Ir Spectra Analysis

IR Spectra Analysis

Primciple of IR

Energies in infra-red radiation correspond to the energies involved in bond vibrations.

That is molecules absorb IR radiation when bonds undergo stretching and bending vibrations transitions. Vibrational transitions- movement from lower to higher vibrational states

Types of molecules that are IR active

Molecules which have a dipole

Molecules that bring about a change in the dipole moment Dipole is formed when molecules within a covalent bond share the electrons unequally

Absorption of IR Radiation The energy absorbed/involved in this vibration depends on things like:  (1) the length of the bond and  (2) the mass of the atoms at either end That means that each different bond will vibrate in a different way, involving different amounts of energy.  each different bond will absorb a different frequency (and hence energy) of infra-red radiation.

Nature of Covalent bonds In covalent bonds, atoms are not joined by rigid links the two atoms are held together because both nuclei are attracted to the same pair of electrons The two nuclei can vibrate backwards and forwards towards and away from each other - around an average position

Types of Vibrations Stretching vibrations The two nuclei involved in a covalent bond can vibrate backwards and forwards towards and away from each other - around an average position.

Bending vibrations- the bond angles within a covalent molecule fluctuates slightly around its average value

Vibrational Modes CO2

SO2

Type of molecule

linear

Non-linear

Polar vs non-polar

Non-polar

polar

Vibrational mode (symmetric stretch)

Non IR active

IR active

Vibrational mode (asymmetric stretch)

IR active

IR active

Vibrational mode (bending)

IR active

IR active

Uses of IR spectroscopy

Determination/ Identification of functional groups the monitoring of air pollutants examples carbon dioxide and sulphur dioxide forensic analysis Determine the degree of unsaturation in polymers

Sample Preparation IR –solid samples Method1 Crush the sample with a mulling agent (usually Nujol) in a marble or agate mortar, with a pestle.

A thin film of the mull is applied onto salt plates made of sodium chlorides and place in sample holder for analysis

Sample Preparation IR –solid samples Method 2 Grind a quantity of the sample with a specially purified salt (usually potassium bromide) finely (to remove scattering effects from large crystals). This powder mixture is then crushed in a mechanical die press to form a translucent pellet Pellet is placed in the sample holder through which the beam of the spectrometer can pass.

Sample Preparation IR – liquid samples Liquid samples can be sandwiched between two plates of a high purity salt (commonly sodium chloride, or common salt, although a number of other salts such as potassium bromide or calcium fluoride are also used). The plates are transparent to the infrared light and will not introduce any lines onto the spectra. Some salt plates are highly soluble in water, so the sample and washing reagents must be anhydrous (without water).

CHARACTERISTIC INFRARED ABSORPTION FREQUENCIES

Bond C-H C-H C-H

C-H C=C CºC C=C C-O C=O

O-H N-H C-N CºN

NO2

Compound Type

Frequency range, cm-1 2960-2850(s) stretch Alkanes 1470-1350(v) scissoring and bending CH3 Umbrella Deformation 1380(m-w) - Doublet - isopropyl, t-butyl 3080-3020(m) stretch Alkenes 1000-675(s) bend Aromatic Rings 3100-3000(m) stretch Phenyl Ring Substitution Bands 870-675(s) bend Phenyl Ring Substitution Overtones 2000-1600(w) - fingerprint region 3333-3267(s) stretch Alkynes 700-610(b) bend Alkenes 1680-1640(m,w)) stretch Alkynes 2260-2100(w,sh) stretch Aromatic Rings 1600, 1500(w) stretch Alcohols, Ethers, Carboxylic acids, Esters 1260-1000(s) stretch Aldehydes, Ketones, Carboxylic acids, 1760-1670(s) stretch Esters Monomeric -- Alcohols, Phenols 3640-3160(s,br) stretch Hydrogen-bonded -- Alcohols, Phenols 3600-3200(b) stretch Carboxylic acids 3000-2500(b) stretch 3500-3300(m) stretch Amines 1650-1580 (m) bend Amines 1340-1020(m) stretch Nitriles 2260-2220(v) stretch 1660-1500(s) asymmetrical stretch Nitro Compounds 1390-1260(s) symmetrical stretch

Interpretation of IR Spetra

IR is not generally used to determine the whole structure of an unknown molecule. We don’t need to analyze every single peak Instead, IR is great for identifying certain specific functional groups,

The peaks represent areas of the spectrum where specific bond vibrations occur. The following gives an overview of the IR window from 4000 cm -1 to 500 cm -1 with various

regions of interest highlighted.

OH Stretch in ALCOHOLS

OH Stretch in CARBOXYLIC ACIDSHydroxyl

groups in carboxylic acids are considerably broader than in alcohols

C=O peaks, in the region around 16301800 cm-1

The C-H Stretch Boundary at 3000 cm-1

N-H Stretch Amines and Amides

the primary amine and primary amide have two “fangs”, while the secondary amine and secondary amide have a single peak. The amine stretches tend to be sharper than the amide stretches; also the amides can be distinguished by a strong C=O stretch

Answer- Problem 1

The band at 1716 indicates a carbonyl, probably a ketone. The bands at 3000-2850 indicate C-H alkane stretches.

Problem 2

Answer- Promlem 2

OH stretch and CH stretch no carbonyl group therefore an ALCOHOL

Problem 3

Answer- Problem 3

OH stretch, C=O stretch and sp3 CH stretch therefore CARBOXYLIC ACID

Problem 4

Answer- Problem 4

Sp3 CH stretch just below 3000 and C=O at around 1700 hence CARBONYL COMPOUND

Problem 5

Answer- Problem 5

The two bands at 3433 and 3354 indicate a PRIMARY AMINE (-NH2). The bands at 3000-2850 indicate C-H alkane stretches

Problem 6

Answer- Problem 6

The bands at 3000-2850 indicate C-H alkane stretches. There really aren't many other bands in the spectrum to indicate functional groups therefore ALKANE

Reference

 Master Organic Chemistry https://www.masterorganicchemistry.com/2016/11/23/quick_analysis_of_ir_spectra/

 Organic Chemistry from CU Boulder http://www.orgchemboulder.com/Spectroscopy/Problems/index.shtml