Infrared spectroscopy

Energies of infrared radiation correspond to vibrational energies of molecules. IR spectroscopy is a vibrational spectroscopy as it is based on the phenomenon of absorption of infrared radiation by molecular vibrations. IR spectroscopy gives information about the molecular structure of the materials. Both inorganic and organic materials can be analyzed. Commonly, the vibrational spectroscopy covers a wave-number range from 200 to 4000 cm^-1 .

Infrared Activity

Among the total number of normal vibration modes in a molecule, only some can be detected by infrared spectroscopy. Such vibration modes are referred to as infrared active. To be infrared active, a vibration mode must cause a change of dipole moment in a molecule.

When a molecule-has a center of positive charge and a center of negative charge and if these two centers are separated by a distance (l), the dipole moment (μ) is defined as μ = el where, e is amount of electrical charge. Mathematically, the requirement of infrared activity is that the derivative of dipole moment with respective to the vibration is not zero.

Where , q = magnitude of normal vibration

Species with polar bonds, such as CO, NO or OH exhibits strong IR bands. Covalent bond or non-polar bonds such as C-C, – C≡C, C=C, N=N, H₂ or N₂ absorb IR weakly or not at all.

Further for polar bonds, in case of symmetric stretching if there is no change in dipole moment the corresponding vibration will not be IR-inactive. But asymmetric stretchings which are associated with change in dipole moment are always IR-active.As a guideline following can be used

• Symmetric stretch: No change in dipole moment so not IR-active
• Asymmetric stretch : Change in dipole moment so IR-active

Analysis by infrared techniques

IR radiation in the range 4000-400 cm^-1 is used to excite stretching and bending molecular vibrations. Stretching vibrations are of highest frequency and most relevant to catalyst studies. A complex molecule is likely to have large no. of vibrations. Normal vibrations are classified in two groups:

1.  Skeletal vibrations: In these vibrations all atoms undergo approximately the same displacement.

Vibrations of the carbon chain in organic molecules, which falls in 1400-1700 cm^-1 range is an example.

Fig. 2 : Examples of skeletal vibrations in hydrocarbons

2.  Group vibrations: In some vibration modes, displacement of a small group of atoms may be much more vigorous than those of the remainder. These are called group vibrations. Group vibration frequencies are almost independent of the structure of the molecule as a whole and generally fall in the region well above or well below the skeletal mode.

Vibrations of light atoms in terminal groups, such as –CH₃ ,-OH, -C ≡ N, >C=O, are of high frequencies. While vibrations of heavy atoms such as -C-Cl, -C-Br, metal-metal etc. are of low frequencies. These vibrations falls in this category. The Table 1 shows the stretching frequencies of some molecular groups.

Overall IR spectrum can be broadly divided in five regions:

• X-H stretch regions ( 4000-2500 cm^-1 ) e.g – CH, NH, OH vibrations
• Triple bonds regions 2500-2000 cm^-1 ; e.g C≡C (2100-2200) C≡N (2240-2280)
• Double bonds region 2000-1500 cm^-1 ; e.g C=O (1680-1750) C=C (1620-1680)
• fingerprint regions 1500-500 cm^-1 ; Single bonds C-N, C-O,C-S, C-Cl etc.
• Metal adsorbate region 450-200 cm^-1 e.g M-X X= C,O,N

Table 1. Stretching frequencies of some molecular group