Infrared (IR) SpectroscopyInfrared (IR) Spectroscopy Lies between microwaves and visible light.Lies between microwaves and visible light. Most used units: Most used units: m and wave numberm and wave number Most useful range: 2.5Most useful range: 2.5m - 16m - 16mm IR region is lower in photon energy than visible light (below red – IR region is lower in photon energy than visible light (below red –

produces heating as with a heat lamp)produces heating as with a heat lamp) 2.5 2.5 10 1066 m to 2.5 m to 2.5 10 1055 m region used by organic chemists for m region used by organic chemists for

identifying functional groups. identifying functional groups. IR energy in a spectrum is usually measured as wavenumber (cmIR energy in a spectrum is usually measured as wavenumber (cm-1-1), ),

the inverse of wavelength and proportional to frequency:the inverse of wavelength and proportional to frequency: Wavenumber (cmWavenumber (cm-1-1) = 1/) = 1/(cm)(cm) Specific IR absorbed by organic molecule related to its structureSpecific IR absorbed by organic molecule related to its structure

IR region and vicinity

Infrared Energy ModesInfrared Energy Modes IR energy absorption corresponds to IR energy absorption corresponds to

specific modes, corresponding to specific modes, corresponding to combinations of atomic movements, combinations of atomic movements, such as bending and stretching of such as bending and stretching of bonds between groups of atoms called bonds between groups of atoms called “normal modes”“normal modes”

Energy is characteristic of the atoms in Energy is characteristic of the atoms in the group and their bondingthe group and their bonding

Corresponds to Corresponds to vibrations and and rotationsrotations

Hukum Hooke :

3700 - 2500 cm-1: X-H stretching (X = C,N,O,S) 2300 - 2000 cm-1: C X stretching (X = C or N) 1900 - 1500 cm-1: C=X stretching (X = C, N, O) 1300 - 800 cm-1: C-X stretching (X = C, N, O)

Molecular Vibrations1. Absorption of infrared radiation corresponds to energy

changes on the order of 8-40 KJ/mole (2-10 Kcal/mole2. The frequencies in this energy range correspond to the

stretching and bending frequencies of the covalent bonds with dipole moments.

3. Stretching (requires more energy than bending)a. Symmetricalb. Asymmetrical

4. Bendinga. Scissoring (in-plane bending)b. Rocking (in-plane bending)c. Wagging (out-of-plane bending)d. Twisting (out of plane bending)

Infrared Energy Modes

Stretching – A rhythmical movement along the bond axis such that the interatomic distance is increasing or decreasing.

C—H HC H

HC H

SymmetricStretch

(2853 cm-1)

AsymmetricStretch

(2926 cm-1)

For the Methylene Group (CH2):

In any group of three or more atoms – at least two of which are identical - there are two modes of stretching or bending: Symmetric and Asymmetric

Bending – A change in bond angle between bonds with a common atom orA movement of a group of atoms with respect to the remainder of the molecule

H H C

H H C

HC H

HC H

Scissoring~1450 cm-1

(In Plane)

Rocking~750 cm-1

(In Plane)

Wagging~1250 cm-1

(Out of Plane)

Twisting~1250 cm-1

(Out of Plane)

Thus, no two molecules of different structure will have exactly the same natural frequency of vibration, each will have a unique infrared absorption pattern or spectrum.

Two Uses:1. IR can be used to distinguish one compound from

another.2. Absorption of IR energy by organic compounds will

occur in a manner characteristic of the types of bonds and atoms in the functional groups present in the compound; thus, infrared spectrum gives structural information about a molecule.

The absorptions of each type of bond (N–H, C–H, OH, C–X, C=O,C–O, C–C, C=C, CC, CN, etc.) are regularly found only in certain small portions of the vibrational infrared region, greatly enhancing analysis possibilities.

Common Structural Units

O H

O H

N H

C Hsp

sp2

sp3

C H

C H

sp2

sp3

C O

C O

1200

1025-1200

2850-2950

3000-3100

3350-3500

3310-3320

2500-3600

3200-3600(alcohols)

(carboxylicacids)

Stretching Vibrations

C C

C O

Aldehydes and ketones

Carboxylic acids

Acid anhydrides

Acyl halides

Esters

Amides

1620-1680

1710-1750

1800-1850 and 1740-1790

1700-1725

1770-1815

1730-1750

1680-1700

Stretching Vibrations (Double Bonds)

Bending Vibrations (Diagnostic)Alkenes:

cis-disubstituted

trans-disubstituted

trisubstituted

665-730

960-980

790-840

Stretching Vibrations Triple Bonds)

C C

C N

Substituted derivatives of benzene

monosubstituted

ortho-disusbstitutedmeta-disubstitutedpara-substituted

730-770 and 690-710

735-770750-810 and 680-730790-840

2100-2200

2240-2280

Serapan gugus fungsional pada Infra merah

Regions of the Infrared SpectrumRegions of the Infrared Spectrum►4000-2500 cm4000-2500 cm-1-1 N-H, C-H, O-H (stretching) N-H, C-H, O-H (stretching)

3300-3600 N-H, O-H3300-3600 N-H, O-H 3000 C-H3000 C-H

►2500-2000 cm2500-2000 cm-1-1 C CC and C C and C N (stretching) N (stretching)►2000-1500 cm2000-1500 cm-1-1 double bonds (stretching) double bonds (stretching)

C=O 1680-1750C=O 1680-1750 C=C 1640-1680 C=C 1640-1680 cmcm-1-1

►Below 1500 Below 1500 cmcm-1-1 “fingerprint” region “fingerprint” region

The Infrared SpectrumA plot of absorption intensity (% Transmittance) on the y-

axis vs. frequency on the x-axis. Transmittance (T) - the ratio of the radiant power

transmitted by a sample to the radiant power incident on the sample.

Absorbance (A) - the logarithm to base 10 of the reciprocal of the Transmittance.

A = log10 (1 / T)

Frequency - The x-axis is represented by two scales:

1. Wavelength (2.5 to 25 ) (Bottom)

2. Wavenumber (4000 cm-1 to 400 cm-1) (Upper)

Methyl Isopropyl Ketone

Spectra Infrared

Regions of the Infrared Spectrum

IR Spectrum Peak Characteristics1.Primary Examination Regions of the Spectrum

High Frequency Region - 4000 to 1300 cm-1

Intermediate (Fingerprint Region) - 1300 to 900 cm-1

2.High Frequency Region (Functional Group Region) Characteristic Stretching frequencies of such groups

as: =CH, OH, NH, C=O, CO, CN, CC, C=C

3.The Fingerprint Region Absorption patterns frequently complex Bands originate from interacting vibrational modes Valuable when used in reference to other regions Absorption unique for every molecular species Effective use comes from experience

Spectra Infrared

IR Spectrum Peak Characteristics (con’t)Shape

Sharp (narrow) Broad

Intensity Weak (w) Medium (m) Strong (s)

Note: Peak intensity is dependent on amount of sample and sensitivity of instrument; therefore, the

actual intensity can vary from spectrum to spectrum

Infrared Spectroscopy (IR)

Infrared Spectra of Hydrocarbons

• C-H, C-C, C=C, C C have characteristic peaks

Hexane

Alkenes

1-Hexene

Alkynes

IR: Aromatic Compounds

• Weak C–H stretch at 3030 cm1

• Weak absorptions 1660 - 2000 cm1 range• Medium-intensity absorptions 1450 to 1600 cm1

Phenylacetylene

Cyclohexane or Cyclohexene?

The infra-red spectrum for a simple carboxylic acid

Ethanoic acid

carbon-oxygen double, C=Ocarbon-oxygen single, C-O

oxygen-hydrogen, O-Hcarbon-hydrogen, C-H

carbon-carbon single, C-C

IR: Alcohols

Infrared Absorption Spectrum of Ethanol

dilute t-butanol in choloroform t-butanol

IR: Carbonyl CompoundsIR: Carbonyl Compounds Strong, sharp C=O peak 1670 to 1780 cmStrong, sharp C=O peak 1670 to 1780 cm11

Exact absorption characteristic of type of Exact absorption characteristic of type of carbonyl compoundcarbonyl compound 1730 cm1730 cm11 in saturated aldehydes in saturated aldehydes 1705 cm1705 cm11 in aldehydes next to double bond or in aldehydes next to double bond or

aromatic ringaromatic ring

C=O in KetonesC=O in Ketones 1715 cm1715 cm11 in six-membered ring and acyclic ketones in six-membered ring and acyclic ketones 1750 cm1750 cm11 in 5-membered ring ketones in 5-membered ring ketones 1690 cm1690 cm11 in ketones next to a double bond or an aromatic ring in ketones next to a double bond or an aromatic ring

C=O in EstersC=O in Esters 1735 cm1735 cm11 in saturated esters in saturated esters 1715 cm1715 cm11 in esters next to aromatic in esters next to aromatic

ring or a double bondring or a double bond

The infra-red spectrum for an esterEthyl ethanoate

Practice problem:

Phenylacetaldehyde

The infra-red spectrum for a ketonePropanone

AminesAmines

The infra-red spectrum for a primary amine1-aminobutane

Analyzing the Spectrum – A Suggested Approach

Step 1. – Check for the presence of the Carbonyl group (C=O) at 1715 cm-1. If molecule is conjugated, the strong (C=O) absorption will be shifted to the right by ~30 cm-1, i.e. ~1685 cm-1

If the carbonyl absorption is present, check for: Carboxylic Acids - Check for OH group (broad

absorptionnear 3300-2500 cm-1)

Amides - Check for NH group (1 or 2 absorptions near 3500 cm-1)

Esters - Check for C-O group (medium absorptions near 1300-1000 cm-1)

Anhydrides - Check for 2 C=O absorptions near 1810 and 1760 cm-1

Aldehydes - Check for Aldehyde CH group (2 weak absorptions near 2850 and 2750 cm-1)

Ketones - Ketones (The above groups have been eliminated)

Isobutyric AcidIR SpectrumCarboxylic Acids

C4H8O2

BenzamideIR SpectrumAmides

C7H7NO

Methyl BenzoateIR SpectrumEsters

C8H8O2

Nonanal

C9H18O

IR SpectrumAldehydes

Methyl Isopropyl KetoneIR SpectrumKetones

C5H10O

Step 2. - If the Carbonyl Group is Absent Check for Alcohols, Amines, or Ethers.

Alcohols & Phenols - Check for OH group (Broad absorption near 3600-3300 cm-1

Confirm present of CO near1300-1000 cm-1

Amines - Check for NH stretch (Medium absorptions) near 3500 cm-1

Primary Amine - 2 Peaks

Secondary Amine - 1 Peak

Tertiary Amine - No peaks

N-H scissoring at 1560 - 1640 cm-1

Ethers - Check for C-O group near 1300- 1000 cm-1 and absence of OH

IR SpectrumAlkynes (CC)

Propargyl Alcohol

HC CCH2OHC3H4O

2-NaphtholIR SpectrumAlcohols & Phenols

C10H9O

n-butylamineIR SpectrumAmines

C4H11N

IR SpectrumEthers

Butyl Ether

C8H18O CH3(CH2)3 – O – (CH2)3CH3

Step 3. – Refine the Structure Possibilities by Looking for Double Bonds, Triple Bonds and Nitro Groups

Double Bonds - C=C is weak absorption near 1650 cm-1

Aliphatics occur on right side of 3000 cm-1.Unsaturated C=C or CC bonds

show an absorption on the left side of 3000 cm-1.

Triple Bonds - CN (medium, sharp absorption near 2250 cm-1

CC (weak, sharp absorption near 2150 cm-1

Nitro Groups - Two strong absorptions 1600-1500 cm-1

and 1390-1300 cm-1

Step 3 (Con’t)Aromatic Ring Absorptions

Aromatic unsaturated C=C bonds show an absorption on the left side of 3000 cm-1, but the aromaticity must be verified in the overtone region (1667 - 2000cm-1) and the out-of-plane (OOP) region (900 - 690 cm-1)

Medium to strong absorption in region 1650 - 1450 cm-1

Many weak combination and overtone absorptions appear between 2000 and 1667 cm-1

The relative shapes and numbers (1 - 4) of the overtone absorptions can be used to tell whether the aromatic ring is monosubstituted or di-, tri-, tetra-, penta-, or hexasubstituted.Positional (ortho-, meta-, para-) isomers can also be distinguished.Note: A strong carbonyl absorption can overlap these ovetone bands, making them unusable.

Step 3 (Con’t)Aromatic Ring Absorptions (Con’t)

The unsaturated =C-H “Out-of-Plane (OOP) bending absorptions in the region 900 – 690 can also be used to determine the type of ring substitution.

The number of absorptions and their relative positions are unique to each type of substitution.

Although these absorptions are in the “fingerprint” region they are particularly reliable for rings with alkyl group substitutions.

They are less reliable for polar substituents.

BenzonitrileIR SpectrumNitriles

C7H5N

IR SpectrumAlkynes (CC)

Propargyl Alcohol

HC CCH2OHC3H4O

Nitro BenzeneIR SpectrumNitro Compounds

C6H5NO2

Step 4. If none of the above apply then the compound is most likely a hydrocarbon.

Generally, very simple spectrum

Hydrocarbons - Check for main absorptions nearright side of 3000 cm-1

Decane

CH3(CH2)8CH3

IR SpectrumAlkane

C10H22

Sample PreparationSample Preparation1.1. IR spectra of liquids. IR spectra of liquids.

IR spectra of liquid compounds may be obtained either IR spectra of liquid compounds may be obtained either from the neat liquid or from a solution of the liquid in an from the neat liquid or from a solution of the liquid in an appropriate solvent. It is generally desirable to obtain the appropriate solvent. It is generally desirable to obtain the spectrum from the neat liquid, if possible, since spectrum from the neat liquid, if possible, since interference by solvent absorption is thereby avoided. interference by solvent absorption is thereby avoided.

2. IR spectra of solids. 2. IR spectra of solids. - The Mull Method. In this method, the solid sample is - The Mull Method. In this method, the solid sample is thoroughly ground up, using an agate mortar and pestle, thoroughly ground up, using an agate mortar and pestle, with a weakly absorbing, non-volatile liquid to form a thick with a weakly absorbing, non-volatile liquid to form a thick paste, called a mull. paste, called a mull.

- - The KBr pellet Method. In this method, the solid sample The KBr pellet Method. In this method, the solid sample is finely pulverized with pure, dry KBr, the mixture is is finely pulverized with pure, dry KBr, the mixture is pressed in a hydraulic press to form a transparent pellet, pressed in a hydraulic press to form a transparent pellet, and the spectrum of the pellet is measured.and the spectrum of the pellet is measured.