Conformations of conjugated hydrocarbons. Part 1. A spectroscopic and thermodynamic study of buta-1,3-diene and 2-methylbuta-1,3-diene (isoprene)

Abstract

Complete vibrational spectra of both buta-1,3-diene and 2-methylbuta-1,3-diene (isoprene) have been recorded over a range of temperatures. It is indicated by spectroscopic and thermodynamic methods that both compounds exhibit an equilibrium (s-trans⇌ s-cis) and the gauche-conformers are not present. Measurement of intensities of selected Raman bands of isoprene over a range of temperatures leads to a value of ΔH^⊖± standard deviation of 4.59 ± 0.16 kJ mol^–1 and a value of ΔS^⊖ of –0.8 J K^–1 mol^–1. The complex methyl torsions of both conformers of isoprene have been assigned and the heights of the three-fold barriers calculated. Complete vibrational assignments for both conformers of isoprene have been made except for the s-cis C–C torsional frequency. Comparison between thermally derived thermodynamic functions and those derived by statistical mechanics leads to a value of ΔH^⊖ of 10.4 kJ mol^–1 for butadiene, and a concentration of less than 3% s-cis-butadiene at normal temperatures. Using the experimental value of ΔH^⊖ obtained for isoprene new values of thermodynamic functions have been calculated over a range of temperatures, indicating a concentration of 11% s-cis-isoprene at normal temperature.