Question

Draw Lewis structures for the following molecules: XeCl4, SiO2, CH3Br, OF2, SO2, I2

Which of the molecules may have a rotational Raman spectrum? List the gross selection for being rotational Raman active.

Answer 1

The selection rule for rotationalRaman spectroscopy is that the molecule must be anisotropically polarisable, which means that the distortion induced in the electron distribution in the molecule by an electric field must be dependent upon the orientation of the molecule in the field.ΔJ = 0 transitions do not lead to a change in the frequency of the scattered photon, and contribute to the unshifted Rayleigh radiation that passes unaltered through the sample.

When the molecule makes a transition with ΔJ = +2, then the interaction has imparted energy to the molecule. The scattered radiation must thus have lost energy, i.e. be at a wavenumber lower than that of the incident radiation. These transitions produce the lines in the lines in the spectrum that are known as Stokes lines.

When the molecule makes a transition of ΔJ = -2, the incoming photon receives energy from the molecule, and thus the scattered radiation must have a higher energy, i.e. be at a higher wavenumber than the incident radiation. These transitions give rise to the anti-Stokes lines.

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