Raman

Spectroscopy

Laser 4880 Å

Raman

Spectroscopy

Selection Rules

Infrared: Intensity of a peak is related to the change in the dipole moment associated in going from the ground state to an excited state; the principle vibrational quantum number changes by ± 1

Raman: Intensity of a peak is related to the polarizability of the stretch. Non-polar bonds are usually more easily polarized than polar bonds.

Comparison of IR and Raman Spectroscopy

1. Bands that are intense in the IR are usually weak in the Raman. The spectral interference associated with hydrogen bonding is greatly reduced. Similar reduction in interference can also be obtain by examining gas phase spectra. Water is a useful solvent in Raman whereas water is a poor solvent for IR studies. The optics in Raman are made from glass or quartz instead of salts (NaCl, KBr, CsI).

NH2CH2CH2OH

Liquid film

KBr

Comparison of IR and Raman Spectroscopy

1. Bands that are intense in the IR are usually weak in the Raman. The spectral interference associated with hydrogen bonding is greatly reduced. Similar reduction in interference can also be obtain by examining gas phase spectra. Water is a useful solvent in Raman whereas water is a poor solvent for IR studies. The optics in Raman are made from glass or quartz instead of salts (NaCl, KBr, CsI).

2. Molecules with a center of symmetry have no coincident IR and Raman bands. Thus a comparison of the two spectra can provide structural information.

3. Raman spectra are generally simpler that IR spectra. Overtones and combination bands frequent in IR are less common.

KBr

powder

powder

KBr

Comparison of IR and Raman Spectroscopy

1. Bands that are intense in the IR are usually weak in the Raman. The spectral interference associated with hydrogen bonding is greatly reduced. Similar reduction in interference can also be obtain by examining gas phase spectra. Water is a useful solvent in Raman whereas water is a poor solvent for IR studies. The optics in Raman are made from glass or quartz instead of salts (NaCl, KBr, CsI).

2. Molecules with a center of symmetry have no coincident IR and Raman bands. Thus a comparison of the two spectra can provide structural information.

3. Raman spectra are generally simpler that IR spectra. Overtones and combination bands frequent in IR are less common.

Comparison of IR and Raman Spectroscopy

2. Molecules with a center of symmetry have no coincident IR and Raman bands. Thus a comparison of the two spectra can provide structural information.

3. Raman spectra are generally simpler that IR spectra. Overtones and combination bands frequent in IR are less common.

4. The entire IR range can be covered by Raman spectroscopy since a laser, usually in the visible region is used and the spectrum is obtained by looking at the frequency differences from the incident frequency. In IR, different optics and beam-splitters are needed to cover the entire useful range from the near IR to the far IR.

Comparison of IR and Raman Spectroscopy

5. IR spectrometers are less expensive and more sensitive instruments. Intensity measurement in Raman are very sensitive to laser power, and cell geometry, and are less reproducible than IR spectra.

6. A small fraction of the incident photons in Raman are scattered, (e. g. 10-8). Broadband fluorescence can obscure the Raman signals.

7. As a result of the simplification in the spectra, Raman spectroscopy provides less structural information.

Liquid film

liquid