Vibrational spectra and relative stability of the van der Waals complexes formed between 1,2-butadiene and HCl: A FTIR and density functional study

Abstract

The formation of weak molecular complexes between 1,2-butadiene (methylallene, CH 3CHCCH 2) and HCl dissolved in liquid argon has been investigated using infrared spectroscopy. In addition, structural and spectral information on the 1:1 and 1:2 complexes was obtained by carrying out DFT calculations at the B3LYP/6-311++G(d,p) level. The calculations predict three isomeric 1:1 complexes with similar energies. In the spectra, at least two different complexes were identified, while indications for a third 1:1 complex were found. Using spectra recorded at temperatures between 92 and 128 K, the average complexation enthalpy for the 1:1 complexes in liquid argon was determined to be –6.8 (3) kJ mol −1, while for the 1:2 complexes an average value of –17.4 (11) kJ mol −1 was obtained. By correcting for solvation, zero-point vibrational and thermal contributions, for both the 1:1 and the 1:2 complexes, the average complexation enthalpy Δ H° derived in liquid argon was converted into a vapor phase complexation energy Δ E exp. The resulting values, −10.7 (6) kJ mol −1 for the 1:1 complexes and –24.5 (22) kJ mol −1 for the 1:2 complexes, are compared with the ab initio values.