Isomerism in van der Waals molecules: A cryospectroscopic study of the complexes of buta-1,2-diene with hydrogen chloride

Abstract

The IR spectra (4000–500 cm⁻¹) of solutions of buta-1,2-diene, CH₃CHCCH₂, and HCl or DCl in liquid argon have been studied in the temperature range from 100 to 125 K. Bands signalling the formation of 1:1 and 1: 2 complexes have been detected. The possible stable structures of the complexes were predicted using DFT calculations at the B3LYP/6-311++ G(d,p) level. Three different stable isomers were found for the 1:1 complexes. For the 1:2 complexes two different types were found to be stable, one in which both HCl molecules are bonded with the carbon–carbon double bonds, and another in which the second HCl is bonded to the first. Of the former, four different stable isomers were found, and three isomers were found for the second type. The spectra provide evidence for the presence of at least two isomers of the 1:1 complexes, and at least two isomers of the second type of 1:2 complexes. The identification of the experimentally detected isomers is discussed. The average complexation enthalpy was found to be − 7.0(6) kJ mol⁻¹ for the 1:1 complexes, and − 12.6(6) kJ mol⁻¹ for the 1: 2 complexes.