Chapter 12 IR Spectroscopy

Transcription

1 Chempocalypse Now! Chapter 12 IR Spectroscopy Page 1 Chapter 12 IR Spectroscopy Topic A3 from the IB HL Chemistry Curriculum A.3.1 Assessment Statement Obj Teacher s Notes Describe the operating principles of a doublebeam IR spectrometer. 2 A schematic diagram of a simple double-beam spectrometer is sufficient. A.3.2 Describe how information from an IR spectrum can be used to identify bonds. 2 A.3.3 Explain what occurs at a molecular level during the absorption of IR radiation by molecules. 3 H 2 O, CH 2, SO 2, and CO 2 are suitable examples. Stress the change in bond polarity as the vibrations (stretching and bending) occur. A.3.4 Analyze IR spectra of organic compounds. 3 Students will be assessed using examples containing up to three functional groups. The Chemistry data booklet contains a table of IR absorptions for some bonds in organic molecules. Students should realize that IR absorption data can be used to identify the bonds present, but not always the functional groups present. The natural frequency of a chemical bond A chemical bond can be thought of as a spring. Each bond vibrates and bends at a natural frequency which depends on the bond strength and the masses of the atoms. Light atoms, for example, vibrate at higher frequencies than heavier atoms and multiple bonds vibrate at higher frequencies than single bonds. Simple diatomic molecules such as HCl, HBr, and HI, can only vibrate when the bond stretches. The HCl bond has the highest frequency of these three as it has the largest bond energy and the halogen atom with the smallest relative atomic mass. The stretching of the bond can be seen at the right in Figure (a). In more complex molecules, different types of vibration can occur, such as bending, so that a complex range of frequencies is present. Bending of a bond can be seen at the right in Figure (b). Using infrared radiation to excite molecules The energy needed to excite the bonds in a molecule to make them vibrate with greater amplitude, occurs in the IR region. A bond will only interact with the electromagnetic infrared radiation, however, if it is polar. The presence of separate areas of partial positive and negative charge in a molecule allows the electric field component of the electromagnetic wave to excite the vibrational energy of the molecule. The change in the vibrational energy produces a corresponding change in the dipole moment of the molecule. The intensity of the absorptions depends on the polarity of the bond. Symmetrical non-polar bonds in N N and O=O do not absorb radiation, as they cannot interact with an electric field.

2 Chempocalypse Now! Chapter 12 IR Spectroscopy Page 2 Stretching and bending in a polyatomic molecule In a polyatomic molecule such as water, it is more correct to consider the molecule as a whole stretching and bending rather than the individual bonds. Water, for example, can vibrate at three fundamental frequencies as shown below. As each of the three modes of vibration results in a change in dipole of the molecule, they can be detected with IR spectroscopy. For a symmetrical linear molecule such as carbon dioxide, there are also three modes of vibration. However, the symmetric stretch is IR inactive as it produces no change in dipole moment. The dipoles of both C=O bonds are equal and opposite throughout the vibration. Exercise: Draw the structure of sulfur dioxide molecule and identify its possible modes of vibration. Predict which of these is likely to absorb IR radiation.

3 Chempocalypse Now! Chapter 12 IR Spectroscopy Page 3 The double-beam IR spectrometer Many spectroscopic methods use a double-beam method in which one beam is passed through the sample under investigation and the other through a reference sample. In the double-beam IR spectrometer, IR radiation from a heated filament is split into two parallel beams. Radiation is absorbed by the sample when it has the same frequency as any of the natural bond frequencies in the sample molecules. Other frequencies simply pass through the sample. The sample and reference beams are analyzed and differences in the intensities of the two beams measured by the detector at each wavenumber and fed into the recorder, which produces a spectrum. When the radiation is not absorbed by the sample, the transmittance is 100% but when radiation is absorbed the transmittance falls to lower values. The baseline of the spectrum corresponds to 100% transmittance and signals are recorded when the transmittance falls as the radiation is absorbed. The purpose of the reference is to eliminate absorptions caused by carbon dioxide and water vapor in the air, or absorptions from the bonds in the solvent used.

4 Chempocalypse Now! Chapter 12 IR Spectroscopy Page 4 Matching wavenumbers with bonds The absorption of particular wavenumbers of IR radiation helps the chemist to identify the bonds in a molecule. The precise position of the absorption depends on the environment of the bond, so a range of wavenumbers is used to identify different bonds. Characteristic infrared absorption bands are shown in the table below. Some bonds can also be identified by the distinctive shapes of their signals: the O H bond gives a broad signal and the C=O bond gives a sharp signal. Exercise: A molecule absorbs IR at a wavenumber of 1720 cm 1. Which functional group could account for this absorption? I. aldehydes II. esters III. ethers A. I only B. I and II C. I, II, and III D. None of these As hydrogen bonding broadens the absorptions, its presence can also be detected. For example, hydrogen bonding between hydroxyl groups changes the O H vibration; it makes the absorption much broader and shifts it to a lower frequency. Molecules with several bonds can vibrate in many different ways and with many different frequencies. The complex pattern can be used as a fingerprint to be matched against the recorded spectra of known compounds in a database. A comparison of the spectrum of a sample with that of a pure compound can also be used as a test of purity. On the right is the IR spectrum of heroin, compared with that of an unknown sample. The near perfect match indicates that the sample contains a high percentage of heroin. Spectral analysis such as this can identify unknown compounds in mixtures or from samples taken from clothing or equipment. The technique is widely used in forensic science.

5 Chempocalypse Now! Chapter 12 IR Spectroscopy Page 5 Consider the spectrum of propanone, seen below. The base line at the top corresponds to 100% transmittance and the key features are the troughs which occur at the natural frequencies of the bonds present in the molecule. The absorption at just below 1800 cm 1 shows the presence of the C=O bond and the absorption near 3000 cm 1 is due to the presence of the C H bond. The more polar C=O bond produces the more intense absorption. The presence of the C H bond can again been seen near 3000 cm 1 in the spectrum of ethanol, seen below. The broad peak at just below 3400 cm 1 shows the presence of hydrogen bonding which is due to the hydroxyl (OH) group.

6 Chempocalypse Now! Chapter 12 IR Spectroscopy Page 6 Exercises: A bond has an IR absorption of 2100 cm 1. What is the wavelength of the radiation and the natural frequency of the bond? State what occurs at the molecular level when infrared radiation is absorbed. Cyclohexane and hex-i-ene are isomers. Suggest how you could use infrared spectroscopy to distinguish between the two compounds. Include diagrams. The intoximeter, used by the police to test the alcohol levels in the breath of drivers, measures the absorbance at 2900 cm 1. ldentify the bond which causes ethanol to absorb at this wavenumber. A molecule has the molecular formula C 2 H 6 O. The infrared spectrum shows an absorption band at cm 1, but no absorption bands above 3000 cm 1. Deduce its structure.

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