Interpretation of Infra red spectra

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Text of Interpretation of Infra red spectra

  • Presented by Y. Narayudu(Pharm. Analysis & Q.A)

  • Features of an ideal IR Spectrum

    Information could be obtained.

    Requirements for the interpretation

    General rules of interpretation


  • Absorption (Band) spectrumSpectrum between percent transmittance (% T) and wave number ()Wave number () represents frequency () of lR 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

  • 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)

  • 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.

  • Literature (Books, Journals, Spectral library)

    Correlation Tables

    Correlation Charts

    Reference Spectra (IP, BP etc.)

    Web Database Source of Data

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  • Typical Infrared Absorption RegionsFREQUENCY (cm-1)WAVELENGTH (mm)O-HC-HN-HC=OC=NVeryfewbandsC=CC-ClC-OC-NC-CX=C=Y(C,O,N,S)C NC CN=O N=O*

  • BASE VALUES(+/- 10 cm-1) These arethe minimumnumber ofvalues tomemorize.O-H3600N-H3400C-H3000C N2250C C2150C=O1715C=C1650C O ~1100large range

  • 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-1Aldehyde (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: CN medium, sharp absorption near 2250 cm-1 CC 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