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Explore the fundamentals of infrared spectroscopy, including the theory, number of vibrational modes, absorption bands, and different regions. Learn how infrared spectroscopy is used to identify and analyze various chemicals in solid, liquid, and gas samples.
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INFRARED SPECTROSCOPY Dr.M.P.Singh Assistant Prof. & Head Department of Chemistry ShriSadguruSaibaba Science College, Ashti-Gadchiroli (M.S)
1-INTRODUCTION 2-THEORY • 3- NO OF VIBRATIONAL MODES 4-ABSORPTION BANDS 5-REGIONS 6-USES AND APPLICATIONS
INTRODUCTION: IR-Spectroscopy involves the interaction of infrared radiation with matter.It covers a range of techniques mostly based on absorption spectroscopy. As with all spectroscopic techniques. It can be used toidentify and study chemicals. Samples may be solid, liquid or gas. • The method or technique of infrared spectroscopy is conducted with an instrument called an infrared spectrometer (or spectrophotometer) to produce an infrared spectrum. An IR spectrum can be visualized in a graph of infrared light absorbance (or transmittance) on the vertical axis v/s frequency or wavelength on the horizontal axis.
Typical units of frequency used in ir spectra are reciprocal centimeters (sometime called wave numbers), with the symbol cm-1.Unit of IR wave length are commonly given in micrometers (formely called “microns”) symbol µm. Which are related to wave numbers in a reciprocal way. A common laboratory instrument that uses this technique, is a Fourier Transform Infrared Spectrometer. Two-dimentional infrared is possible as discussed below. Typical units of frequency used in infrared spectra are reciprocal centimeters (sometime called wave numbers), with the symbol cm-1. Unit of IR wave length are commonly given in micrometers (formely called “microns”) symbol µm. Which are related to wave numbers in a reciprocal way. A common laboratory instrument that uses this technique, is a Fourier Transform Infrared Spectrometer. Two-dimentional infrared is possible as discussed below.
The mid-IR, approximately 4000-400 cm-1 (2.5-25µm), may be used to study the fundamental vibrations and associated rotational vibrational structure. The Far-IR, approximately 400-10 cm-1 (25-1000µm),lying adjacent to the microwave region, has low energy and may be used for rotational spectroscopy. The name and classifications of these sub regions are conventions, and are only loosely based on the relative molecular or electromagnetic properties. THEORY: IR-Spectroscopy exploits the fact that molecule absorb frequencies characteristic of their structure. These absorptions occur at resonance frequencies i.e., the frequency of the absorbed radiation matches the vibrational frequency. The energies are affected by the shape of the molecular potencial energy surfaces, the masses of the atoms and the associated vibronic coupling.
Number of Vibrational Modes: In order for a vibrational mode in a sample to be “IR-active”. It must be associated with changes in the dipole moment. A permanent dipole is not necessary, as the rule requires only a change in dipole moment. A molecule can vibrate in many ways, and each way is called a vibrational mode. For molecules with N-number of atoms. Linear molecules have 3N-5 degrees of vibrational modes, whereas, non linear molecules have 3N-6 degrees of vibrational modes (also called vibrational degree of freedom) as an examples H2O, a non-linear molecule, will have 3×3-6=3 degree of vibrationalfreedom or modes. Simple diatomic molecules have only one bond and only one vibrational band. If the molecule is symmetrical, e.g., N2. The band is not observed in the infrared spectrum. Asymmetric diatomic molecules, e.g., CO, absorb in the IR-Spectrum. More complex molecules have many bonds, and their vibrational spectra are correspondingly more complex, i.e., big molecules have many peaks in their infrared spectra.
The atoms in a CH2X2 group, commonly found in organic compounds, and where X can represent any other atom, can vibrate in nine different ways. Six of these vibrations involve only the CH2 portion. Symmetric and asymmetric stretching, scissoring, rocking, wagging and twisting as shown below. Structure that do not have the additional X group attached, have fewer modes because , some modes are defind by specific rotationship to those other attached groups. For examples in water, the rocking, wagging and twisting modes, do not exist because , these types of motions of the H-represent simple rotation of the whole molecule rather than vibrations within it.
Absorption Bands: IR-Spectroscopy is often used to identify structures, because functional groups give rise to characteristic bands both in terms of intensity and position (frequency). The position of these bands are summarized in correlation table as shown below. Regions: A spectrograph is often interpreted as having two regions. 1- Functional Group Region: In this region there are one to a few troughs per functional group. 2- Finger Print Region: In this region there are many troughs which form an intricate pattern which can be used like a fingerprint to determine the compound.
USES AND APPLICATIONS: IR-Spectroscopy is a simple and reliable technique widely used in both organic and inorganic chemistry, in research and industry. It is used in quality control, dynamic measurement and monitoring applications suchas the long term unattend of CO2 concentrations in the green houses and growth chambers by IR gas analyser. It is also useful in forensic analysis in both criminal and civil cases For example in identifying polymer degradation. It can be used in Determining the blood alcohol content of a suspected drunk driver. It hase been successfully used in analysis and identification of pigments in paintings and other arts objects suchas illuminated Manuscipts.
It is also useful in measuring the degree of polymerisation in polymerManufacture. Chances in the character or quantity of a particular bondAreassesed by measuring at a specific frequency over time. ModernInstruments can take IR measurements across the range of interest asFrequently as 32 times a second. This can be done whilst simultaniousMeasurements are made using other techniques. This make theobservations of chemical reactions and processes quicker and most accurate. It has also been successfully utilised in the field of semiconductor Microelectronics. For example, IR-Spectroscopy can be applied to Semiconductors like silicon, gallium arsenide, Zinc Selenide, amorphous silicon, silicon nitride etc. It is also applicable in food industry to measure the concentration Of various compounds in different food products.
It is also useful in gas leak detection devices suchas the DP-IR and Eye CGas. These devices detect hydrocarbon gas leaks in the transportation of natural gas and crude oil. In feb.2014, NASA announced a greately updated database, based on IR-Spectroscopy, for traking polycyclic hydrocarbons (PAHS) in the universe. According to scientists, more than 20% of the carbon in the universe may be associated with PAHS possible starting materials for the formation of life.
