Infrared Spectroscopy Theory and Interpretation of IR spectra

Prepared ByDr. Khalid Ahmad Shadid

Islamic University in MadinahDepartment of Chemistry

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Spectroscopy and the Electromagnetic Spectrum

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• 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

Basic Theory of IR Absorption

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• Changes in interatomic vibrations of a molecule are brought about through the absorption of IR light

E

r , interatomic distance

E = h

Zero point energy

bond dissociationenergy

h2

1E

0

Basic Theory of IR Absorption

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• 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

The IR Spectrum

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• In IR spectroscopy, there is interaction between molecules and radiations from the IR region of the EMR spectrum (IR region = 4000 - 400 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: 4000-1000 cm-1– Fingerprint region: < 1000 cm-1 (more complex and much

harder to assign)

IR Spectroscopy

Regions of Frequencies

After Table 16-1 of Skoog and West, et al. (Chapter 16)

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 InfraredFundmental bands for organic molecules

1.2 x 1014 to 6.0

x 1012

4000 to 200 2.5 to 50

Far IRInorganics organometallics

6.0 x 1012 to 3.0

x 1011

200 to 10 50 to 1000

Spectral Region Frequency(Hz) Wavenumber(cm-1) Wavelength (,m)

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We need to talk about IR energy modes (Types of vibration in molecules) Band positions Band Intensity

Basic Theory of IR Absorption

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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

Vibrational modes leading to IR absorptions:

Many possible absorptions per molecule exist: stretching, bending,…

Asymmetrical StretchingSymmetrical Stretching

Scissoring Rocking Wagging Twisting

Bending:

لولبية مقصية او الخيل تارجحية التوائية ذيل

Bond length changes

Bond angle changes

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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

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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/cmdouble 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 / (12 + 12)])1/2 = 1682 cm-1 (calculated) compare to experimental value1650 cm-1

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• 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.

Factors affecting Absorption Frequency

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• 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:

• .

Factors Affecting Absorption Frequency: Bond Strength

k ,stronger bond

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21

mm

mm

k

ccm

2

1)( 1

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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)

C H C O C S

R F R Cl R Br

~ 3000cm -1 1100cm -1 650cm -1

heavier atoms

,

21

21

mm

mm

k

ccm

2

1)( 1

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• H-bonding• For example: free OH is observed at 3600cm-1 while H-

bonded –OH is observed at 3400cm-1

Factors Affecting Absorption Frequency: Hydrogen Bonding

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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

Factors Affecting Absorption Frequency: Resonance

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• Internal factors: Bond angle or ring strain

Factors Affecting Absorption Frequency: Bond Strain

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• 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

Factors Affecting Absorption Frequency: Hybridization

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• The more polar a chemical bond is, the higher the intensity of the band

• Low dipole moments results in a weak bands

• 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).

Factors Affecting Absorption Frequency: Polarity of Bond

C CC

HCC

CC

C C ONC O

HO

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Band intensity

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

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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 1600 - 2000 cm-1– coupling: interaction between 2 vibrating groups in close

proximity; e.g.:

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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)

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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

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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)

• 2.5 106 m to 2.5 105 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

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Regions of the Infrared Spectrum• 4000-2500 cm-1 N-H, C-H, O-H

(stretching)– 3300-3600 N-H, O-H– 3000 C-H

• 2500-2000 cm-1 CºC and C º N (stretching)

• 2000-1500 cm-1 double bonds (stretching)– C=O 1680-1750– C=C 1640-1680 cm-1

• Below 1500 cm-1 “fingerprint” region

IR ABSORPTION RANGEThe typical IR absorption range for covalent bonds is 600 - 4000 cm-1. The graph shows the regions of the spectrum where the following types of bonds normally absorb. For example a sharp band around 2200-2400 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

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Regions of the Infrared Spectrum

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Regions of the Infrared Spectrum

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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

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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

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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 3000 - 2840 cm–1CH3: 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. 1375 cm–1CH2: ca. 1465 cm–1

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n-Alkanes

• n-Hexane CH3(CH2)4CH3

C-HStretches

CH3 (as) CH3 (s) C-H

Bends

CH3 (as)

CH3 (s)

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Finger printing

The IR of C10H22 and C12H26

are Similarbut NotIdentical

C10H22

C12H26

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Unconjugated Alkenes

• Linear alkenes:– C=C–H stretch: ≥ 3000 cm-1

– C=C–H bending in the range 1000-650 cm-1 – C=C stretch: moderate to weak at 1680-1600 cm-1

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Unconjugated Alkenes

• Example: 1-Hexene

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• The C=C stretch is sensitive to ring strain (size)

Cyclic Alkenes

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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

1650 cm–1 (as)1600 cm–1 (s)

1600 cm–1 (as)

Me

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• 2-methylbutadiene

C-H stretch3090 cm–1

SymmetricalC=C stretch1640 cm–1

(weak)AsymmetricalC=C stretch1598 cm–1 (strong)

Out of plane C=C–H bends

990, 892 cm–1

Conjugated alkenes

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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 2260-2100 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

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Terminal Alkynes

• Example: 1-octyne

AlkyneC-H stretch3310 cm–1

AlkyneCC stretch2119 cm–1

AlkyneC-H bend630 cm–1

AlkyneC-H bend overtone1260 cm–1

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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: 1600-1585; 1500-1400 cm-1

• C=C out of plane ring bending: 600-420 cm-1

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Aromatic rings

• Example: Toluene

AromaticC-H out ofPlane bends728 cm–1

694 cm-1

AromaticC-H Stretches3087, 3062,3026 cm–1

Overtone bands2000-1650 cm–1

Aromatic C-C Stretches1600-1585; 1500-1400 cm–1

Aromatic C-H in plane bends1300-1000 cm–1

out ofplane ring bending428 cm–1

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Alcohols and phenols

• The value is strongly dependent on hydrogen-bonding– Free non-hydrogen bonded

O-H groups absorb strongly in the 3700-3600 cm-1 range

– H-bonded O-H band is broad at 3400-3300 cm-1

• C-O-H bending appears at 1440-1220 cm-1 as broad and weak peak often obscured by CH3 bending

• C-O (alcohol) stretching at 1260-1000 cm-1. Used to assign primary secondary and tertiary alcohols)

• C-O (phenol) stretching at 1800-1260 cm-1

OHnot H-bondedeven when 'neat'too hindered

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C–O stretching Vibrations of Alcohols

• Primary alcohol: 1050-1085 cm-1

• Secondary alcohol: 1085-1125 cm-1

• Tertiary alcohol: 1125-1200 cm-1

1073 cm-1

1110 cm–1

1202 cm–1

OH

OH

OH

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Ethers• C–O–C stretching bands are very prominent (1300-1000 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, 1150-1085 cm-1

(usually 1125 cm-1)– weak band due to symmetrical stretching (lower freq)

• Alkyl aryl ethers: – asymmetrical stretch at 1275-1200 cm-1

– symmetrical stretch at 1075-1020 cm-1

• Vinyl alkyl ethers:– asymmetrical stretch at 1225-1200 cm-1

– symmetrical stretch at 1075-1020 cm-1

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C=O In Aldehydes• C=O stretch appears at 1740-1725 cm1 for normal aliphatic

aldehydes• Conjugation of C=O with a,b C=C: 1700-1680 cm1 for C=O

and 1640 cm1 for C=C• Conjugation of C=O with phenyl: 1700-1660 cm1 for C=O

and 1600-1450 cm1 for the ring• C-H stretch of aldehyde H ( in CHO): show pair of weak

bands at 2860-2800 cm1 and 2760-2700 cm1

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C=O in Ketones• C=O stretch: 1720-1708 for aliphatic ketones• Conjugation of C=O with a,b C=C: 1700-1675 cm-1 for C=O and

1644-1617 cm1 for C=C• Conjugation of C=O with phenyl: 1700-1680 cm-1 for C=O and

1600-1450 cm-1 for the ring• In strained rings, interaction with the adjacent C-C bonds

increases the frequency of C=O stretching

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C=O in Ketones• 2-Heptanone

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C=O in Carboxylic Acids

• O-H stretch usually very broad (H-bonding) occurs at 3400-2400 cm1 and often overlaps the C-H absorptions

• C=O stretch, strong broad, occurs at 1730-1700 cm1

• C-O stretch occurs in the range 1320-1210 cm1 in medium intensity

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Acids and alcohols are well distinguished

C=O in Carboxylic Acids

• Hexanoic acid

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C=O in Esters

C=O stretch appears in range 1750-1735 for normal aliphatic esters

Conjugation of C=O with a,b C=C: 1740-1715 cm1 for C=O and 1640-1625 cm1 for C=C

Conjugation of C=O with phenyl: 1740-1715 cm1 for C=O and 1600-1450 cm1 for the ring

C-O stretch in two or more bands, one stronger and broader than the other, occurs at 1300-1000 cm1

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C=O in EstersO

O

1763 cm–1

1199, 1164, 1145 cm–1 C–O

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C=O in Amides

• C=O stretch appears in range 1680-1630 cm1 • N-H stretch in primary amides (NH2) gives two bands near

3350 and 3180 cm1. Secondary amides have one band ca. 3300 cm1

• N-H bending occurs at ca. 1640-1550 cm1 for primary and secondary amides

NH

O

NH

O

NH

O

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C=O in Amides

Et

O

NH

H

H3C

O

NCH3

H

1662 cm-1 (I)

1655 cm-1 (I)1565 cm-1 (II)

(II)

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Acid Chlorides

• C=O stretch appears in range 1810-1775 cm1 in unconjugated chlorides. Conjugation lowers the frequency to 1780-1760 cm1

• C-Cl stretch occurs in the range 730-550 cm1

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Alkyl Halides

• C-F: Stretch (strong) at 1400-1000 cm-1

• C-Cl: Stretch (strong) in aliphatic chlorides at 785-540 cm-1

• C-Br: Stretch (strong) in aliphatic bromides at 650-510 cm-1. Aryl bromides absorb between 1075 and 1030 cm-1

• C-I: Stretch (strong) in aliphatic iodides at 600-485 cm-1

• CH2-X bending at 1300 -1150 cm-1

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Anhydrides

• C=O stretch always has two bands at 1830-1800 cm1 and 1775-1740 cm1 with variable relative intensities

• C-O stretch (multiple bands) occurs in the range 1300-900 cm1

O alkyl

OO

alkyl

1818; 1750 cm–1

O

OO

1775; 1720 cm–1R R

OO O

1865; 1782 cm–1

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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. 1640-1560 cm-1. Secondary amines absorbs near 1500 cm-1

• C-N stretch occurs near 1350-1000 cm-1

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Amines

• Example: Propylamine

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Others

• Nitrile: medium intensity, sharp absorption at 2250 cm1

• Imine -C=N- at ca. 1690 -1640 cm-1

• Aliphatic nitro compounds: strong asymmetrical stretch at 1600-1530(s) cm-1 and medium symmetrical stretch 1390 -1300(s) cm-1

• Aromatic nitro compounds: strong asymmetrical stretch at 1550 -1490(s) cm-1 and strong symmetrical stretch 1355 -1315(s) cm-1

C N

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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

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Analysis of IR spectrum: What To Do!

• Look if a C=O group is present (1820-1660 cm-1)• If present look for:

– Is O-H also present? (3400-2400 cm-1): acid – Is N-H also present? (ca. 3400 cm-1): amide– Is C-O also present? (1300-1000 cm-1): Ester– Two C=O present? (1810-1760 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 (3400-3300 cm-1) and C-O (1300-1000 cm-1) present?

Alcohol C-O-H– Is N-H (~3400 cm-1) present? amine– Is C-O (1300-1000 cm-1) present? ether

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Analysis of IR spectrum: Remember….

• Double bonds/aromatic rings:– C=C (~1650 cm-1)– aromatic C=C (1600-1450 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(1600-1530 & 1390-1300 cm-1)

• Hydrocarbons

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• 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 1640-1680 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

Remember…..

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• 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

Remember…..

Good Luck!