Intensity of vibrational bands

To determine the vibrational structure of electronic transitions of polyatomics, we can make the same approximation [Equation (7.21)] as for diatomics of replacing Ptl by some sort of average Pel, which is independent of the Qi s. The intensities of vibrational bands in an electronic transition then depend on the vibrational overlap integral, which is like... 

Thus far, this contribution has been concerned mainly with the energies of vibrational transitions. Intensities were considered only in connection with Fermi resonance and RR spectroscopy. In this section a few short comments about the intensity of vibrational bands in normal IR and Raman spectra are presented. The intensity depends on the electronic stnicture of the species, since changes in the dipole moment (dfi) and in the polarizability (da) during a vibration are caused by changes in the electron density. 

Table 3.48 Magnifications of intensity of vibrational bands occurring upon dimerization of water .

Interpreting the intensities of vibrational bands in a vibrational spectrum is not straightforward. The band intensity is not simply proportional to the number of atoms involved with such specific vibration. The intensity varies with types of bonds and types of spectroscopy, either IR or Raman. The intensity of an IR absorption band is proportional to the square of its change... 

Raman spectroscopy displays the same property as NIR regarding the intensities of vibrational bands. Raman bands are much less sensitive to H-bonds than IR bands, as seen in Ch. 4. For a long time, Raman spectra were therefore the most frequently used method to study liquid water, before the advent of ATR set-ups in IR spectroscopy. It has thus been possible to show that the band of liquid water is composed of two bands (68). Their significance has also for long been an object of discussion, but as indicated in Ch. 9, they are now seen as corresponding to two different states of rotations or librations of individual HjO molecules. 

Vibrational sum frequency spectroscopy in conjunction with interfacial pressure measurements provide the first direct information about the structure of phospholipid monolayers composed of DLPC, DMPC, DPPC, and DSPC, adsorbed to the interface between two immiscible liquids. Temperature controlled experiments carried out with aqueous solutions of DSPC show the lipid bilayer gel to liquid crystalline phase transition temperature to play a pivotal role in determining interfacial monolayer concentration and alkyl chain structure. Even at equivalent interfacial concentrations longer chain phosphocho-line species form more disordered monolayers with a greater number of gauche defects than shorter chain phosphocholine species, as determined from relative intensities of vibrational bands in the CH stretching region. 

Various structural information from band shift, relative peak intensities and/or width, e.g., drug-polymer H-bonding (red shift), monomer-multimer ratio of API Relative crystallinity can be quantified from the ratio of peak intensities of vibrational bands unique to amorphous and crystalline forms or the ratio of peak width of partially to completely crystalline samples of IR or Raman sftectra Contribution from the alteration of the specific molar absorbance on newer bands (e.g., drug-polymer interactions) difficult to account for absolute quantification... 

---