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

The formation of weak molecular complexes between 1,2-butadiene (methylalle ne, CH3CH=C=CH2) and HCl dissolved in liquid argon has been investigated us ing 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 isomer ic 1:1 complexes with similar energies. In the spectra, at least two differ ent complexes were identified, while indications for a third 1:I complex we re found. Using spectra recorded at temperatures between 92 and 128 K, the average complexation enthalpy for the 1:1 complexes in liquid argon was det ermined to be -6.8 (3) kJ mol(-1), while for the 1:2 complexes an average v alue of -17.4 (11) kJ mol(-1) was obtained. Hy correcting for solvation, ze ro-point vibrational and thermal contributions, for both the 1:1 and the 1: 2 complexes, the average complexation enthalpy Delta H degrees derived in l iquid argon was converted into a vapor phase complexation energy Delta E-ex p. 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 v alues. (C) 1999 Elsevier Science B.V. All rights reserved.