Dielectric and raman spectroscopic studies on the rotational isomerism of t-butyl formate

Abstract

Static dielectric constants, refractive indices and densities have been measured for t-butyl formate and acetate in cyclohexane at increasing concentrations and at two temperatures, 20°C and 60°C. The Δ H value for the cis-trans equilibrium of t-butyl formate and the dipole moment of the trans conformation has been calculated from the dielectric measurements taking the Δ S value calculated from the sum-over-states for the cis and trans conformations and assuming the dipole moment of the cis conformation to be equal to 1.94 D as found for t-butyl acetate, which has the cis conformation only. The relative intensities of the Raman bands corresponding to specific vibrations modes of the cis and trans conformations of t-butyl formate are measured in cyclohexane and acetonitrile at various concentrations. The enthalpy difference Δ H is also measured in the liquid state and in acetonitrile by variation of the intensity ratio of the bands with temperature. All thermodynamic quantities obtained from dielectric or Raman intensity measurements are compared with each other and with the theoretical values. The solvation energy difference between cis and trans conformations in cyclohexane and acetonitrile as well as in the liquid state are also compared with theoretical values. The large deviation of solvation energy difference between the experimental value and Onsager's model value are well described by an additional term, which considers dipole-dipole interaction.