Vibrational spectra, phase transitions and conformers of 1,4‐dichlorobut‐2‐yne

The Raman and IR spectra of 1,4-dichlorobut-2-yne (DCB) as a liquid at various temperatures and as an amorphous and as metastable and stable crystalline solids at low temperatures have been recorded in the 4000–20 cm−1 range. Additional IR and Raman spectra of a high-pressure crystalline phase were obtained at ca 17 kbar pressure. The broad unsymmetrical bands of DCB in the liquid and in the amorphous solid below 500 cm−1 suggest nearly free rotation in these phases at all of the temperatures investigated. Below ca 190 K a crystal containing the anti and above 190 K a crystal containing the gauche conformer of DCB were formed; the latter crystal is believed to be thermodynamically more stable at all temperatures. Only one high-pressure crystal with molecules in the anti conformation was formed at room temperature. A phase transition reported at 232 K in the liquid, involving a conversion to an anti conformer, was not detected. Complete vibrational assignments for both anti and gauche conformers are presented, supported by force constant calculations.