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INFRARED SPECTROSCOPY

INFRARED SPECTROSCOPY. INFRARED SPECTROSCOPY. the spectroscopy that deals with the  infrared  region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency  than visible light. Infrared of electromagnetic spectrum. Near-infrared Mid-infrared Far-infrared.

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INFRARED SPECTROSCOPY

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  1. INFRAREDSPECTROSCOPY

  2. INFRAREDSPECTROSCOPY • the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light.

  3. Infrared ofelectromagnetic spectrum Near-infrared Mid-infrared Far-infrared http://www.nasa.gov/images/content/56638main_Infrared_Region.jpg

  4. Near-infrared • approximately 14000-4000cm−1(0.8–2.5μm wavelength) • Can exciteovertoneorharmonicvibrations. • Has high energy

  5. mid-infrared • approximately 4000-400cm−1(2.5–25μm) • used to study the fundamental vibrations and associated rotational-vibrational structure.

  6. far-infrared • approximately 400–10 cm−1 (25–1000 μm) • lying adjacent to the microwave region • has low energy and may be used for rotational spectroscopy. 

  7. The vibrational infrared spectrum • 2.5x10-6to 2.5x10-5m • Vibrational infared region – the portion of the infrared region that extends from 4000 to 400 cm-1 • Wavenumber (v) – radiation in the vibrational infrared region. The number of waves per centimeter.

  8. Molecular vibrations • Atoms joined by covalent bonds are not permanently fixed in one position, but instead undergo continual vibrations relative to each other • The bond undergoing vibration must be polar. • It’s vibration must cause a periodic change in the bond dipole. • The greater the polarity of the bond, the more intense is the absorption.

  9. 3n – 6 (fundamental vibrations) Example: • Ethanol CH3CH2OH • Hexanoic acid CH3(CH2)4COOH

  10. STRETCHING MOTIONS • Change in band length Assymmetric stretching Symmetrical stretching

  11. bendING MOTIONS • Change in band angle. Rocking Scissoring Twisting Wagging

  12. Correlation tables • Data on absorption patterns of selected functional groups are collected.

  13. Example: Determine the functional group that is likely present if a compound shown IR absorption at • 1705 cm-1 • 2950 cm-1

  14. facts • absorption of IR radiation only occurs if there is a change in dipole moment • the larger the change in dipole moment, the more intense the absorption band • the greater the electronegativity differences between atoms in the molecule, the more polarized, the larger change in dipole moment

  15. INTERPRETINGINFRAREDSPECTRA

  16. INFRARED spectrum Transmittance Wavenumber (cm-1)

  17. ALKANES decane

  18. ALKENES cyclopentene

  19. ALKYNES 1-octyne

  20. ARENES toluene

  21. ALCOHOLS 2-pentanol

  22. ETHERS Diethyl ether

  23. KETONES menthone

  24. AMINES butanamine

  25. AMIDES N,N-dimethyldodecanamide

  26. AMIDES N-methylbenzamide

  27. AMIDES butanamide

  28. CARBOXYLIC ACIDS Butanoic acid

  29. ESTERS Ethyl butanoate

  30. Index of hydrogen deficiency • The sum of the number of rings and pi bonds in a molecule. • Compare the number of hydrogens in the molecular formula of a compound of unknown structure with the number of hydrogen in a reference compound with the same number of carbon atoms and with no rings or pi bonds.

  31. CnH2n+2 Index of Hydrogen = (Hreference – Hmolecule) 2

  32. Example Index of hydrogen deficiency = 4

  33. UV-VISSpectroscopy Ultraviolet-visible Spectroscopy

  34. Uv-visSPECTROSCOPY • Measures the attenuation of a beam of light after it passes through a sample or after reflection from a sample surface. • Spectral range of 190 nm to 900 nm • 190 – 400 nm, UV • 400 – 750 nm, Vis

  35. Electronic Transition Vis = 36 – 72 kcal/mol UV = >72 – 143kcal/mol

  36. chromophore • identifiable part of the molecule • responsible for any spectral feature like bands • use to deduce presence of structural fragment or element in the molecule • corresponds to a functional group, an atom or group of atoms • Molar absorptivity > 200 for UV-Vis

  37. UV-Vis Spectrum • UV - electronic transition in 200 – 380 nm • Vis - electronic transition in 380 – 800 nm

  38. Effects of Conjugation

  39. Effects of Conjugation

  40. Schematic of a wavelength-selectable, single-beam UV-Vis spectrophotometer

  41. UV-Vis spectrophotometer

  42. References • http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/UV-Vis/spectrum.htm Thank You!

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