Infra Red spectroscopy CH143 executive summary Use to Pharmaceutical analysis (Watson Chapter 5). Structural elucidation. Finger print region. Video’s at –

Different covalent bonds have different strengths due to the masses of different atoms at either end of the bond. As a result, the bonds vibrate at different frequencies The frequency of vibration can be found by detecting when the molecules absorb electro-magnetic radiation. Various types of vibration are possible. Examples include... STRETCHING and BENDING INFRA RED SPECTROSCOPY SYMMETRIC BENDING ASYMMETRIC STRETCHING STRETCH Slide courtesy of Knockhardy Publishing

Types of Functional Groups

Example of a IR Spectrum C-O stretching at cm -1 (strong) C=O stretching near 1735 cm -1 C=O C-O

SYMMETRIC STRETCHING BENDING AND STRETCHING IN WATER MOLECULES Slide courtesy of Knockhardy Publishing

ASYMMETRIC STRETCHING BENDING AND STRETCHING IN WATER MOLECULES Slide courtesy of Knockhardy Publishing

BENDING AND STRETCHING IN WATER MOLECULES BENDING Slide courtesy of Knockhardy Publishing

JOG 2008 a beam of infra red radiation is passed through the sample a similar beam is passed through the reference cell the frequency of radiation is varied bonds vibrating with a similar frequency absorb the radiation the amount of radiation absorbed by the sample is compared with the reference the results are collected, stored and plotted The Infra-red Spectrophotometer Slide courtesy of Knockhardy Publishing

A bond will absorb radiation of a frequency similar to its vibration(s) The Infra-red Spectrophotometer normal vibrationvibration having absorbed energy Slide courtesy of Knockhardy Publishing

IDENTIFICATION OF PARTICULAR BONDS IN A MOLECULE INFRA RED SPECTRA - USES The presence of bonds such as O-H and C=O within a molecule can be confirmed because they have characteristic peaks in identifiable parts of the spectrum. IDENTIFICATION OF COMPOUNDS BY DIRECT COMPARISON OF SPECTRA The only way to completely identify a compound using IR is to compare its spectrum with a known sample. The part of the spectrum known as the ‘Fingerprint Region’ is unique to each compound. Slide courtesy of Knockhardy Publishing

Infra-red spectra are complex due to the many different vibrations taking place in each molecule. Total characterisation of a substance based only on its IR spectrum is almost impossible unless one has computerised data handling facilities for comparison of the obtained spectrum with one in memory. The technique is useful when used in conjunction with other methods - nuclear magnetic resonance spectroscopy and mass spectroscopy. Peak position depends on bond strength masses of the atoms joined by the bond strong bonds and light atomsabsorb at lower wavenumbers weak bonds and heavy atoms absorb at high wavenumbers INFRA RED SPECTRA - INTERPRETATION Slide courtesy of Knockhardy Publishing

Vertical axisAbsorbance the stronger the absorbance the larger the peak Horizontal axisFrequencywavenumber (waves per centimetre) / cm -1 Wavelengthmicrons (m); 1 micron = 1000 nanometres INFRA RED SPECTRA - INTERPRETATION Slide courtesy of Knockhardy Publishing

FINGERPRINT REGION organic molecules have a lot of C-C and C-H bonds within their structure spectra obtained will have peaks in the 1400 cm -1 to 800 cm -1 range this is referred to as the “fingerprint” region the pattern obtained is characteristic of a particular compound the frequency of any absorption is also affected by adjoining atoms or groups. Slide courtesy of Knockhardy Publishing

IR SPECTRUM OF A CARBONYL COMPOUND carbonyl compounds show a sharp, strong absorption between 1700 and 1760 cm -1 this is due to the presence of the C=O bond Slide courtesy of Knockhardy Publishing

IR SPECTRUM OF AN ALCOHOL alcohols show a broad absorption between 3200 and 3600 cm -1 this is due to the presence of the O-H bond Slide courtesy of Knockhardy Publishing

IR SPECTRUM OF A CARBOXYLIC ACID carboxylic acids show a broad absorption between 3200 and 3600 cm -1 this is due to the presence of the O-H bond they also show a strong absorption around 1700 cm -1 this is due to the presence of the C=O bond Slide courtesy of Knockhardy Publishing

IR SPECTRUM OF AN ESTER esters show a strong absorption between 1750 cm -1 and 1730 cm -1 this is due to the presence of the C=O bond Slide courtesy of Knockhardy Publishing

WHAT IS IT? O-H STRETCH C=O STRETCH O-H STRETCH C=O STRETCH AND ALCOHOL ALDEHYDE CARBOXYLIC ACID One can tell the difference between alcohols, aldehydes and carboxylic acids by comparison of their spectra. Slide courtesy of Knockhardy Publishing

O-HC=OC-O N-H Aromatic C-CC-H C=CC-C alkanes CNCN C-Cl CHARACTERISTIC FREQUENCIES Slide courtesy of Knockhardy Publishing

BondClass of compoundRange / cm -1 Intensity C-HAlkane strong C-CAlkane weak C=CAlkene variable C=OKetone strong Aldehyde strong Carboxylic acid strong Ester strong Amide strong C-OAlcohol, ester, acid, ether strong O-HAlcohol (monomer) variable, sharp Alcohol (H-bonded) strong, broad Carboxylic acid (H-bonded) variable, broad N-HAmine, Amide3500 (approx)medium C  NNitrile medium C-XChloride strong Bromide strong Iodide500 (approx)strong CHARACTERISTIC ABSORPTION FREQUENCIES Slide courtesy of Knockhardy Publishing

Quick Test 1 The fingerprint region is made use of in which of the following techniques? A) UV spectroscopy. B) Gas analysis. C) Fluorescence spectroscopy. D) IR spectroscopy to confirm structure.

Quick Test 2 A compound has undergone a transformation to an ester from a carboxylic acid. Which of the following would be expected in the IR spectrum? A) The spectra would be identical. B) There would be a hydroxyl absorption around 3500 cm -1 in the acid that would not be seen in the ester spectrum. C) The fingerprint region would be missing from the spectrum. D) The carbonyl absorption would disappear.

Quick test 3 A carbonyl group has a characteristic stretch around 1700 cm -1 in the infra red spectrum. Which of the following types of compounds has this functional group? A)Alkanes B)Esters C)Amines D)Alkenes

Quick test 4 1.When using spectroscopy techniques to identify the structure of a drug molecules it is important to a.Only ever use titrations, no other techniques have any merit. b.Never use I.R spectroscopy. c.Use as many spectroscopic techniques as possible including UV and IR spectroscopy. d.Rely up fluoresces spectroscopy only. e.Never use a fingerprint region associated with I.R. spectroscopy.