inductive effect of substituents on the dipole moment of α-substituted vinyl methyl ether

Dipole moments of several α-substituted vinyl methyl ethers R(OMe)C:CH₂; R = Me, Et, i-Pr, t-Bu, cyclopropyl, vinyl, Ph) have been determined by the Halverstadt-Kumler method in benzene solution at 293 K. The square of the total dipole moment μ_r was found to be a linear function of the Taft's inductive constant σ_r^*: μ_r²/D²=(0.619±0.033)+(1.092±0.10) σ_r^*. The inductive contribution of the substituent R on the total dipole moment may be expressed by the equation μ_j/D = ?0.52 σ^* + 0.25. This is in good agreement with the corresponding equation for the dipole moments of alkyl-substituted ethenes: μ_i/D = ?0.58 σ^* + 0.28 (based on dipole moments obtained by PCILO calculations).