INFRAREDSPECTROSCOPY

INFRAREDSPECTROSCOPY

the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light.

INFRARED OFELECTROMAGNETIC SPECTRUM

Near-infrared

Mid-infrared

Far-infrared

http://www.nasa.gov/images/content/56638main_Infrared_Region.jpg

NEAR-INFRAREDapproximately 14000-4000 cm−1 (0.8–2.5 μm wavelength) Can excite overtone or harmonic vibrations.Has high energy

MID-INFRARED approximately 4000-400 cm−1 (2.5–25 μm) used to study the fundamental vibrations and associated rotational-vibrational structure.

approximately 400–10 cm−1  (25–1000 μm)

lying adjacent to the microwave regionhas low energy and may be used for rotational spectroscopy. 

FAR-INFRARED

2.5x10-6 to 2.5x10-5 mVibrational infared region – the portion of the infrared region that extends from 4000 to 400 cm-1

Wavenumber (v) – radiation in the vibrational infrared region. The number of waves per centimeter.

THE VIBRATIONAL INFRARED SPECTRUM

Atoms joined by covalent bonds are not permanently fixed in one position, but instead undergo continual vibrations relative to each otherThe bond undergoing vibration must be polar. It’s vibration must cause a periodic change in the bond dipole.The greater the polarity of the bond, the more intense is the absorption.

MOLECULAR VIBRATIONS

3n – 6 (fundamental vibrations)

Example:Ethanol CH3CH2OHHexanoic acid CH3(CH2)4COOH

STRETCHING MOTIONS

Change in band length

Symmetrical stretching

Assymmetric stretching

• Change in band angle.BENDING MOTIONS

Scissoring

Rocking

Twisting

Wagging

Data on absorption patterns of selected functional groups are collected.

CORRELATION TABLES

Characteristic IR Absorptions of Selected Functional GroupsBond Frequency (cm-1) Intensity O-H 3200-3500 Strong and broad

N-H 3100-3500 Medium

C-H 2850-3100 Medium to strong

C≡C 2100-2260 Weak

C=O 1630-1800 Strong 

C=C 1600-1680 Weak 

C-O 1050-1250 Strong 

Example:Determine the functional group that is likely present if a compound shown IR absorption at

1705 cm-1

2950 cm-1

FACTS absorption of IR radiation only occurs if there is a change in dipole moment the larger the change in dipole moment, the more intense the absorption band the greater the electronegativity differences between atoms in the molecule, the more polarized, the larger change in dipole moment

INTERPRETINGINFRAREDSPECTRA

INFRARED SPECTRUMTr

ansm

itta

nce

Wavenumber (cm-1)

ALKANES

decane

ALKENES

cyclopentene

ALKYNES

1-octyne

ARENES

toluene

ALCOHOLS

2-pentanol

ETHERS

Diethyl ether

KETONES

menthone

AMINES

butanamine

AMIDES

N,N-dimethyldodecanamide

AMIDES

N-methylbenzamide

AMIDES

butanamide

CARBOXYLIC ACIDS

Butanoic acid

ESTERS

Ethyl butanoate

INDEX OF HYDROGEN DEFICIENCY

The sum of the number of rings and pi bonds in a molecule.Compare the number of hydrogens in the molecular formula of a compound of unknown structure with the number of hydrogen in a reference compound with the same number of carbon atoms and with no rings or pi bonds.

CnH2n+2

Index of Hydrogen = (Hreference – Hmolecule)

2

Example

Index of hydrogen deficiency = 4

UV-VISSPECTROSCOPY

Ultraviolet-visible Spectroscopy

Measures the attenuation of a beam of light after it passes through a sample or after reflection from a sample surface. Spectral range of 190 nm to 900 nm

190 – 400 nm, UV 400 – 750 nm, Vis

UV-VIS SPECTROSCOPY

ELECTRONIC TRANSITION

Vis = 36 – 72 kcal/mol UV = >72 – 143kcal/mol

CHROMOPHORE• identifiable part of the molecule• responsible for any spectral feature like bands• use to deduce presence of structural fragment

or element in the molecule• corresponds to a functional group, an atom or

group of atoms• Molar absorptivity > 200 for UV-Vis

UV-VIS SPECTRUM

• UV - electronic transition in 200 – 380 nm• Vis - electronic transition in 380 – 800 nm

EFFECTS OF CONJUGATION

EFFECTS OF CONJUGATION

SCHEMATIC OF A WAVELENGTH-SELECTABLE, SINGLE-BEAM UV-

VIS SPECTROPHOTOMETER

UV-VIS SPECTROPHOTOMETER

References• http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/UV-Vis/spectrum.htm

Thank You!