Infrared Spectroscopy

What is Infrared Spectroscopy?

Infrared spectroscopy (IR spectroscopy) is 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. It covers a range of techniques, mostly based on absorption spectroscopy. As with all spectroscopic techniques, it can be used to identify and study chemicals.

The infrared portion of the electromagnetic spectrum is usually divided into three regions; the near-, mid- and far- infrared, named for their relation to the visible spectrum. The higher-energy near-IR, approximately 14000–4000 cm−1 (0.8–2.5 μm wavelength) can excite overtone or harmonic vibrations. The mid-infrared, approximately 4000–400 cm−1 (2.5–25 μm) may be used to study the fundamental vibrations and associated rotational-vibrational structure. The far-infrared, approximately 400–10 cm−1 (25–1000 μm), lying adjacent to the microwave region, has low energy and may be used for rotational spectroscopy.

What are the purpose of IR spectroscopy?

Identification of all types of organic and many types of inorganic compounds Determination of functional groups in organic materials Determination of the molecular composition of surfaces Identification of chromatographic effluents Quantitative determination of compounds in mixtures Nondestructive method Determination of molecular conformation (structural isomers) and

stereochemistry (geometrical isomers) Determination of molecular orientation (polymers and solutions).

How does it work?

Infrared spectroscopy exploits the fact that molecules absorb specific frequencies that are characteristic of their structure. These absorptions are resonant frequencies, i.e. the frequency of the absorbed radiation matches the frequency of the bond or group that vibrates. The energies are determined by the shape of the molecular potential energy surfaces, the masses of the atoms, and the associated vibronic coupling.

Infrared Spectroscopy of Organic Molecules:

1. IR region is lower in photon energy than visible light (below red – produces heating as with a heat lamp)

2. 2.5 ´ 10-6 m to 2.5 ´ 10-5 m region used by organic chemists for structural analysis

3. IR energy in a spectrum is usually measured as wave number (cm -1), the inverse of wavelength and proportional to frequency:

4. Wave number (cm-1) = 1/l(cm)

5. Specific IR absorbed by organic molecule is related to its structure

Infrared Energy Modes:

1. IR energy absorption corresponds to specific modes, corresponding to combinations of atomic movements, such as bending and stretching of bonds between groups of atoms called “normal modes”

2. Energy is characteristic of the atoms in the group and their bonding

3. Corresponds to molecular vibrations

Characteristic IR absorptions of some functional groups:

Single Bonds to Hydrogen:

Bond Wavenumber (cm-1) Notes

C-H 3000-2850Saturated alkanes, limited value as most organic compounds contain C-H

=C-H 3100 - 3000 Unsaturated alkene or aromatic

ºC-H 3300 Terminal Alkyne

O=C-H 2800 and 2700 Aldehyde, two weak peaks

O-HO-H (free)

3400 - 3000~3600

Alcohols and Phenols. If hydrogen bondingpresent peak will be broad 3000-2500 (e.g.carboxylic acids)

N-H 3450 - 3100Amines: Primary - several peaks, Secondary -one peak, tertiary - no peaks

Double Bonds:

Bond Wavenumber (cm-1) Notes

C=O1840 - 1800 &1780 - 1740

Anhydrides

C=O 1815 – 1760 Acyl halides

C=O 1750 – 1715 Esters

C=O 1740 – 1680 Aldehydes

C=O 1725 – 1665 Ketones

C=O 1720 – 1670 Carboxylic acids

C=O 1690 – 1630 Amides

C=C 1675 – 1600 Often weak

C=N 1690 - 1630 Often difficult to assign

N=O1560 - 1510 & 1370 - 1330

Nitro compounds

Triple Bonds:

Bond Wavenumber (cm-1) Notes

CºC 2260 – 2120 Alkynes, bands are weak

CºN 2260 - 2220 Nitriles

Single Bonds (not to Hydrogen):

Bond Wavenumber (cm-1) Notes

C-C Variable No diagnostic value

C-O, C-N 1400 – 1000 Difficult to assign

C-Cl 800-700 Difficult to interpret

C-Br, C-I Below 650 Often out of range of instrumentation

Bending Vibrations:

Bond Wavenumber (cm-1) Notes

R-N-H 1650 – 1500 Take care not to confuse N-H bend with the C=O stretch in amides

R-C-H 1480 – 1350 Saturated alkanes and alkyl groups

R-C-H 1000 - 680 Unsaturated alkenes and aromatics

Regions of the Infrared Spectrum:

1. 4000-2500 cm-1 N-H, C-H, O-H (stretching)

a) 3300-3600 N-H, O-H

b) 3000 C-H

2. 2500-2000 cm-1 CºC and C º N (stretching)

1. 2000-1500 cm-1 double bonds (stretching)

a) C=O 1680-1750

b) C=C 1640-1680 cm-1

2. Below 1500 cm-1 “fingerprint” region

Differences in Infrared Absorptions:

1. Light objects connected to heavy objects vibrate fastest (at higher frequencies): C-H, N-H, O-H

2. For two heavy atoms, stronger bond requires more energy (higher frequency): C º C, C º N > C=C, C=O, C=N > C-C, C-O, C-N, C-halogen

3. Molecules vibrate and rotate in normal modes, which are combinations of motions (relates to force constants)

4. Bond stretching dominates higher energy (frequency) modes

Infrared Spectra of Hydrocarbons:

C-H, C-C, C=C, C º C have characteristic peaks:

Hexane:

Infrared Spectra of Some Common Functional Groups:

Alcohols:

Amines:

Carbonyl Compounds:

1. Strong, sharp C=O peak 1670 to 1780 cm-1

2. Exact absorption characteristic of type of carbonyl compound

a) 1730 cm-1 in saturated aldehydes

b) 1705 cm-1 in aldehydes next to double bond or aromatic ring

C=O in Ketones:

1. 1715 cm-1 in six-membered ring and acyclic ketones

2. 1750 cm-1 in 5-membered ring ketones

3. 1690 cm-1 in ketones next to a double bond or an aromatic ring

C=O in Esters:

1. 1735 cm-1 in saturated esters

2. 1715 cm-1 in esters next to aromatic ring or a double bond

Aromatic Compounds:

1. Weak C–H stretch at 3030 cm-1

2. Weak absorptions 1660 - 2000 cm-1 range

3. Medium-intensity absorptions 1450 to 1600 cm-1

Phenylacetylene:

Uses and applications:

Infrared spectroscopy is a simple and reliable technique widely used in both organic and inorganic chemistry, in research and industry. It is used in,

Quality control Dynamic measurement Monitoring applications Forensic analysis in both criminal and civil cases Measuring the degree of polymerization in polymer manufacture.