Conformational and electronic interaction studies of some alpha-ethylsulfinyl p-substituted acetophenones

The preferred conformations of the α-ethylsulfinyl p-substituted acetophenones Y_PhC(O)CH2S(O)Et ( Y=OMe 1, Me 2, H 3, Cl 4, Br 5, CN 6, NO2 7) are determined by νCO IR analysis and ab initio HF/6-31G** computations. In solvents of low relative permittivity, carbon tetrachloride and chloroform, the cis2 rotamer prevails over the gauche3 one, excepting for 6 and 7, in chloroform, for which the gauche3 rotamer becomes the most stable. In both solvents the c2/g3 population ratio gradually increases on going from electron-attracting to electron-donating substituents. These trends are discussed in terms of the increasing contribution of the CT and electrostatic and interactions which stabilise the cis2 rotamer along with the simultaneous decreasing contribution of the , orto- and interactions which stabilise the gauche3 rotamer. In the high relative permittivity acetonitrile the gauche3 rotamer becomes the most stable one for the whole series. The good linear correlation of the c2/g3 ratio with both the σp+ substituent constants and the negative cis2 carbonyl frequency shifts (Δνc2), in chloroform indicate that the cis2-gauche3 equilibrium is mainly determined by the interactions which act in the cis2 rotamer, and only to a minor extent by those which operate in the gauche3 rotamer.