VIBRATIONAL-SPECTRA AND CONFORMATION OF 1-CHLORO-4-FLUOROBUT-2-YNE AND ABINITIO CALCULATIONS ON 1,4-DIHALOBUT-2-YNES

Raman spectra were recorded in the 3200-5 cm-1 range for 1-chloro-4-fluorobut-2-yne as a liquid at various temperatures and as amorphous and different crystalline solids at low temperature. In the liquid, the Raman spectra showed broad, asymmetrical band shapes as previously reported for other 1,4-dihalobut-2-ynes. Additional IR and far-IR spectra were obtained of the liquid, vapour, amorphous and crystalline solids at low temperature, and of the compound isolated in argon and nitrogen matrices at 13 K. The vapour and liquid spectra were interpreted in terms of a low barrier to internal rotation and the crystal spectra of a single conformer, gauche. The unusual band shapes in the low-frequency Raman spectra of the 1,4-dihalobut-2-ynes are discussed and are reproduced fairly well by a simple model of a molecule exhibiting nearly free internal rotation. The agreement between the observed and calculated spectra is good for all the molecules studied when ab initio potentials are applied in the model calculations. Temperature effects in the Raman spectra of liquid 1-chloro-4-fluorobut-2-yne can also be derived from the model. Internal rotation in 1,4-difluoro-, 1,4-dichloro-, 1-chloro-4-fluoro-, 1,4-dibromo- and 1,4-diiodobut-2-yne was studied with the aid of ab initio calculations. The barriers were found to be low and decreasing through the series from 1,4-difluoro- to 1,4-diiodobut-2-yne accompanied by an increase in the equilibrium dihedral angle, X-CCCC-Y, from 101-degrees in 1,4-difluorobut-2-yne to 180-degrees in 1,4-diiodobut-2-yne. The systematic variations through the series were correlated with the local dipole moments of the halomethyl groups and the electronegativities of the halogen substituents.