INTERPRETATION OF INFRARED SPECTRA

GUIDED BY: PRESENTEDBY: Dr.Sk.Abdul Rehman R.Keerthi M.Pharm, Ph D M.Pharm 1st

year

DEPARTMENT: Pharmaceutical analysis

Outline…Features of an ideal IR Spectrum

Information could be obtained

Requirements for the interpretation

General rules of interpretation

Illustration

Features of an IR SpectrumAbsorption (Band) spectrumSpectrum between percent transmittance (% T)

and wave number (ύ)Wave number (ύ) represents frequency (υ) of IR

absorptionTwo regions of absorption : FGR (4000-1500 cm-1): Identification of groups FPR (1500-400 cm-1): Molecular fingerprint

(Spectral matching)Nature (Intensity and Shape) of spectral bands :

Strong (s), medium (m), weak (w); Broad and Narrow

Different types of IR Bands

Information Obtained from an Unknown IR Spectra

IR is most useful in providing information about the presence or absence of specific functional groups (FGR)

IR can provide a molecular fingerprint that can be used for the identification of samples (FPR)

Interpretation of Spectra Spectral Requirements

The spectrum must be adequately resolved and of adequate intensity.

The spectrum should be of a pure compound.The spectrophotometer should be calibrated so

that the bands are observed at their proper frequencies or wavelength.

The method of sample handling must be specified. If a solvent is employed, the solvent concentration, and the cell thickness should be indicated.

Correlation Chart and Tables

9

O-H 3400cm-1 C≡C 2150cm-1

N-H 3400cm-1 C=O 1715cm-1

C-H 3000cm-1 C=C 1650cm-1

C≡N 2250cm-1 C-O 1100cm-1

Typical Infrared AbsorptionTypical Infrared AbsorptionRegionsRegions

2.5 4 5 5.5 6.1 6.5 15.4

4000 2500 2000 1800 1650 1550 650

FREQUENCY (cm-1)

WAVELENGTH (m)

O-H C-H

N-H

C=O C=NVeryfewbands

C=C

C-ClC-O

C-NC-CX=C=Y

(C,O,N,S)

C N

C C

N=O N=O*

Absorption Regions

BASE VALUESBASE VALUES(+/- 10 cm-1)

These arethe minimumnumber ofvalues tomemorize.

O-H 3600N-H 3400C-H 3000

C N 2250C C 2150

C=O 1715

C=C 1650

C O ~1100 large range

Simple Approach to the Analysis of Spectrum at a Glance

( General Rules)

Look if carbonyl group is present?...... The C=O gives strong absorption in the region 1820-1660 cm-1

If C=O is present, check for the presence of following group.....

Acids (O-H)

a broad band near 3400-2400 cm-1

Amides (N-H)

medium absorption near 3400 cm-1 & some times double dips with equivalent halves

Ester (C-O)

strong absorption near 1300-1000 cm-1.

Anhydrides(c-o)

Gives two C-O absorption near 1810-1760 cm-1

Aldehyde (C-H)

show two weak absorption near 2850 and 2750 cm-1.

Ketones(C-R)

If none of the above is present, conforms the presence of ketones

If C=O is absent, Alcohols and Phenols: Check for O-H,

show broad absorption near 3400-3300 cm-1. confirm this by finding C-O near 1300-1000cm-1.

Amines: N-H stretch, show medium absorption near 3400 cm-1 (1O – 2 bands, 2O-1 band)

Ethers: C-O near 1300-1000cm-1,and also absence of O-H near 3400cm-1

Double bonds and / or Aromatic ring:

C=C is a weak absorption near 1650 cm-1

Medium to strong absorption 1600-1450 cm-1

Aromatic C-H occurs to left of the 3000 cm-1

Aliphatic C-H occurs to right of 3000 cm-1

Triple bonds: C≡N medium, sharp absorption near 2250 cm-1 C≡C weak sharp absorption near 2150 cm-1

Nitro groups

Two absorptions at 1600-1530 cm-1 & 1390-1300 cm-

1

Hydrocarbons None of the preceding is found Major absorptions are in C-H region near 3000 cm-1

Very simple spectrum

IllustrationsTaking example of drugs

References :www2.ups.edu/faculty/hanson/Spectroscopy/IR/IRInterpretation.htm

chemwiki.ucdavis.edu › ... › Infrared Spectroscopy

chemistry.oregonstate.edu/courses/ch361-464/ch362/irinterp.htm

infrared.als.lbl.gov/BLManual/IR_Interpretation.pdf