IR Spectroscopy PDF

INFRARED SPECTROSCOPY SPECIFIC OBJECTIVES: 6.1. explain the origin of absorption in IR spectroscopy 6.2. describe the basic steps involving in analysing samples by IR spectroscopy 6.3. comment on the limitations associated with the use of IR spectroscopy 6.4. deduce the functional groups p...

INFRARED SPECTROSCOPY SPECIFIC OBJECTIVES: 6.1. explain the origin of absorption in IR spectroscopy 6.2. describe the basic steps involving in analysing samples by IR spectroscopy 6.3. comment on the limitations associated with the use of IR spectroscopy 6.4. deduce the functional groups present in organic compounds from IR spectra 6.5. cite examples of the use of IR spectroscopy in the monitoring of air pollutants  THE SPECTRUM AND MOLECULAR EFFECTS 2 The term ‘infrared’ describes a region of the electromagnetic spectrum with frequencies lower than visible light. Infrared spectroscopy is the measurement of the absorption of IR radiation by compounds. A molecule is not a rigid assembly of atoms, there is always some vibration motion associated with a molecule. For a molecular vibration to interact with the absorb IR radiation, the motion of the atoms must produce a change in the dipole moment of the molecule. Chapter 12 How does the mass influence the vibration? H2 I2 MM =2 g/mole MM =254 g/mole The greater the mass - the lower the wavenumber The wavenumber is used to quantify the energy of vibrations. Higher wavenumbers correspond to higher vibration frequencies and, thus, higher energy vibrations. 5 Chapter 12 EXAMPLES OF DIFFERENT VIBRATIONAL MODES IN A MOLECULE => 6  PRINCIPLES OF INFRARED SPECTROSCOPY (ORIGEN OF ABSORPTIONS) Molecules exhibit two types of radiation-induced transitions: vibrational transitions and rotational transitions within the ground electronic state of the molecule. Absorption of infrared wavelengths can cause bonds to stretch, bend and twist in characteristic ways.  The number of ways in which the molecule can vibrate is related to the number of atoms and thus the number of bonds it contains. A molecule absorbs only those frequencies of IR light that match vibrations that cause a change 7 in the dipole moment of the molecule.  Bonds in symmetric O2, N2 and H2 molecules does not absorb IR because stretching does not change the dipole moment, and bending cannot occur with only two atoms in the molecule. Some motions do not change the dipole moment for the molecule; some are so much alike that they coalesce into one band. The frequency of a vibration is directly proportional to the strength of the bond. The IR spectrum for a molecule is a graphical display of the frequencies of IR radiation absorbed (in wavenumber or cm-1) and the % of the incident light that passes A TYPICAL ‘SET-UP’ FOR THE INFRARED INSTRUMENT REFEREN CE HEAT DETECTO DISPLA SOURCE R Y SAMPLE  IR is a far more powerful tool than UV-Vis spectroscopy in qualitative analysis. IR is less satisfactory tool for quantitative analyses than its ultraviolet and visible counterparts because the narrow peaks that characterize infrared absorption usually lead to deviations from Beer’s law; infrared absorbance measurements are consider less precise.  METHODS USED TO ANALYZED SOLID SAMPLES Mix the sample Mix the solid sample into with powdered KCl a thick paste in a drop of or KBr paraffin oil (nujol). A portion of this A drop of this thick mixture is paste or mull is than compressed under pressed between two vacuum into a disc. NaCl plates or disc. The disc is inserted into a sample holder and placed in the sample compartment in the IR instrument. The result of the sample rum is recorded as a plot of transmittance versus wavenumber (1/ wavelength). Wavenumber is measured in cm - 1  ANALYSIS OF LIQUID AND VAPOUR SAMPLES Analysis of liquid samples can be done by smearing the sample between NaCl plates or by using special liquid cell. Generally the liquid samples are prepared about 0.2 M using a suitable organic solvent for necessary dilutions, such as CCl4 and CS2. WATER IS NEVER USED in the preparation of the samples. A cell containing pure solvent is generally placed in the reference beam of the spectrometer, so that solvent IR bands are not obtained in the desired spectrum. If a reference cell is not used, the solvent bands are ignored when interpreting the resulting INTERPRETING AN INFRA-RED SPECTRUM An IR spectrum is a chart showing downward peaks (% transmittance) on the y-axis, plotted against wavenumber (cm-1) on the x-axis. An IR spectrum can contain more than 30 bands. The vast majority of the bands are not interpreted, these result from combination of different stretching and bending modes in the molecule. The identifiable group vibrations are found -1 The spectrum is divided into two regions: The firgerprint region is unique for a molecule. The functional group region is similar for molecules with the same functional groups.  THE POSITIONS OF CHARACTERISTIC ABSORPTION PEAKS FOR SOME COMMON FUNCTIONAL GROUPS Functional group Wavenumber (cm-1 ) Alkyl C-H Stretch 2950-2850 Alkene C-H Stretch 3100-3010 C=H 1680-1620 Stretch Alkyne C≡C 2260-2100 Stretch Aromatic C-H 860-680 Bending 1700-1500 C=C Bending Alcohol / Phenol O-H Stretch 3550-3200 Carboxylic Acid O-H Stretch 3300-2500 Amine N-H Stretch 3500-3300 Amide N-H Stretch 3700-3500 Nitrile 15 C≡ N 2280-2220 Stretch  INTERPRETING AN INFRA-RED SPECTRUM Ethanoic acid has the structure: It contains the following bonds: carbon-oxygen double, C=O carbon-oxygen single, C-O oxygen-hydrogen, O-H carbon-hydrogen, C-H carbon-carbon single, C-C  USES OF INFRARED SPECTROSCOPY Infrared spectroscopy is widely applicable as a sensitive, rapid and highly specific tool for the monitoring of various atmospheric pollutants including carbon disulphide, pyridine, sulphur dioxide, hydrogen cyanide and carbon dioxide. 18 Chapter 12 OTHER USES OF INFRARED TECHNOLOGY Heat-sensitive thermal imaging cameras. These can be used to study human and animal body heat patterns, but more often, they are used as night-vision cameras. (security cameras and in nocturnal animal research) Most remote controls operate by sending pulses of infrared, spelling out codes that an electronic device will recognize. Infrared is also widely used in astronomy. Taking pictures of the universe in infrared can lead to some amazing discoveries.  LIMITATIONS OF INFRARED SPECTROSCOPY With IR spectroscopy is not possible to know molecular weight of substance. Generally does not provide information of the relative position of different functional groups on a molecule. From single IR spectrum is not possible to know whether it is pure compound or a mixture of compound. 21 Chapter 12  Key points Particular groups can be identified from IR spectrum by their typical wavenumbers. Typical wavenumbers for functional groups are 1680-1750cm-1 for aldahydes and ketones and 3580-3650cm-1 for the O-H group in alcohols. A wide peak in the 2800-3500cm-1 regions indicate hydrogen bonding in alcohols or carboxilic acids. Infrared spectroscopy is used in monitoring air pollution such as carbon monoxide and sulphur dioxide. The concentration of carbon dioxide in the air can also be measured.  O-H Stretch of a Carboxylic Acid This O-H absorbs broadly, 2500-3500 cm -1, due to strong hydrogen bonding. => 23  An Alcohol IR Spectrum => 24  An Alkane IR Spectrum => 25  An Alkene IR Spectrum => 26  A Ketone IR Spectrum => 27  An Aldehyde IR Spectrum => 28  QUESTIONS 1. Explain the origin of absorption in infrared spectroscopy. 2. Outline the basic steps involved in the analysis of each of the following samples by infrared spectroscopy: a. a solid sample b. a liquid sample c. a gas sample 29  3. a. Explain how infrared spectroscopy can be used to distinguish between functional groups in organic molecules. b. Use infrared spectroscopy to distinguish between the following compounds: i. CH3CH2COOH and CH3CH2CHO ii. CH3CH=CHNH2 and CH3CH=CHCN iii. C6H5CH2CHO and C6H5COOH 4. Explain why three characteristic absorption peaks are seen in an IR spectrum of sulphur dioxide but only two peaks are seen in that of carbon dioxide. 5. Discuss the significance of the bond spotting 30 and the fingerprint regions of the IR spectrum. 2009.5