The moments of inertia of the linear mercury (II) halides are very large, so the °and S branches of their vibrational Raman spectra show little rotational structure. Nevertheless, the peaks of both branches can be identified and have been used to measure the rotational constants of the molecules (R.J, H.
Clark and D.M. Rippon, f. Chem. Sac. Faraday Soc. II, 69, 1496 (1973)) Show, from a knowledge of the value of J corresponding to the intensity maximum, that the separation of the peaks of the °and S branches is given by the Placzek- Teller relation ∂v = (32BkT/ hc) 1/2 The following widths were obtained at the temperatures stated:
HgC12 HgBr2 HgI2
θ/oC 282 292 292
∂v/cm-1 23.8 15.2 11.4