Published on March 7, 2014
INFRARED SPETROSCOPY (IR) λ = (2.5 μm-25 μm) ῡ = 400-4000cm-1 IR no sufficient energy to cause excitation of e-s. It causes atoms and groups to vibrates about covalent bonds. Bonds vibrate like spring 1
% Transmittance = (Is/Ir)*100% Is (intensity of sample beam) Ir (intensity of reference beam) 2
Uses of IR-Spectrum 3
Factors affecting the frequency location in IR spectrum 1. Masses of bonded atoms. Light atoms such as hydrogen vibrate at higher frequencies than heavier ones. Hooke's law of spring υ = 1/2π(k/μ)1/2 k (force constant in dynes/cm) μ (reduced mass) = (m1m2)/(m1+m2); masses of atoms in grams bond C-H C-C C-O C-Cl C-Br C-I ῡ (cm-1) 3000 1200 1100 800 550 ~500 7
2. Mode of motions Bending motions tend to be easier than stretching motions, and the force constant k is smaller. C-H stretching ~3000 cm-1 C-H bending ~1340 cm-1 3. Hybridization Hybridization affect the force constant k. Bonds are stronger in the order sp>sp2>sp3. sp sp2 sp3 ≡C-H =C-H -C-H 3300 cm-1 3100 cm-1 2900 cm-1 8
4. Resonance stabilization Resonance affect the strength and length of a bond and, hence, its force constant k. 9
The absorption band of a particular group may be shifted by several structural features: Conjugation Electron withdrawal by a neighboring groups Angle strain Van der Waals strain (steric effect) H-bonding 11
Number of absorption peaks in IR spectrum Theoretically, # of peaks = # of vibrational modes # of peaks = 3n – 6 Practically, # of peaks < 3n – 6 CH4 # of beaks = 3 (5) - 6 = 9 C6H6 # of beaks = 3(12) - 6 = 30 12
Not all molecular vibrations result in the absorption of IR energy. In order for a vibration to occur with absorption of IR energy, the dipole moment of the molecule must change as the vibration occurs. Vibrational absorption may occur outside the region measured by IR spectrophotometer. Vibrational absorptions may occur so closely together that the peaks fall on the top of peaks (overlapping). 13
Cases of symmetrical vibrations without absorption of IR energy: 1.When all hydrogen atoms of methane vibrate symmetrically. 2. Symmetrical vibration of carbon-carbon double and triple bond of ethene and ethyne. 14
INTERPRETATION of IR SPECTRA Position (??cm-1) Every bond in a molecule has its own characteristic features so it absorbs IR radiation and vibrates at a specific wavenumber. The same bond in different molecules may absorbs IR radiation at slightly different positions. 15
Intensity (I%) (strong, medium, weak) C=O 1850-1630 cm-1 strong C=C 1680-1620 cm-1 weak 16
Shape (sharp, broad, singlet, doublet, …??) O-H 3650-3200 cm-1 broad (only one peak for pure liquid) N-H 3500-3300 cm-1 sharp (two peaks for primary amines) 17
HYDROCARBONS: ALKANES, ALKENES, AND ALKYNES 1. ALKANES The spectrum is usually simple, with few peaks. Sp3 C-H Stretch occurs around 2840-3000 cm−1. CH2 Methylene groups have a characteristic bending absorption of approximately 1465 cm−1. CH3 Methyl groups have a characteristic bending absorption of approximately 1375 cm−1. CH2 The bending (rocking) motion associated with four or more CH2 groups in an open chain occurs at about 720 cm−1 (called a long-chain band). C-C Stretch not interpretatively useful; many weak peaks. 18
2.ALKENES =C-H Stretch for sp2 C-H occurs at values greater than 3000 cm−1 (3095–3010 cm−1). =C-H Out-of-plane (oop) bending occurs in the range 1000–650 cm−1. C=C Stretch occurs at 1660–1600 cm−1; conjugation moves C=C stretch to lower frequencies and increases the intensity. 22
Molecular vibrational levels and overtones 2730 cm-1 1820 cm-1 910 cm-1 ground state 23
3.ALKYNES ≡C-H Stretch for sp C-H usually occurs near 3300 cm−1. C≡C Stretch occurs near 2150 cm−1; conjugation moves stretch to lower frequency. Disubstituted or symmetrically substituted triple bonds give either no absorption or weak absorption. 27
AROMATIC RINGS =C-H Stretch for sp2 CIH occurs at values greater than 3000 cm−1 (3050–3010 cm−1). =C-H Out-of-plane (oop) bending occurs at 900–690 cm−1. C=C Ring stretch absorptions often occur in pairs at 1600 cm−1 and 1475 cm−1. Overtone/combination bands appear between 2000 and 1667 cm−1. 30
ALCOHOLS AND PHENOLS The free O-H stretch is a sharp and weak peak at 3650–3600 cm−1. The hydrogen-bonded O-H stretch is a broad and strong peak at 3400– 3300 cm−1. The above mentioned peaks appear together when the alcohol is dissolved in a solvent. C-O Stretching vibration usually occurs in the range 1260–1000 cm−1. 37
Solvent = CCl4 Solvent = CCl4 41
ETHERS The most prominent band is that due to C-O stretch, 1300–1000 cm−1. Absence of C-O and OIH is required to ensure that C-O stretch is not due to an ester or an alcohol. Phenyl alkyl ethers give two strong bands at about 1250 and 1040 cm−1, while aliphatic ethers give one strong band at about 1120 cm−1. 42
CARBONYL COMPOUNDS Carbonyl group is present in aldehydes, ketones, acids, esters, amides, acid chlorides, and anhydrides. Carbonyl group absorbs strongly in the range from 1850 to 1650 cm−1. 45
1760 → 1710 cm-1 E-withdrawing effect H-bonding Ketones: tow E-releasing group Aldehyde: one E-releasing group 46
CARBOXYLIC ACIDS O-H Stretch, usually very broad (strongly H-bonded), occurs at 3400–2400 cm−1 and often overlaps the CIH absorptions. C=O Stretch, broad, occurs at 1730 –1700 cm−1. Conjugation moves the absorption to a lower frequency. C-O Stretch occurs in the range 1320 –1210 cm−1, medium intensity. 55
Nujol: It is a mineral oil [CnH(2n + 2)] used in infrared spectroscopy. It has the following absorption peaks: 2950-2800, 1465-1450, and 1380–1370 cm−1. It is used to prepare a thin film of the sample squeezed between two KBr plates. The KBr pellet is placed in the sample holder of the IR-instrument. 57
AMIDES: C=O Stretch occurs at approximately 1680 –1630 cm−1. N-H Stretch in primary amides (INH2) gives two bands near 3350 and 3180 cm−1. Secondary amides have one band (INH) at about 3300 cm−1. N-H Bending occurs around 1640 –1550 cm−1 for primary and secondary amides. 62
ACID CHLORIDES C=O Stretch occurs in the range 1810–1775 cm−1 in unconjugated chlorides. Conjugation lowers the frequency to 1780–1760 cm−1. C-Cl Stretch occurs in the range 730–550 cm−1. 65
ANHYDRIDES C=O Stretch always has two bands, 1830–1800 cm−1 and 1775–1740 cm−1, with variable relative intensity. Conjugation moves the absorption to a lower frequency. Ring strain (cyclic anhydrides) moves the absorptions to a higher frequency. C-O Stretch (multiple bands) occurs in the range 1300–900 cm−1. 68
AMINES N-H Stretch occurs in the range 3500–3300 cm−1. Primary amines have two bands. Secondary amines have one band: a vanishingly weak one for aliphatic compounds and a stronger one for aromatic secondary amines. Tertiary amines have no NIH stretch. N-H Bend in primary amines results in a broad band in the range 1640–1560 cm−1. Secondary amines absorb near 1500 cm−1. N-H Out-of-plane bending absorption can sometimes be observed near 800 cm−1. C-N Stretch occurs in the range 1350–1000 cm−1. 70
NITRILES, ISOCYANATES, ISOTHIOCYANATES, AND IMINES NITRILES R-C≡N -C≡N Stretch is a medium-intensity, sharp absorption near 2250 cm−1. Conjugation with double bonds or aromatic rings moves the absorption to a lower frequency. ISOCYANATES R-N=C=O -N=C=O Stretch in an isocyanate gives a broad, intense absorption near 2270 cm−1. ISOTHIOCYANATES R-N=C=S -N=C=S Stretch in an isothiocyanate gives one or two broad, intense absorptions centering near 2125 cm−1. IMINES R2C=N-R C=N- Stretch in an imine, oxime, and so on gives a variable-intensity absorption in the range 1690–1640 cm−1. 75
NITRO COMPOUNDS Aliphatic nitro compounds: asymmetric stretch (strong), 1600–1530 cm−1; symmetric stretch (medium), 1390–1300 cm−1. Aromatic nitro compounds (conjugated): asymmetric stretch (strong), 1550–1490 cm−1; symmetric stretch (strong), 1355–1315 cm−1. 79
CARBOXYLATE SALTS, AMINE SALTS, AND AMINO ACIDS 81
SULFUR COMPOUNDS 84
PHOSPHORUS COMPOUNDS 90
ALKYL AND ARYL HALIDES 91
THE BACKGROUND SPECTRUM The infrared energy beam passes not only through the sample being measured but also through a length of air. Air contains two major infrared-active molecules: carbon dioxide and water vapor. Absorptions from these two molecules are contained in every spectrum. Carbon dioxide: two absorptions at 2350 cm−1 are due to the asymmetric stretching modes. Water molecules: groups of peaks centered at 3750 cm−1 and 1600 cm−1 are due to the stretching and bending modes of atmospheric (gaseous). Background spectrum: shows the absorptions of CO2 and H2O. After a sample spectrum is determined, the computer subtracts the background spectrum from that of the sample, effectively removing the air peaks. 94
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