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Interpretation of IR Spectra
- 1. INTERPRETATION OF IR SPECTRA Submitted to – Mr. KRISHAN KUMAR VERMA ( Asst. Professor) Submitted by – PRANJALI YADAV B.pharm IV year (Sec-B) Roll no- 1709350064
- 2. CONTENTS Interpretation Means Principle Modes of Vibrations Components of IR instrument Features of IR Spectrum IR spectrum Use of IR Spectra Diff types of IR Bands Interpretation of Various Functional Groups Simplified Correlation chart
- 3. Interpretation of IR Spectrum Means An Explanation or Understanding of INFRA RED SPECTRUM (Graph) . Infrared (IR) radiation is a type of Electromagnetic radiation . Infrared means below Red IR spectra tells what types of vibrational modes (motion) the molecule responds with after it absorbs that light , and figures out which peaks correspond to which motions , can figure out what functional groups the molecules has and (almost) what the molecule is.
- 4. PRINCIPLE Infrared spectroscopy is based on a simple fact that a chemical substance shows marked selective absorption in IR region. After absorption of IR radiation the molecules of chemical substance vibrate at many rates of vibration giving rise to closely packed absorption spectrum called as IR absorption spectrum which may extend over a wide wavelength range. IR region extends from RED end of visible region to microwave region. Has less energy content than UV/Visible light. Vibrational – Rotational Spectra occur. REQUIREMENTS Frequency Match Dipole moment
- 5. Modes of Vibration It can be divided into Two principal groups . 1. Stretching vibrations : changes in bond length but atom remain in same bond axis. a) Symmetrical stretching b) Asymmetrical stretching 2. Bending vibration : changes in bond angle but no change in bond length. a) In plane bending: i) Scissoring ii) Rocking b) Out of plane bending: i) Wagging ii) Twisting
- 6. Components of IR Spectrophotometer
- 7. Features of an IR Spectrum An IR spectrum is a plot of percent transmittance ( or absorbance ) against Wavenumber ( frequency or wavelength ). A 100 percent transmittance in the spectrum implies no absorption of IR radiation. When a compound absorbs IR radiation, the intensiy of transmitted radiation decreases. This results in a decrease of percent transmittance and hence a dip in spectrum. The dip is often called an absorption peak or absorption band. * Different types of groups atoms ( C-H , O-H , N-H ,etc… ) absorbs infrared radiation at different characteristics wavenumbers . * No two molecules will give exactly the same IR spectrum ( except Enatiomers )
- 8. IR SPECTRUM Simple stretching ( Functional group region ) : 1500 – 4000 cm-1 Complex Vibrations : approx 1500 – 600 cm-1 , called the ’’FINGERPRINT REGION”
- 9. The functional group region runs from 4000 cm-1 to 1500 cm-1, and the fingerprint region from 1500 cm-1 to 600 cm-1. A typical IR spectrum looks something like the one below. The functional group region contains relatively few peaks. These are typically associated with the stretching vibrations of functional groups. The stretching vibrations of a functional group vary within a narrow range.
- 10. o In the table below, notice how almost all the stretching vibrations are all above approx 1500cm-1.
- 11. In the fingerprint region, the spectra usually consist of bending vibrations within the molecule. The pattern of peaks is more complicated, and it is much more difficult to pick out individual bonds in this region. The fingerprint region is important because each different compound produces its own unique pattern of peaks (like a fingerprint) in this region. • Once you think you have identified an unknown compound, you can compare its IR spectrum with that of a known sample of the compound. If all peaks in the fingerprint region match, you have identified your compound.
- 12. Compare the IR spectra of propan-1-ol and propan-2-ol Both spectra are quite similar in the functional group region, but their absorption patterns in the fingerprint region are completely different.
- 16. Different Types of IR Bands
- 18. Approach:
- 20. Carboxylic acids: Gives the messiest of IR spectra C=O band occurs between 1700-1725 cm-1 The highly dissociated O-H bond has a broad band from 2400- 3500 cm-1 covering up to half the IR spectrum in some cases
- 22. Amides: Display features of amines and carbonyl compounds • C=O stretch at 1640-1680 cm-1 If the amide is primary (-NH2 ) the N-H stretch occurs from 3200- 3500 cm-1 as a doublet If the amide is secondary (-NHR) the N-H stretch occurs at 3200- 3500 cm-1 as a sharp singlet
- 24. Esters: C=O stretch at 1735-1750 cm-1 Strong band for C-O at a higher frequency than ethers or alcohols at 1150-1250 cm-1
- 26. Acid anhydrides : Coupling of the anhydride though the ether oxygen splits the carbonyl band into two with a separation of 70 cm-1 Bands are at 1740-1770 cm-1 and 1810-1840 cm-1 Mixed mode C-O stretch at 1000-1100 cm-1
- 28. Aldehydes: C=O (carbonyl) stretch from 1720-1740 cm-1 Band is sensitive to conjugation, as are all carbonyls A highly unique sp2 C-H stretch appears as a doublet, 2720 & 2820 cm-1 called a “Fermi doublet”
- 30. Ketones: Simplest of the carbonyl compounds as far as IR spectrum – carbonyl only C=O stretch occurs at 1705-1725 cm-1
- 32. Amines (N-H stretch) :
- 33. Primary amines: Shows the –N-H stretch for NH2 as a doublet between 3200-3500 cm-1 (symmetric and antisymmetric modes) -NH2 has deformation band from 1590-1650 cm-1 Additionally there is a “wag” band at 780-820 cm-1 that is not diagnostic
- 35. Secondary amines & Tertiary amines : N-H band for R2N-H occurs at 3200-3500 cm-1 as a single sharp peak weaker than –O-H Tertiary amines (R3N) have no N-H bond and will not have a band in this region
- 38. Nitro group (-NO2) :
- 39. Nitro group : • Proper Lewis structure gives a bond order of 1.5 from nitrogen to each oxygen • Two bands are seen (symmetric and asymmetric) at 1300-1380 cm-1 and 1500-1570 cm-1 • This group is a strong resonance withdrawing group and is itself vulnerable to resonance effects
- 41. Alcohols: Strong, broad O-H stretch from 3200-3400 cm-1 Like ethers, C-O stretch from 1050-1260 cm-1 The shape is due to the presence of hydrogen bonding
- 42. Nitriles (cyano): Principle group is the carbon nitrogen triple bond at 2100-2280 cm-1 This peak is usually much more intense than that of the alkyne due to the electronegativity difference between carbon and nitrogen
- 43. Acid halides: Clefted band at 1770-1820 cm-1 for C=O Bonds to halogens, due to their size (see Hooke’s Law derivation) occur at low frequencies, only Cl is light enough to have a band on IR, C-Cl is at 600- 800 cm-1
- 57. IR INSTRUMENT : IR SPECTROPHOTOMETER
- 58. Conclusion The IR interpretation is the QUALITATIVE tool widely useful in : Determination of functional groups. Differentiation between isomers. Determination of air pollutants. Differentiation between primary, secondary and tertiary amines. Monitoring of chemical reactions. Detection of impurities. (e.g. ketones in alcohols) Detection of moisture in drugs or samples.
- 59. REFERENCES Introduction to Spectroscopy , Fourth edition , Donald L. Pavia , page no – 26 – 73. Instrumental method of Analysis , Seventh edition , Williard Merritt Dean Settle , Pg. no. 305-310. WWW. Google.com WWW. Pubmed.com