INFRARED ABSORPTION SPECTROSCOPY LECTURE 4

Polystyrene Spectrum

IR SPECTROSCOPY Mostly for qualitative analysis. Absorption spectra is recorded as transmittance. Absorption in the infrared region arise from molecular vibrational transitions Absorption for every substance are at specific wavelengths where IR spectra provides more specific qualitative information. IR spectra is called “fingerprints” because no other chemical species will have similar IR spectrum.

Transmittance vs Absorbance The transmittance spectra provide better contrast between intensities of strong and weak bands compared to absorbance spectra.

Electromagnetic Spectrum Energy of IR photon insufficient to cause electronic excitation but can cause vibrational excitation

Vibrations

IR SPECTROSCOPY Infrared (IR) spectroscopy deals with the interaction of infrared radiation with matter. IR spectrum provides….. Important information about its chemical nature and molecular structure IR applicability for….. Analysis of organic materials Polyatomic inorganic molecules Organometallic compounds

IR region subdivided into 3 sub-regions Near IR region (Nearest to the visible) 780 nm to 2.5 μm (12,800 to 4000 cm-1) Mid IR region 2.5 to 50 μm (4000 – 200 cm-1) Far IR region 50 to 1000 μm (200 – 10cm-1) visible NEAR MID infrared FAR microwave

When IR Absorption occur? IR absorption only occurs when IR radiation interacts with a molecule undergoing a change in dipole moment as it vibrates or rotates. Infrared absorption only occurs when the incoming IR photon has sufficient energy for the transition to the next allowed vibrational state. No absorption can occur if both rules above are not met.

What happen when a molecule absorbs IR radiation? Absorption of IR radiation corresponds to energy changes on the order of 8 to 40 kJ/mole. Radiation in this energy range corresponds to stretching and bending vibrational frequencies of the bonds in most covalent molecules. In the absorption process, those frequencies of IR radiation which match the natural vibrational frequencies of the molecule are absorbed. The energy absorbed will increase the amplitude of the vibrational motions of the bonds in the molecule.

What happen when molecules absorbs IR ? NOT ALL bonds in a molecule are capable of absorbing IR energy. Only those bonds that have change in dipole moment are capable to absorb IR radiation. The larger the dipole change, the stronger the intensity of the band in an IR spectrum.

What is dipole moment? is a measure of the extent to which a separation exists between the centers of positive and negative charge within a molecule. δ- δ+ δ+

What is dipole moment? In heteronuclear diatomic molecule, because of the difference in electronegativities of the two atoms, one atom acquires a small positive charge (δ+), the other a negative charge (δ-). This molecule is then said to have a dipole moment whose magnitude, μ = qd distance of separation of the charge

Molecular Species That Absorb IR Compound absorb in IR region Organic compounds, carbon monoxide Compounds DO NOT absorb in IR region O2, H2, N2, Cl2

Molecular vibration divided into stretching bending wagging scissoring involves change in bond angles back & forth movement stretching bending wagging scissoring symmetrical asymmetrical rocking twisting out of plane vibration in-plane vibration

STRETCHING

STRETCHING

BENDING

Sample Handling Techniques Gases Using evacuated cylindrical cells equipped with suitable windows. Liquid sodium chloride windows. “neat” liquid Solid Pellet (KBr) Mull

2. LIQUIDS a drop of the pure (neat) liquid is squeezed between two rock-salt plates to give a layer that has thickness 0.01mm or less. 2 plates held together by capillary mounted in the beam path.

What is ……..? What is meant by “neat” liquid? Neat liquid is a pure liquid that do not contain any solvent or water. Neat liquid method is applied when the amount of liquid is small or when a suitable solvent is unavailable.

3. SOLIDS There are 2 ways to prepare solid sample for IR spectroscopy. Solid that is soluble in solvent . The most commonly IR solvent is carbon tetrachloride, CCl4. Solid that is insoluble in CCl4 or any other IR solvents can be prepared either by KBr pellet or Mulls.

KBr PELLET The finely ground solid sample is mixed with potassium bromide (KBr). The mixture is pressed under high pressure (10,000 – 15,000 psi) in special die to form a pellet. KBr pellet then can be inserted into a holder in the IR spectrometer.

MULLS 2-5 mg finely powdered sample is ground (grind) together with the presence 1 or 2 drops of a heavy hydrocarbon oil called Nujol to form a Mull. Mull is then examined as a film between flat salt plates. Mulls method is applied when solid not soluble in an IR transparent solvent and solid is not convenient to be pelleted with KBr.

What is …… ? What is Mull What is Nujol A thick paste formed by grinding an insoluble solid with an inert liquid and used for studying spectra of the solid. What is Nujol A trade name for a heavy medicinal liquid paraffin. Extensively used as a mulling agent in spectroscopy.

Instrumentation

IR Instrument Dispersive spectrometers Fourier Transform spectrometers sequential mode Fourier Transform spectrometers simultaneous analysis of the full spectra range using inferometry.

Dispersive IR Instrument Important components in IR dispersive spectrometer 5 1 2 3 4 signal processor & readout source lamp sample holder λ selector detector Detector: - Thermocouple - Pyroelectric transducer - Thermal transducer Source: - Nernst glower - Globar source - Incandescent wire - Nichrome wire

Radiation Sources Generate a beam with sufficient power in the λ region of interest to permit ready detection & measurement. Provide continuous radiation which made up of all λ’s with the region (continuum source). Provide stable output for the period needed to measure both P0 and P.

Schematic Diagram of IR Spectrophotometer

FTIR Fourier Transform Infrared

FTIR Why FTIR is developed? To overcome limitations encountered with the dispersive instruments. Dispersive IR spectrophotometer has slow scanning speed due to measurement of individual molecules/atom. It utilize the use of an interferometer.

FTIR

FTIR

Interferometer Special instrument which can read IR frequencies simultaneously. Faster method than dispersive instrument. Interferograms are transformed into frequency spectrums by using mathematical technique called Fourier Transformation. FT Calculations interferograms IR spectrum

Components of FTIR Majority of commercially available FTIR instruments are based upon Michelson interferometer. 3 4 1 2 5 6

Advantages FTIR High sensitivity. High resolution. Quick data acquisition ( data for an entire spectrum can be obtained in 1 s or less).

Interpretation Infrared Spectra

Infrared Spectra IR spectrum is due to specific structural features, a specific bond, within the molecule, since the vibrational states of individual bonds represent 1 vibrational transition. From IR spectrum we could predict the present of atoms or group of atoms or functional groups such as the present of an O-H bond or a C=O or an aromatic ring.

Infrared Spectra

How to Interpret IR Spectra?

How to analyze IR spectra Begin by looking in the region from 4000-1300. Look at the C–H stretching bands around 3000. Indicates Are any or all to the right of 3000? alkyl groups (present in most organic molecules) Are any or all to the left of 3000? a C=C bond or aromatic group in the molecule

a carboxylic acid group Look for a carbonyl in the region 1760-1690. If there is such a band: Indicates Is an O–H band also present? a carboxylic acid group Is a C–O band also present? an ester Is an aldehyde C–H band also present? an aldehyde Is an N–H band also present? an amide Are none of the above present? a ketone (also check the exact position of the carbonyl band for clues as to the type of carbonyl compound it is)

Look for a broad O–H band in the region 3500-3200 cm-1 Look for a broad O–H band in the region 3500-3200 cm-1. If there is such a band: Indicates Is an O–H band present? an alcohol or phenol Look for a single or double sharp N–H band in the region 3400-3250 cm-1. If there is such a band: Indicates Are there two bands? a primary amine Is there only one band? a secondary amine

How to analyze IR spectra Other structural features to check for Indicates Are there C–O stretches? an ether (or an ester if there is a carbonyl band too) Is there a C=C stretching band? an alkene Are there aromatic stretching bands? an aromatic Is there a C≡C band? an alkyne Are there -NO2 bands? a nitro compound

How to analyze IR spectra If there is an absence of major functional group bands in the region 4000-1300 cm-1 (other than C–H stretches), the compound is probably a strict hydrocarbon. Also check the region from 900-650 cm-1. Aromatics, alkyl halides, carboxylic acids, amines, and amides show moderate or strong absorption bands (bending vibrations) in this region. As a beginning student, you should not try to assign or interpret every peak in the spectrum. Concentrate on learning the major bands and recognizing their presence and absence in any given spectrum.

ALKANE

CH Stretch for sp3 C-H around 3000 – 2840 cm-1. CH2 Methylene groups have a characteristic bending absorption at approximate 1465 cm-1 CH3 Methyl groups have a characteristic bending absorption at approximate 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

ALKENE

ALKENE =C-H Stretch for sp2 C-H occurs at values greater than 3000 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; often conjugation moves C=C stretch to lower frequencies and increases the intensity.

ALKYNE

ALKYNE Stretch for sp C - H occurs near 3300 cm-1. Stretch occurs near 2150 cm-1; conjugation moves stretch to lower frequency.

AROMATIC RINGS Stretch for sp2 C-H occurs at values greater than 3000 cm-1. Ring stretch absorptions occur in pairs at 1600 cm-1 and 1475 cm-1. Bending occurs at 900 - 690cm-1.

AROMATIC RINGS

C-H Bending ( for Aromatic Ring) The out-of-plane (oop) C-H bending is useful in order to assign the positions of substituents on the aromatic ring. Monosubstituted rings this substitution pattern always gives a strong absorption near 690 cm-1. If this band is absent, no monosubstituted ring is present. A second strong band usually appears near 750 cm-1. Ortho-Disubstituted rings one strong band near 750 cm-1. Meta- Disubstituted rings gives one absorption band near 690 cm-1 plus one near 780 cm-1. A third band of medium intensity is often found near 880 cm-1. Para- Disubstituted rings - one strong band appears in the region from 800 to 850 cm-1.

Ortho-Disubstituted rings Bending observed as one strong band near 750 cm-1.

Meta- Disubstituted rings - gives one absorption band near 690 cm-1 plus one near 780 cm-1. A third band of medium intensity is often found near 880 cm-1.

Para- Disubstituted rings - one strong band appears in the region from 800 to 850 cm-1.

ALCOHOL Primary alcohol 10 Secondary alcohol 20 Tertiary alcohol 30

ALCOHOL O-H The hydrogen-bonded O-H band is a broad peak at 3400 – 3300 cm-1. This band is usually the only one present in an alcohol that has not been dissolved in a solvent (neat liquid). C-O-H Bending appears as a broad and weak peak at 1440 – 1220 cm-1 often obscured by the CH3 bendings. C-O Stretching vibration usually occurs in the range 1260 – 1000 cm-1. This band can be used to assign a primary, secondary or tertiary structure to an alcohol.

PHENOL

PHENOL

ETHER C-O The most prominent band is that due to C-O stretch, 1300 – 1000 cm-1. Absence of C=O and O-H 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 – 1040 cm-1, while aliphatic ethers give one strong band at about 1120 cm-1.

CARBONYL COMPOUNDS cm-1 1810 1800 1760 1735 1725 1715 1710 1690 Anhydride Acid Chloride Anhydride Ester Aldehyde Ketone Carboxylic acid Amide (band 1) (band 2) Normal base values for the C=O stretching vibrations for carbonyl groups.

ALDEHYDE C=O stretch appear in range 1740-1725 cm-1 for normal aliphatic aldehydes Conjugation of C=O with phenyl; 1700 – 1660 cm-1 for C=O and 1600 – 1450 cm-1 for ring (C=C) C-H Stretch, aldehyde hydrogen (­­-CHO), consists of weak bands, one at 2860 - 2800 cm-1 and the other at 2760 – 2700 cm-1.

KETONE C=O stretch appear in range 1720-1708 cm-1 for normal aliphatic ketones Conjugation of C=O with phenyl at 1700 – 1680 cm-1 for C=O and 1600 – 1450 cm-1 for ring (C=C)

CARBOXYLIC ACID

ESTER C=O stretch appear in range 1750-1735 cm-1 for normal aliphatic esters Conjugation of C=O with phenyl; 1740 – 1715 cm-1 for C=O and 1600 – 1450 cm-1 for ring (C=C) C – O Stretch in two or more bands, one stronger and one broader than the other, occurs in the range 1300 – 1000 cm-1

AMIDE 10 20 30

AMIDE

Acid chloride show a very strong band for the C=O group. Stretch appear in range 1810 -1775 cm-1 in conjugated chlorides. Conjugation lowers the frequency to 1780 – 1760 cm-1 Stretch occurs in the range 730 -550 cm-1 Acid chloride show a very strong band for the C=O group.

ANHYDRIDE 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 anhydride) moves absorptions to a higher frequency. Stretch (multiple bands) occurs in the range 1300 -900 cm-1

AMINE Secondary amine , 20 Tertiary amine, 30 Primary amine, 10

AMINE Stretching 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 N – H stretch. N – H Bending 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 N – H C – N Stretch occurs in the range 1350 – 1000 cm-1

Primary Amine Secondary Amine

Tertiary Amine Aromatic Amine