High-temperature infrared and Raman spectra of aluminium chloride dimer and monomer in the vapour phase

The infrared spectra of aluminium chloride vapour (Al2Cl6 and AlCl3) including Al 235Cl6 and Al 237Cl6 at 473–843 K were measured in the region 700–50 cm−1 with an evacuable Fourier transform spectrometer by transmission and emission techniques. Evacuable cells of nickel were employed having windows of type IIa diamond and sealed with gold O‐rings. Raman spectra of Al2Cl6 vapour at ca 500 K and of AlCl3 at 1075 K with pressures from 0.3 to 10 atm were recorded, polarization measurements were carried out and additional spectra of the 35Cl and 37Cl compounds were obtained. The dimer spectra were interpreted in terms of D2h symmetry. All the nine Raman‐active and seven of the infrared‐active dimer fundamentals were assigned. The IR active mode ν10(B1u), expected below 40 cm−1, and the inactive mode ν5(Au) were left unassigned. The monomer spectra were interpreted in terms of D3h symmetry and all four fundamentals were assigned. A dimer force field, involving 12 independent force constants, was derived. The calculated frequencies were fitted with a least‐squares programme to 30 observed fundamentals of Al 235Cl6 and Al 237Cl6 and 14 additional isotopic shifts transferred from previous matrix isolation spectra. A complete force field for the monomer was derived, using the four observed fundamentals and seven isotopic shifts transferred from previously reported matrix isolation spectra.