Vibrational spectra and conformations of 1, 4‐dibromobut‐2‐yne and 1,4‐diiodobut‐2‐yne

The Raman spectra of 1,4‐dibromobut‐2‐yne (BrBr) as a liquid at various temperatures and as an amorphous and crystalline solid at low temperatures were recorded in the 3200‐10 cm −1 region. IR spectra of the liquid at room temperature and of the amorphous and crystalline solids at low temperatures (4000‐50 cm −1 ) were obtained. Additional IR spectra of the vapour were recorded (4000‐500 cm −1 ). The spectra of the liquid phase showed characteristic, broad bands below 500 cm −1 . No temperature dependence was detected in these, and the spectra of the amorphous solid at 90 K are almost identical with the room temperature spectra of the liquid. A shoulder on the Rayleigh line was observed at ca. 20 cm −1 in the Raman spectrum of the amorphous BrBr at 90 K which tentatively, has been interpreted as the torsional mode. In the crystal an anti conformer was present, while the spectra of the vapour and the liquid were interpreted in terms of large torsional freedom. The Raman and IR spectra of the very unstable 1,4‐diiodobut‐2‐yne (II) as a crystalline solid were recorded in the 4000‐20 cm −1 range at low temperatures. Spectra of carbon disulphide solutions at temperatures 250–260 K were obtained and additional IR spectra of II in Nujol mulls and in KBr pellets were also recorded. In the region below 500 cm −1 , Where other 1,4‐dihalobut‐2‐ynes have characteristic, broad bands, a high irregular background was observed in the Raman spectrum of II in solution. The spectra agree with an anti conformer present in the crystal and nearly free internal rotation in the liquid. The vibrational assignments are supported by force constant calculations and by a correlation diagram for the vibrations of the six skeletal bending modes in 1,4‐dichlorobut‐2‐yne, BrBr and II.