Study of the conformational equilibria between rotational isomers using ultrasonic spectroscopy. Part 1.—1-Chloro-2-methylpropane and 1-Bromo-2-methylpropane

Abstract

Ultrasonic absorption spectra of 1-chloro-2-methylpropane and 1-bromo-2-methylpropane in hexane solutions have been measured as a function of temperature and concentration. Ultrasonic relaxation spectra of these solutions are well expressed by a single relaxation equation. The relaxation is attributed to the rotational isomerism of these molecules. Combining the ultrasonic data and the enthalpy and volume differences associated with the conformational changes of these molecules obtained earlier by Raman spectroscopy, the entropy difference between the two conformers has been obtained. Thus, all the thermodynamic quantities, ΔH, ΔV, ΔS, ΔH^‡ and ΔS^‡, in the energy diagram and the kinetic parameters, k_f and k_b, of the conformational equilibria of these molecules in their pure and dissolved states have been completely determined. We have estimated the conformational entropy difference and discuss the relationship between the volume difference and the conformational entropy difference.