Infrared Spectroscopy PDF

INFRARED SPECTROSCOPY IR spectroscopy : Infrared spectroscopy is the subset of spectroscopy that deals with the infrared region of the electromagnetic spectrum. It covers a range of techniques, the most common being a form of absorption spectroscopy. Wavelength The wavelength range of infrared ra...

INFRARED SPECTROSCOPY IR spectroscopy : Infrared spectroscopy is the subset of spectroscopy that deals with the infrared region of the electromagnetic spectrum. It covers a range of techniques, the most common being a form of absorption spectroscopy. Wavelength The wavelength range of infrared radiation lies between 14000–10 cm−1 (0.8–1000 μm). The most useful I.R. region lies between 4000 - 670cm-1. Regions Near infrared :- approximately 14000–4000 cm−1 (0.8–2.5μm) Mid infrared :- approximately 4000–400 cm−1 (2.5–30μm) Far infrared :- approximately 400–10 cm−1 (30–1000 μm) @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg  CORRECT WAVELENGTH OF RADIATION  In any molecule, atoms or groups of atoms are connected by bonds ,which will be in a continuous motion & will exhibit some frequency. This frequency is called as natural frequency.  A molecule absorbs radiation only when natural frequency = incident frequency. After absorbing the correct wavelength of radiation, the molecule vibrates and absorption of infrared radiation takes place and a peak is observed. @Sinaiunieg [email protected] www.su.edu.eg  A molecule can absorb IR radiation when its absorption causes a change in its electric dipole.  Molecule is said to have an electric dipole when there is a slight positive and negative charge.  When the molecule having an electric dipole is kept in the electric field ,this field will exert forces on the electric charges in the molecules. @Sinaiunieg [email protected] www.su.edu.eg  For a molecule to absorb IR, the vibrations within a molecule must cause a net change in the dipole moment of the molecule. Dipole moment  measures the polarity of a chemical bond, or molecule. depends on  Electronegativity, e.g. H-Cl  Direction of withdrawing (vector sum)  Sym st. No absorption  asymst. Absorption occurs @Sinaiunieg [email protected] www.su.edu.eg  When charged atoms vibrate, they absorb infrared radiation from radiation source.  If rate of vibration at charged atoms in a molecule is fast, absorption of radiation is intense and thus IR spectrum will have intense absorption bands.  If rate of vibration is slow, weak bands will appear in IR spectrum. @Sinaiunieg [email protected] www.su.edu.eg  1. STRETCHING : Change in inter-atomic distance along bond axis.  It is of two types: Symmetrical Asymmetrical Bonds increase or decrease One bond length increases in length symmetrically. and the other decreases. @Sinaiunieg [email protected] www.su.edu.eg  2. BENDING: Change in angle between two bonds. There are four types of bend. Rocking Scissoring Bond angle is maintained, but Bond angle decreases. both bonds move within the plane. @Sinaiunieg [email protected] www.su.edu.eg  Twisting Wagging Both atoms move to one side One atom is above the plane and the of plane. other is below the plane @Sinaiunieg [email protected] www.su.edu.eg 1. Sources 2. Monochromators 3. Sample preparation 4. Detectors 5. Typical method @Sinaiunieg [email protected] www.su.edu.eg  IR CHART Most of Most of stretching bending Fnctional fingerprint group region frequency region @Sinaiunieg [email protected] www.su.edu.eg Force constant (Bond strength) Reduced mass @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg (Fermi Resonance) @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg  Infrared Absorption Frequencies Structural unit Frequency, cm-1 Stretching vibrations (single bonds) sp C—H 3310-3320 sp2 C—H 3000-3100 sp3 C—H 2850-2950 sp2 C—O 1200 sp3 C—O 1025-1200 @Sinaiunieg [email protected] www.su.edu.eg  Infrared Absorption Frequencies Structural unit Frequency, cm-1 Stretching vibrations (multiple bonds) C C 1620-1680 —C C— 2100-2200 —C N 2240-2280 @Sinaiunieg [email protected] www.su.edu.eg Structural unit Frequency, cm-1 Stretching vibrations (carbonyl groups) C O Aldehydes and ketones 1710-1750 Carboxylic acids 1700-1725 Acid anhydrides 1800-1850 and 1740-1790 Esters 1730-1750 Amides 1680-1700 @Sinaiunieg [email protected] www.su.edu.eg Structural unit Frequency, cm-1 Bending vibrations of alkenes RCH CH2 910-990 R2C CH2 890 cis-RCH CHR' 665-730 trans-RCH CHR' 960-980 R2C CHR' 790-840 @Sinaiunieg [email protected] www.su.edu.eg Structural unit Frequency, cm-1 Bending vibrations of derivatives of benzene Monosubstituted 730-770 and 690-710 Ortho-disubstituted 735-770 Meta-disubstituted 750-810 and 680-730 Para-disubstituted 790-840 Stretching vibrations (single bonds) O—H (alcohols) 3200-3600 O—H (carboxylic acids) 3000-3100 N—H 3350-3500 @Sinaiunieg [email protected] www.su.edu.eg  Assoc. prof/Miral Ahmed Associate professor of organic chemistry