Infrared (IR) Spectroscopy: Basics

INFRARED SPECTROSCOPY: BASICS

• This technique is used for the structural analysis of molecule.
•  After absorption of IR radiations, molecules vibrate at many rates of vibrations, which shows various characteristic peaks in the spectrum.

Various wavelength ranges (IR):

REGION	WAVELENGTH (μ)	WAVE NUMBER (cm-1)
Photographic region	Visible to 1.2	Visible to 8333.33
Very near IR (Overtone) region	1.2 to 2.5	8300 to 4000
Near IR (Vibration) region	2.5 to 25	4000 to 400
Far IR (Rotation) region	25 to 300-400	400 to 33.33-25

Conversion of wavelength into wavenumber and vice versa:

Remember, 1μ = 10^-6 m, 

and cm^-1 = 1/cm, 

Simply, multiply (1/wavelength) or (1/wavenumber) with 10⁴.

Principle of IR spectroscopy:

1. Correct wavelength of radiation:

Natural frequency of vibration of a molecule should match with the incident IR radiation. 

Example: natural frequency of vibration of HCl is 8.7 x 10¹³ sec^-1.

Only this frequency from incident IR radiation is absorbed by HCl molecule. Other frequency is transmitted.

2. Electric dipole:

Absorption of radiation should cause a change in the electric dipole moment of the molecule.
Fast rate of vibration ∝ Intense peaks
Slow rate of vibration ∝ Weak peaks

Symmetric diatomic molecules like O2, N2, etc do not possess electric dipole, therefore cannot be excited by IR radiations. 

Types of molecular vibrations:

INSTRUMENTATION:
1. Radiation Sources:
a. Incandescent lamp:
- fails in far IR beacuse of its glass enclosed structure.
- low spectral emissivity.
b. Nernst glower:
- a hollow rod
- composed of rare earth oxides (Zirconia, yttria, thoria)
- non-conducting at room temperature
- heated externally to bring in conducting state 
- heated at temperature 1000-1800℃
- maximum radiation at 7100 cm^-1

- Disadvantage: frequent mechanical failure.

c. Globar source:

- rod of sintered silicon carbide (4 mm x 50 mm)

- temperature of heating: 1300-1700℃

- maximum radiation at 5200 cm^-1

^{- self-starting}

^{- less intense source than nernst glower}

^{d. Mercury arc:}

^{- at shorter wavelength heated quartz envelop emits radiation}

^{- at longer wavelength the mercury plasma provides radiation through quartz}

^{2. Monochromators:}

^{a. Prism:}

^{- it is composed of dispersive elements,} 

^{- composed of metal halide salts which transmit in the IR}

^{- Sodium chloride (NaCl) is the most common prism salt}

^{- Disadvantage: due to salts this monochromator is subjeted to thermal, mechanical instabilty and water solubility.}

^{b. Grating monochromator:}

^{- these are reflection gratings}

^{- works according to Bragg's equation: nλ = d(sin i ± sin 𝛳)} 

^{- grating is composed of aluminium (not attacked by moisture).}

^{3. Sampling cells and sampling substances:}

^{a. Solids run in solution:}

^{- solid is added in non-aqeous solvent}

^{- this solution is evaporated on alkali metal disc}

^{- thin film of solute /solution is placed in cell}

^{b. Solid films:}

^{- amorphous solid sample is deposited on KBr or NaCl cell by evaporating the solution}

^{- helps in rapid qualitative analysis but not quantitative analysis}

c. Mull technique:
- finely ground solid sample is mixed with Nujol (mineral oil)
- Nujol is transparent throughout IR region but shows absorption at 2915, 1462, 1376 and 719 cm<sup>-1
- these absorption bands may hide the absorption bands of sample and will create problems in interpretation.
- Nujol+ hexachlorobutadiene is used
-</sup> hexachlorobutadiene absorbs in the region 1630-1510, 1200-1140, 1010-760 cm<sup>-1
d. Pressed pellet technique:
- calculated amount of fine sample is mixed with 50-100 times its weight of powdered KBr 
- resultant powder mixture is pressed under high pressure to obtain a pellet
- this pellet is subjected to IR radiations
- KBr is transparent to the IR radiations 
- quantitative estimation is possible.
e. Sampling of gases:
- sample cell in similar to the liquid sampling cell but larger.
- multiple reflection technique is used to make effective path length.
- the gas must not react with the cell windows or the reflecting surfaces.

4. Detectors:
a. Bolometer:
- it works on the principle of degree of change in the resistance upon fluctuations in the temperature (due to falling of radiations on metallic wire)
- thus the amount of radiation is measured
b. Thermocouple:
- it works on the basis of temperature differences between the two ends of metal wires.
c. Thermister:
- fused mixture of metal oxides is used
- increase in the temperature of these metal oxides (due to incident radiations) causes decrease in the electrical resistance.
- changes in the resistance implies amount of radiation.
d. Golay cell:
- this is a metal cylinder filled with xenon gas.
-  its one end is closed by blackened metal plate and other end is closed by flexible metallic diaphragm.
- incident radiations increases the temperature of metallic plate which expands the gas and movement of diaphragm occurs.
- light from the lamp is made to fall on diaphragm, whose movememt reflects the light on photocell as an output.
e. Photoconductivity cell:
- non-thermal detector, greater sensitivity (0.5 sec response time)
- lead sulphide or lead telluride layer on glass enclosed in a glass cell
- IR radiation increases the conductance and current flow (showing intensity of falling radiation).


Which part of this article did you find useful? Any improvisation required?
Let me know about this article in the comments.</sup>