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Bsc Chemistry SUBJECT FORENSIC SCIENCE Paper No. and Title PAPER No.4: Instrumental Methods and Analysis Module No. and Title MODULE No.16: Microwave Spectroscopy Module Tag FSC_P4_M16 FORENSIC SCIENCE PAPER No.4: Instrumental Methods and Analysis MODULE No.16: Microwave Spectroscopy TABLE OF CONTENTS 1. Learning Outcomes 2. Introduction- Microwave Spectroscopy 3. Difference between Infrared and Microwave Spectroscopy 4. Instrumentation for Microwave spectroscopy 5. Working of microwave spectrometer 6. Applications of Microwave Spectroscopy 7. Summary FORENSIC SCIENCE PAPER No.4: Instrumental Methods and Analysis MODULE No.16: Microwave Spectroscopy 1. Learning Outcomes After studying this module, you shall be able to know about- Introduction to Microwave Spectroscopy Differences between infrared and microwave spectroscopy Instrumentation for microwave spectroscopy Applications of microwave spectroscopy 2. Introduction: Microwave Spectroscopy Microwave spectroscopy is proving in recent years a great asset in the study of various problems in physics, chemistry, electronics and even astronomy. Particularly, this has been found to be very useful in the determination of the structures of those molecules which do not give good results by using Raman and infrared spectroscopy. The ability to measure frequencies more precisely in the microwave region allows very accurate calculations, than in the infrared-visible-ultraviolet region and accuracy of the order of 0.001 to 0.005 Aº is possible. These spectra result from transitions between the rotational energy levels of a gaseous molecule on the absorption of radiations falling in the microwave region. These spectra are shown by molecules which possess a permanent dipole moment, e.g. HCl, CO, H2O vapor, NO, etc. Homo-nuclear diatomic molecules such as H2, Cl2, etc., and linear polyatomic molecules such as CO2, which do not possess a dipole moment, do not show microwave spectra. The microwave spectroscopy explores that part of the electromagnetic spectrum which is extending from 100µm (3x1013 Hz) to 1 centimeter (3x1010 Hz). This region of electromagnetic spectrum is designated as the microwave region. This lies between the far infrared and conventional radio frequency regions. Spectroscopic applications of microwave consist almost exclusively of absorption works, rather than the emission type. In most of the cases, absorption of microwave energy represents changes of the absorbing molecule from one rotational level to another. FORENSIC SCIENCE PAPER No.4: Instrumental Methods and Analysis MODULE No.16: Microwave Spectroscopy Therefore, the microwave spectroscopy deals with the pure rotational motion of the molecules and is also known as rotational spectroscopy. The condition for observing resonance in that region is that a molecule must possess permanent dipole moment. When a molecule having dipole moment rotates, it generates an electric field which can interact with the electric component of the microwave radiation. During the interaction, energy can be absorbed or emitted and thus the rotation of the molecule gives rise to a spectrum. If molecules are not having dipole moment, interactions are not possible and these molecules are said to be “microwave inactive”. Examples of such molecules are H2, Cl2, etc. On the other hand, the molecules like HCl, CH3Cl, etc. are having dipole moments and their interaction will give rise to a spectrum. Such molecules are said to be “microwave active”. 3. Generally, the microwave spectra obtained in most of the molecules are absorption spectra. Theoretical considerations reveal that the probability of microwave spectra is only about one per cent of the optical absorption. 3. Differences between Infrared and Microwave spectroscopy 4. The main differences between infrared and microwave spectroscopy are as follows: - 1. The absorption spectrum in the microwave region is characteristic of the absorbing molecule as a whole, whereas spectrum in the infrared region is characteristic of the functional groups present in the absorbing molecule. 2. The resolution of the lines in the microwave spectrum is very much greater than that obtained by the infrared method. 3. In microwave spectroscopy, the substance must be in the gaseous state. On the other hand, in the infrared spectrum the substance must be in the solid, liquid or gaseous state. 4. In microwave spectroscopy, the spectra observed are nearly always absorption spectra. But in the infrared spectroscopy the spectra observed may be absorption or emission spectra. FORENSIC SCIENCE PAPER No.4: Instrumental Methods and Analysis MODULE No.16: Microwave Spectroscopy 4. Instrumentation for Microwave Spectroscopy A microwave spectrometer consists of the following essential components: 1. The source and Monochromator- Reflex klystron valve is the main source of radiations in the microwave region. As the klystron valve emits radiations of a very narrow frequency range (called monochromatic), it acts as its own Monochromator. Furthermore, the frequency of the emitted radiation depends on the voltage that is applied to the klystron valve. As the voltage is varied over a given range, the emitted radiation can thus be made to sweep through a region of the microwave range. Klystrons are readily available from 3000 to 5000 Mc/second and weaker signals up to 250000 Mc/second may be obtained with harmonic generators. One slight disadvantage of the klystron valve is that it radiates out very small energy which is of the order of 30 milliwatts. However, since the energy radiated is concentrated into very narrow frequency band, a sharply tuned detector may be activated to produce a strong signal. 2. The Beam Direction – The radiation emitted by the klystron cannot be handled with mirrors and lenses, but can be most advantageously transmitted through hollow metallic conductors of such geometry that the electric and magnetic fields can be utilized to the greatest extent. These are known as wave guides. These are hollow tubes of copper or silver of rectangular cross section inside which the radiation is confined. In order to maintain the direction of beam as well as its focusing, the wave guides may be bent or tapered. The wave guides are generally evacuated because if air is present in them, considerable absorption of the radiation will occur. The wave guides used in the microwave spectrometer is now commonly used in the chemical research facilities. FORENSIC SCIENCE PAPER No.4: Instrumental Methods and Analysis MODULE No.16: Microwave Spectroscopy 3. Sample and Sample Space- The sample is placed in a piece of evacuated wave- guide which is closed at both ends by thin mica windows. Round holes are made in the tube for evacuation purposes and for introduction of the gas under test. The pressure of the gas is adjusted to make the absorption line sharp. The sample must be in the gaseous state for studying in the microwave region. The pressure of the order of 0.01 mm mercury is generally required to give absorption spectrum. Many solid or liquid substances can be studied by the microwave techniques provided their vapor pressures are above the value of 0.01 mm of mercury. 4. Detector – A Quartz crystal is generally used as a detector. It is mounted on a cartridge made up of a tungsten whisker held in point contact with the crystal. In place of crystal detector, an ordinary superheterodyne radio receiver can be used provided it may be tuned to the appropriate high frequency. But a simple quartz crystal is more sensitive and easier to use. 5. Spectrum analyzer- It consists of an amplifier of detected energy and an indicator which may be either a cathode ray oscillography or a pen and ink-recorder. The vibrations emitted by the quartz crystal produces an electric signal which is amplified and then displayed as a pattern on an oscilloscope screen or a recording on a chart by the pen-and-ink recorder. FORENSIC SCIENCE PAPER No.4: Instrumental Methods and Analysis MODULE No.16: Microwave Spectroscopy A diagrammatic sketch of one of the simplest type of microwave spectrometers is shown in the following figure- Figure 1- Simplified diagram of Microwave spectrometer. 5. Working of microwave spectrometer Monochromatic radiations of various wavelengths in the microwave region emitted by klystron valve are allowed to pass through the sample space containing the gaseous sample of the substance under investigation. Then, the radiations are made to conduct along a rectangular tube called a waveguide. After this the radiations are received by the quartz crystal detector which is situated at the far end of the waveguide. After receiving the radiations from the waveguide it vibrates and produces an electrical signal which is amplified by amplifier and then displayed either as a recording on a chart or as a pattern on an oscilloscope screen. The pattern obtained on the chart or on the screen of the oscillography enables one to determine the frequency or the range of frequencies of the detected microwave radiation. FORENSIC SCIENCE PAPER No.4: Instrumental Methods and Analysis MODULE No.16: Microwave Spectroscopy S.No. Molecules Bond r(A) Bond r(A) 1 HCN C-H 1.06317 C-N 1.15535 2 ClCN C-Cl 1.629 C-N 1.163 3 BrCN C-Br 1.790 C-N 1.159 4 NNO N-N 1.126 N-O 1.191 5 OCS C-O 1.164 C-S 1.559 6 CH3Cl C-H 1.0959 C-Cl 1.178 7 CH3F C-H 1.109 C-F 1.385 8 SiH3Br Si-H 1.570 Si-H 2.209 Table 1: Molecular Data determined by Microwave Spectroscopy The microwave spectrometer described above is usually used for the measurements of the highest accuracy because the absorption lines are narrow and fairly faithful in shape and relative intensities. The use of oscilloscope poses a serious problem that the amplifier bandwidth cannot be narrowed to remove noise and, thus, the sensitivity is not exceptionally high. At the same time the new lines for unknown substances cannot be obtained very easily unless their frequencies are known within the narrow limits.
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