Vibrational spectroscopy polarised absorption

Figure 4.13 illustrates the use of polarised absorption spectroscopy to measure orientation for a uniaxially oriented polymer. For a single molecule, the absorbance is proportional to the square cosine of the angle between the vibration transition dipole moment and the IR electric field vector E). Thus if 0 = 90°, the absorbance is zero (note that the measurement is sensitive to the alignment of the vibrational dipole moment rather than the chain axis itself). The dichroic ratio R is calculated by ratioing the absorbance measured with E parallel toXs with that measured perpendicular toXs R = A A ). For an assembly... 

Mid-IR absorption and Stokes Raman deal with the same vibrations but are subject to different selection rules (and consequently the spectra differ). IR and RS provide complementary images of molecular vibrations. Vibrations which modulate the molecular dipole moment are visible in the IR spectrum, while those which modulate the polarisability appear in the Raman spectrum. Compositions that do not absorb in the IR range generally give a Raman spectrum and strong IR absorbers will produce a weak spectrum by Raman. Examples of silent Raman vibrational modes are specific point groups (e.g. C(, De, Cev, C4h, D, >3h. Den, etc.). Other vibrations may be forbidden in both spectra. Raman spectroscopy complements IR spectroscopy, particularly for the study of non-polar bonds and functional groups e.g. C=C, C—S, S—S, metal-metal bonds). 

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