1. Infrared Spectroscopy Theory and Interpretation of IR spectra
Infrared Spectroscopy. Prepared by Dr. Khalid A. Shadid
Islamic University in Madinah
Department of Chemistry
Infrared Spectroscopy Theory and Interpretation of IR spectra
Prepared By
Dr. Khalid Ahmad Shadid

2. Spectroscopy and the Electromagnetic Spectrum

3. Basic Theory of IR Absorption
Infrared: exciting from one vibrational level to another
UV/Vis: exciting from one electronic level to another
Microwaves: exciting from one rotational level to another

4. Basic Theory of IR Absorption
Changes in interatomic vibrations of a molecule are brought about through the absorption of IR light

5. The IR Spectrum
The vibrational spectrum of a molecule is a unique physical property and is characteristic of the molecule
IR spectrum can be used as for identification by the comparison of ‘‘unknown’’ spectrum with reference spectra
IR spectrum can lead to characterization, and possibly even identification of an unknown samples
The IR information can indicate:
Linear or branched backbone
If chains are (un)saturated
Aromatic rings in the structure and substitution
Functional groups

6. IR Spectroscopy
In IR spectroscopy, there is interaction between molecules and radiations from the IR region of the EMR spectrum (IR region = cm-1)
IR radiation causes the excitation of the vibrations of covalent bonds within that molecule. These vibrations include the stretching and bending modes
In practice, it is the polar covalent bonds that are IR "active" and whose excitation can be observed in an IR spectrum
Generally, it is convenient to split an IR spectrum into two approximate regions:
Functional group region: cm-1
Fingerprint region: < 1000 cm-1 (more complex and much harder to assign)

7. Regions of Frequencies
Spectral Region Frequency(Hz) Wavenumber(cm-1) Wavelength (,m)
Near -to visible- IR (NIR)
Combination bands
3.8 x 1014 to 1.2 x 1014
12800 to 4000
0.78 to 2.5
Mid Infrared
Fundmental bands for organic molecules
1.2 x 1014 to 6.0 x 1012
4000 to 200
2.5 to 50
Far IR
Inorganics organometallics
6.0 x 1012 to 3.0 x 1011
200 to 10
50 to 1000
After Table 16-1 of Skoog and West, et al. (Chapter 16)

8. Basic Theory of IR Absorption
We need to talk about
IR energy modes (Types of vibration in molecules)
Band positions
Band Intensity

9. Infrared Energy Modes
IR energy absorption corresponds to specific modes, corresponding to combinations of atomic movements, such as bending (change in bond angle) and stretching (change in bond length) of bonds between groups of atoms
Energy is characteristic of the atoms in the group and their bonding
Corresponds to vibrations and rotations

10. Many possible absorptions per molecule exist: stretching, bending,…
Vibrational modes leading to IR absorptions:

11. Bond length changes
Symmetrical Stretching
Asymmetrical Stretching
Bond angle changes
Bending:
Scissoring
Rocking
Wagging
Twisting
تارجحية
ذيل الخيل
مقصية
التوائية او لولبية

12. Bands Position: Hookes' Law
A vibrating bond In IR can be compared to the physical model of a vibrating spring system that can be described by Hooke's Law of harmonic oscillation
Using the force constant k (which reflects the stiffness of the spring) and the two masses m1 and m2, then the equation indicates how the frequency, u, of the absorption should change as the properties of the system change

13. Bands Position: Hookes' Law
Hooke’s Law can be used to estimate the wavenumber (n) of light that will be absorbed by chemical bonds
n = 4.12 * (K / m)1/2
K is the force constant (in dynes / cm) and for:
single bond: K = 5 x 105 dynes/cm
double bond: K = 10 x 105 dynes/cm
triple bond: K = 15 x 105 dynes/cm
m = reduced mass
For example in C=C bond:
n = 4.12 * (10 x 105 / [12 * 12 / ( )])1/2 = 1682 cm-1 (calculated) compare to experimental value1650 cm-1

14. Factors affecting Absorption Frequency
Strength of chemical bond
Masses of attached atoms to the bond
Hydrogen bonding
Resonance
Bond angle or ring strain
Hybridization
Polarity of bonds
External factors: eg. state of measurements, conc., temp., solvent used etc.

15. Factors Affecting Absorption Frequency: Bond Strength
For a stronger bond (larger k value), u increases Compare (increasing bond strength) :
CC bonds : C-C (1000 cm-1), C=C (1600 cm-1) and CºC (2200 cm-1)
CH bonds: C-C-H (2900 cm-1), C=C-H (3100 cm-1) and CºC-H (3300 cm-1)
Strength of the chemical bond: .

16. Factors Affecting Absorption Frequency: Masses of Atoms
Masses of the attached atoms to the bond
For heavier atoms (larger m value), u decreases compare (increasing reduced mass):
C-H  (3000 cm-1)
C-C  (1000 cm-1)
C-Cl (800 cm-1)
C-Br (550 cm-1)
C-I   (500 cm-1)

18. Factors Affecting Absorption Frequency: Hydrogen Bonding
H-bonding
For example: free OH is observed at 3600cm-1 while H-bonded –OH is observed at 3400cm-1

20. Factors Affecting Absorption Frequency: Resonance
Resonance: electronic factors
Conjugation lowers the energy to vibrate bond
isolated ketones: 1710 cm-1
a,b-unsaturated ketones: 1690 cm-1
a,b,g,d-unsaturated ketones: 1675 cm-1

22. Factors Affecting Absorption Frequency: Bond Strain
Internal factors: Bond angle or ring strain

23. Factors Affecting Absorption Frequency: Hybridization
Bonds are stronger in the order sp > sp2 > sp3
C-H (sp): 3300 cm-1
C-H (sp2): 3100 cm-1
C-H (sp3): 2900 cm-1

24. Factors Affecting Absorption Frequency: Polarity of Bond
The more polar a chemical bond is, the higher the intensity of the band
Low dipole moments results in a weak bands
Band intinsity is qualitatively described as: very strong (vs), strong (s), medium (m), weak (w) and variable (var).

26. Band intensity
Symmetrical bonds have no dipole moments and thus no IR bands observed in the spectrum (ie. infrared inactive)

27. Band intensity Absorption other than fundamental modes of vibration
overtones: exactly 2x or 3x of a fundamental frequency
combination bands where freq. = (1  2). For example: aromatics between cm-1
coupling: interaction between 2 vibrating groups in close proximity; e.g.:

28. Band intensity
An IR spectrophotometer is made of an IR light source, a sample container, a prism to separate light into different wavelengths, a detector, and a recorder (to produces the infrared spectrum)

29. Spectroscopy and the Electromagnetic Spectrum
Radiant energy is proportional to its frequency (cycles/s = Hz) as a wave (Amplitude is its height)
Different types are classified by frequency or wavelength ranges

30. Infrared Spectroscopy of Organic Molecules
Organic compounds when exposed to electromagnetic radiation, can absorb energy of only certain wavelengths (unit of energy)
Transmits, energy of other wavelengths.
IR region lower energy than visible light (below red – produces heating as with a heat lamp)
2.5  106 m to 2.5  105 m region used by organic chemists for structural analysis
IR energy in a spectrum is usually measured as wave number (cm-1), the inverse of wavelength and proportional to frequency

31. Regions of the Infrared Spectrum
cm-1 double bonds (stretching)
C=O
C=C cm-1
Below 1500 cm-1 “fingerprint” region
cm-1 N-H, C-H, O-H (stretching)
N-H, O-H
3000 C-H
cm-1 CºC and C º N (stretching)

34. IR ABSORPTION RANGE
The typical IR absorption range for covalent bonds is cm-1. The graph shows the regions of the spectrum where the following types of bonds normally absorb. For example a sharp band around cm-1 would indicate the possible presence of a C-N or a C-C triple bond.
Graphics source: Wade, Jr., L.G. Organic Chemistry, 5th ed. Pearson Education Inc., 2003

35. Regions of the Infrared Spectrum

36. Regions of the Infrared Spectrum

37. Differences in Infrared Absorptions
Molecules vibrate and rotate in normal modes, which are combinations of motions (relates to force constants)
Bond stretching dominates higher energy modes
Light objects connected to heavy objects vibrate fastest: C-H, N-H, O-H
For two heavy atoms, stronger bond requires more energy: C º C, C º N > C=C, C=O, C=N > C-C, C-O, C-N, C-halogen

38. 12.8 Infrared Spectra of Hydrocarbons
C-H, C-C, C=C, C º C have characteristic peaks
absence helps rule out C=C or C º C

39. Infrared Spectra of Some Common Functional Groups
n-Alkanes
- look for stretching and bending of C–H and C–C bonds
• C–C bends: ca. 500 cm–1 (out of spectral window)
• C–C stretches: 1200–800 cm–1, weak bands not of value for interpretation (fingerprint)
More characteristic
• C–H stretches: occurs from cm–1
CH3: 2962 cm–1, asymmetrical stretch
2872 cm–1, symmetrical stretch
CH2: 2926 cm–1, asymmetrical stretch
2853 cm–1, symmetrical stretch
• C–H bends:
CH3: ca cm–1
CH2: ca cm–1

40. n-Alkanes n-Hexane CH3(CH2)4CH3 C-H Bends C-H Stretches CH3 (s)
CH3 (as)
CH3 (s)
C-H
Bends
CH3 (as)
C-H
Stretches

41. Finger printing C10H22 C12H26 The IR of C10H22 and C12H26 are Similar
but Not
Identical
C10H22
C12H26

42. Unconjugated Alkenes Linear alkenes: C=C–H stretch: ≥ 3000 cm-1
C=C–H bending in the range cm-1
C=C stretch: moderate to weak at cm-1

43. Unconjugated Alkenes
Example: 1-Hexene

44. Cyclic Alkenes
The C=C stretch is sensitive to ring strain (size)

45. Conjugated alkenes
often conjugation moves C=C stretch to lower frequencies and increases the intensity
The alkene bond stretching vibrations in alkenes without a center of symmetry, e.g. 1-methylbutadiene, gives to two C=C stretches
For symmetrical molecules, e.g. butadiene, only the asymmetric stretch is observed

46. Conjugated alkenes 2-methylbutadiene Symmetrical C=C stretch 1640 cm–1
(weak)
C-H stretch
3090 cm–1
Out of plane
C=C–H bends
990, 892 cm–1
Asymmetrical
C=C stretch
1598 cm–1 (strong)

47. Alkynes C C–H bend: 700-610 cm-1: broad, strong
C C–H stretch: 3333–3267 cm-1, strong and narrow (as compared to OH or NH)
C C stretch: weak absorption at cm-1
not observed for symmetrical alkynes
terminal alkynes (R-C C-H) absorptions are stronger than internal (R-C C-R) absorptions
Disubstituted or symetrically substituted triple bonds give either no absorption or weak absorption

48. Terminal Alkynes Example: 1-octyne Alkyne CC stretch 2119 cm–1 Alkyne
C-H bend overtone
1260 cm–1
Alkyne
C-H bend
630 cm–1
Alkyne
C-H stretch
3310 cm–1

49. Aromatic Rings C=C-H stretch occurs at value 3100-3000 cm-1
C=C-H out of plane bending occurs at 900–690 cm-1
intense bands, strongly coupled to adjacent hydrogens on the ring
position and number of bands gives information about the ring substitution pattern
C=C stretch occurs in pairs: ; cm-1
C=C out of plane ring bending: cm-1

50. Aromatic rings Example: Toluene Overtone bands 2000-1650 cm–1 Aromatic
C-H Stretches
3087, 3062,
3026 cm–1
out of
plane
ring
bending
428 cm–1
Aromatic C-H in plane bends
cm–1
Aromatic
C-H out of
Plane bends
728 cm–1
Aromatic C-C Stretches
; cm–1
694 cm-1

51. Alcohols and phenols
The value is strongly dependent on hydrogen-bonding
Free non-hydrogen bonded
O-H groups absorb strongly in
the cm-1 range
H-bonded O-H band is broad at cm-1
C-O-H bending appears at cm-1 as broad and weak peak often obscured by CH3 bending
C-O (alcohol) stretching at cm-1. Used to assign primary secondary and tertiary alcohols)
C-O (phenol) stretching at cm-1

52. C–O stretching Vibrations of Alcohols
Primary alcohol: cm-1
Secondary alcohol: cm-1
Tertiary alcohol: cm-1
1073 cm-1
1110 cm–1
1202 cm–1

53. Ethers
C–O–C stretching bands are very prominent ( cm-1) due to strong dipole moment. C=O and O-H must be absent to ensure C-O stretch is not due to ester or alcohol
aliphatic ethers:
strong band due to asymmetrical stretching, cm-1 (usually 1125 cm-1)
weak band due to symmetrical stretching (lower freq)
Alkyl aryl ethers:
asymmetrical stretch at cm-1
symmetrical stretch at cm-1
Vinyl alkyl ethers:
asymmetrical stretch at cm-1

54. C=O In Aldehydes
C=O stretch appears at cm1 for normal aliphatic aldehydes
Conjugation of C=O with a,b C=C: cm1 for C=O and 1640 cm1 for C=C
Conjugation of C=O with phenyl: cm1 for C=O and cm1 for the ring
C-H stretch of aldehyde H ( in CHO): show pair of weak bands at cm1 and cm1

55. C=O in Ketones C=O stretch: 1720-1708 for aliphatic ketones
Conjugation of C=O with a,b C=C: cm-1 for C=O and cm1 for C=C
Conjugation of C=O with phenyl: cm-1 for C=O and cm-1 for the ring
In strained rings, interaction with the adjacent C-C bonds increases the frequency of C=O stretching

56. C=O in Ketones
2-Heptanone

57. C=O in Carboxylic Acids
O-H stretch usually very broad (H-bonding) occurs at cm1 and often overlaps the C-H absorptions
C=O stretch, strong broad, occurs at cm1
C-O stretch occurs in the range cm1 in medium intensity

58. C=O in Carboxylic Acids
Hexanoic acid
Acids and alcohols are well distinguished

59. C=O in Esters
C=O stretch appears in range for normal aliphatic esters
Conjugation of C=O with a,b C=C: cm1 for C=O and cm1 for C=C
Conjugation of C=O with phenyl: cm1 for C=O and cm1 for the ring
C-O stretch in two or more bands, one stronger and broader than the other, occurs at cm1

60. C=O in Esters
1763 cm–1
1199, 1164, 1145 cm–1 C–O

61. C=O in Amides C=O stretch appears in range 1680-1630 cm1
N-H stretch in primary amides (NH2) gives two bands near 3350 and 3180 cm1. Secondary amides have one band ca cm1
N-H bending occurs at ca cm1 for primary and secondary amides

62. C=O in Amides
(II)
1662 cm-1 (I)
1565 cm-1 (II)
1655 cm-1 (I)

63. Acid Chlorides
C=O stretch appears in range cm1 in unconjugated chlorides. Conjugation lowers the frequency to cm1
C-Cl stretch occurs in the range cm1

64. Alkyl Halides C-F: Stretch (strong) at 1400-1000 cm-1
C-Cl: Stretch (strong) in aliphatic chlorides at cm-1
C-Br: Stretch (strong) in aliphatic bromides at cm-1. Aryl bromides absorb between 1075 and 1030 cm-1
C-I: Stretch (strong) in aliphatic iodides at cm-1
CH2-X bending at cm-1

65. Anhydrides
C=O stretch always has two bands at cm1 and cm1 with variable relative intensities
C-O stretch (multiple bands) occurs in the range cm1

66. Amines N-H stretch ca. 3500-3300 cm-1
Primary amine have two bands, secondary amines have one band while tertiary amines have no N-H stretch
N-H bend in primary amines results in a broad band ca cm-1. Secondary amines absorbs near 1500 cm-1
C-N stretch occurs near cm-1

67. Amines
Example: Propylamine

68. Others Nitrile: medium intensity, sharp absorption at 2250 cm1
Imine -C=N- at ca cm-1
Aliphatic nitro compounds: strong asymmetrical stretch at (s) cm-1 and medium symmetrical stretch (s) cm-1
Aromatic nitro compounds: strong asymmetrical stretch at (s) cm-1 and strong symmetrical stretch (s) cm-1

69. Remember…..
The absence of an absorption band can often provide more information about the structure of a compound than the presence of a band
Be careful to avoid focusing on selected absorption bands and overlooking others
Look for absorption bands in decreasing order of importance

70. Analysis of IR spectrum: What To Do!
Look if a C=O group is present ( cm-1)
If present look for:
Is O-H also present? ( cm-1): acid
Is N-H also present? (ca cm-1): amide
Is C-O also present? ( cm-1): Ester
Two C=O present? ( cm-1): anhydride
Is aldehyde C-H present? (2840 & 2720 cm-1): aldehyde
If all of the above absent, then think of ketone!
If C=O is absent:
Is O-H ( cm-1) and C-O ( cm-1) present? Alcohol C-O-H
Is N-H (~3400 cm-1) present? amine
Is C-O ( cm-1) present? ether

71. Analysis of IR spectrum: Remember….
Double bonds/aromatic rings:
C=C (~1650 cm-1)
aromatic C=C ( cm-1)
vinyl C-H (>3000 cm-1)
Triple bonds
C,N triple bond (~2250 cm-1)
C,C triple bond (~2150 cm-1)
acetylenic C-H (~3300 cm-1)
Nitro groups
N-O( & cm-1)
Hydrocarbons

72. Remember….. The C-H absorption(s) 3100 - 2850 cm-1
Absorption above 3000 cm-1 indicates C=C, either alkene or aromatic
Confirm the aromatic ring by finding peaks at 1600 and 1500 cm-1 and C-H out-of-plane bending to give substitution patterns below 900 cm-1
Confirm alkenes with an absorption at cm-1
C-H absorption between 3000 and 2850 cm-1 is due to aliphatic hydrogens
If the main absorptions are approximately 2935 and 2860 cm-1 and there are also absorptions at 1470 and 720 cm-1 then the compound probably contains a long linear aliphatic chain

73. Remember…..
Hydroxy or Amino groups appear at 3650–3250 cm-1 while the C-H stretch of a terminal alkyne (acetylene) exhibits a relatively narrow absorption at 3300 cm-1
If the main absorption band in the area is broad, the compound probably contain a hydroxyl or amino group. For -NH2 a doublet will be observed
The C-O absorption between 1080 and 1300 cm-1. These peaks are normally rounded like the O-H and N-H peak and are prominent. Carboxylic acids, esters, ethers, alcohols and anhydrides all containing this peak.
A methyl group may be identified with C-H absorption at 1380 cm-1. This band is split into a doublet for isopropyl(gem-dimethyl) groups

74. Good Luck!