Raman and Infrared Spectra of 1,4-Dichloro-2-butyne.

Nielsen

1992

Raman spectra were recorded in the 3200-5 cm-' range for l-chloro-Qfluorobut-2yne 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-Z-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 l,ddihalobut-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 goad for all the molecules studied when ab initio potentials are applied in the model calculations. Temperature effects in the Raman spectra of liquid lchloro-4-fluorobut-2-yne can also be derived from the model. Internal rotation in l,Uifluoro-, 1,4-dichloro-, lchloro-Qfluoro-, 1,ddibromo-and 1,Q diiodobut-2-yne was studied with the aid of ub initio calculations. The barriers were found to be low and decreasing through the series from 1,Qdifluoro-to 1,4-diiodobut-2-yne accompanied by an increase in the equilibrium dihedral angle, X-CCCC-Y, from 101" in 1,4-difluorobut-2-yne to 180" 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.

Section: Band Shapes Of the Liquid Phasesmentioning

confidence: 99%

Vibrational spectra and conformation of 1‐chloro‐4‐fluorobut‐2‐yne and ab initio calculations on 1,4‐dihalobut‐2‐ynes

Nielsen

1992

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

Nielsen

1987

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-' 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-' 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 232K 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.

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

Bauza

et al. 1992

The Raman spectra of l,ddibromobut-%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-' region. IR spectra of the liquid at room temperature and of the amorphous and crystalline solids at low temperatures (4000-50 cm-') were obtained. Additional IR spectra of the vapour were recorded (4000-500 cm-I). The spectra of the liquid phase showed characteristic, broad bands below 500 cm-'. 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 cu. 20 cm-' 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 1,4diiodobut-%yne (11) as a crystalline solid were recorded in the 4000-20 cm-' range at low temperatures. Spectra of carbon disulphide solutions at temperatures 250-260 K were obtained and additional IR spectra of I1 in Nujol mulls and in KBr pellets were also recorded. In the region below 500 cm-', where other 1,4-dihalobut-2-ynes have characteristic, broad bands, a high irregular background was observed in the Raman spectrum of I1 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 11.