Infrared Spectroscopy(IR)

1)  What is the principle of IR?

Infra-red spectroscopy is an important spectroscopy which gives almost complete information about the structure of a compound. The infra-red spectrum is containing a large number of absorption bands which provides much information about the molecule's structure. 

2)   What is the range of IR?

Region	Wavelength range (λm)	Wave number range (cm-1)
Near	0.78 - 2.5	12800 - 4000
Middle	2.5 - 50	4000 - 200
Far	50 - 1000	200 - 10

The absorption of infrared radiations can be expresses either in terms of wavelength or wave number. 

Wave length = 1/wave length (cm)

The intensity of bands observed in IR spectrum is expressed in terms of transmittance (T) or absorbance (A).

A = log10 (1/T)

Because of absorption of infra-red radiation, molecules become excited and move to lower to higher vibrational level. Each vibrational level is associated with a number of spaced rotational levels. Hence the absorption of infra-red caused to effect vibrational as well as rotational level and also called as vibrational- rotational spectra.

                                  

*Some common bands with their intensity values are as follows.

Typical Infrared Absorption Frequencies
	Stretching Vibrations			Bending vibrations
Functional class	Range  (cm-1)	Intensity	Assignment	Range (cm-1)	Intensity	Assignment
Alknes	2850-3000	strong	CH3, CH2 & CH 2 or 3 bands	1350-1470 1370-1390 720-725	Medium Medium Weak	CH2 & CH3 deformation CH2 rocking
Alkenes	3020-3100 1630-1680 1900-2000	medium strong	=C-H & =CH2 (usually sharp) C=C (symmetry reduces intensity) C=C asymmetric stretch	880-995 780-850 675-730	Strong Medium Medium	+CH & =CH2 ( out of plane bending) cis-RCH=CHR
Alkynes	3300 2100-2250	Strong	CH (usually sharp) C=C (Symmetry reduces intensity)	600-700	strong	C-H deformation
Arenes	3030 1600 & 1500	Medium-weak	C-H (may be several bands) C=C (in ring)(2 bands) (3 if conjugated)	690-900	Strong-medium	C-Hbending & ring puckering
Alcohols & phenols	3580-3650 3200-3550 970-1250	Strong strong	O-H (free), usually sharp O-H (H-bonded) usually broad C-O	1330-1430 650-770	medium Var-weak	O-H bending (in plane) O-H bend (out of palne)
Amines	3400-3500 (dil.solution) 3300-3400 (dil.solution) 1000-1250	Weak weak Medium	N-H (1o amines), 2 bands N-H (2o amines) C-N	1550-1650 660-900	Medium-strong Var	NH2 scissoring (1oamines) NH2 & NH wagging (shifts on H-bonding)
Aldehyde & ketones	2690-3840(2 bands) 1720-1740 1710-1720	Medium strong strong strong	C-H (aldehyde C-H) C=O (saturated ketone) aryl ketone Cyclo pentanone cyclobutanone	1350-1360 1400-1450-1100	Strong Strong Medium	α-CH3 bending α-CH2 bending C-C-C bending
Carboxylic acids &derivatives	2500-3300(acids) overlap C-H 1705-1720(acids) 1210-1320(acids) 1785-1815(acyl halides) 1750&1820 (anhydrides) 1040-1100 1735-1750(esters) 1000-1300 1630-1695(amides)	Strong Strong medium-Strong Strong Strong Strong Strong Strong Strong	O-H (very broad) C=O (H-bonded) O-C (Sometimes 2-peaks) C=O C=O (2-bands) O-C C=O O-C(2-bands) C=O(amide I band)	1395-1440 1590-1650 1500-1560	Medium Medium Medium	C-O-H bending N-H (1oamide) II band N-H (2o amide) II band
Nitriles Isocyanates, Isothiocyanates, Diimides, Azides &ketenes	2240-2260 2100-2270	Medium Medium	C=N (sharp) -N=C=O, -N=C=S -N=C=N-, -N3 C=C=O

3) What are the applications of UV?

*Identification of functional group and structure elucidation

·      The identity of an organic compound can be established by using its finger print region (1400-1900 cm-1). 

·      The identity of an organic compound is confirmed of its finger print region exactly matches with the known spectrum of that compound. 

·      As compounds contain same functional group may have similar absorptions above 1500 cm-1 but they differ considerably in finger print region.

·      Each functional group absorbs a certain frequency of radiation and show corresponding peak. Hence the position of peak gives the information regarding the presence of certain functional group.

*Identification of structure of substances

·      Infra red spectroscopy is used to establish the structure of an unknown compound. 

·      All functional groups present in molecule absorbed at their characteristic wave number and the shift due to environment effect can be easily observed in spectrum. 

·      Form the data available, the whole structure of molecule can be predicated. 

·      If the IR spectrum of two compounds is exactly same, it shows that they are identical compounds. Hence must be samples of the same substances.

·      IR spectrum cannot identify the presence of Enantiomers as they show similar IR spectrum.

*Study of the progress of a chemical reaction 

·      Infra red spectroscopy is quite useful for studying the rate and progress of a chemical reaction. 

·      For example the reduction of ketone to secondary alcohol can be detected easily. As the absorption band of ketone is observed at about 1710 cm-1.

·      With the progress of reaction the band of >C=O group diminished and band of -O-H group at 3300 cm-1 appears. 

·      The progress of reaction can be determined by using the rate of disappearance of a certain absorption band of the reactant molecule and the rate of appearance of the absorption band of the product group. 

·      IR spectroscopy is also used to predict the products formed in photochemical reaction.

*Detection of impurities Infra-red

·      Infra red spectroscopy is also useful in the detection of the impurity in a compound by comparing its spectrum with the spectrum of the authentic sample of the compound.

·      A pure sample always consists of sharp peaks and bands while the impure sample will have poor bands with some additional bands.

*Quantitative analysis

·      The quantitative analysis of an organic mixture can be done by measuring the intensities of the absorption bands of each component and by knowing the optical density of the absorption bands for the pure substance.