INFRARED SPECTROSCOPY Infrared rays is the region between red rays in the visible rays and microwave spectrum in electromagnetic radiation. The energy of infrared rays less than red rays but more than microwaves. The frequency is vice versa. Infrared is thermal radiation and could be emitted from the lamp, or any heating body as well as emitted from the earth, and the sun or from the human body, animals and plants. Infrared cannot be seen with the naked eye, but can imaging in total darkness, because it depends on the thermal radiation emitted from objects. Infrared application: 1. Doctors use infrared to treat skin diseases, and pain relief, which may affect the muscles, where they are shed infrared on the patient's body, as implemented through the skin and works to a certain degree warmed to stimulate circulation. 2. Infrared ovens used in some special paint dry surfaces such as leather, metal, and paper, and fabrics. 3. Some photographers use films sensitive to infrared imaging in the dark using infrared spectrum. The infrared portion of the electromagnetic spectrum is usually divided into three regions, named for their relation to the visible spectrum. 1. Near- infrared The higher-energy near-IR, approximately 14000–4000 cm−1 (0.8– 2.5 μm wavelength) can excite overtone or harmonic vibrations. 2. Mid- infrared The mid-infrared, approximately 4000–400 cm−1 (2.5–25 μm) may be used to study the fundamental vibrations and associated rotational-vibrational structure. 3. Far- infrared The far-infrared, approximately 400–10 cm−1 (25–1000 μm), lying adjacent to the microwave region, has low energy and may be used for rotational spectroscopy. The names and classifications of these sub regions are conventions, and are only loosely based on the relative molecular or electromagnetic properties. The Mid IR region is the most commonly used devices in the spectral analysis of the infrared. Infrared spectroscopy (IR spectroscopy) is the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light. It covers a range of techniques, mostly based on absorption spectroscopy. As with all spectroscopic techniques, it can be used to identify and study chemicals. A common laboratory instrument that uses this technique is a Fourier transform infrared (FTIR) spectrometer. Theory Sample of an IR spec. reading; this one is from BROMOMETHANE, showing peaks around 3000, 1300 and 1000(on the x-axis). Infrared spectroscopy exploits the fact that molecules absorb specific frequencies that are characteristic of their structure. These absorptions are resonant frequencies, i.e. the frequency of the absorbed radiation matches the transition energy of the bond or group that vibrates. The energies are determined by the shape of the molecular potential energy surfaces, the masses of the atoms, and the associated vibronic coupling. IR absorption and the number of vibrational modes A molecule can vibrate in many ways, and each way is called a vibrational mode. For molecules with N atoms in them, linear molecules have 3N – 5 degrees of vibrational modes, whereas nonlinear molecules have 3N – 6 degrees of vibrational modes (also called vibrational degrees of freedom). As an example H2O, a non-linear molecule, will have 3 × 3 – 6 = 3 degrees of vibrational freedom, 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 IR spectrum, but only in the Raman spectrum. Asymmetrical 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 IR 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 involve only the CH2 portion: symmetric and antisymmetric stretching, scissoring, rocking, wagging and twisting, as shown below. (Note, that because CH2 is attached to X2 it has 6 modes, unlike H2O, which only has 3 modes. The rocking, wagging, and twisting modes do not exist for H2O, since they are rigid body translations and no relative displacements exist.) Symmetrical stretching Anti-symmetrical stretching Scissoring Rocking Wagging Twisting Practical IR spectroscopy The infrared spectrum of a sample is recorded by passing a beam of infrared light through the sample. When the frequency of the IR is the same as the vibrational frequency of a bond, absorption occurs. Examination of the transmitted light reveals how much energy was absorbed at each frequency (or wavelength). This can be achieved by scanning the wavelength range using a monochromator. Vibrational energy levels in water molecule: Water molecule is non-linear and contains 3 atoms, and therefore the number of vibrational changes is 3 according to the previous law. 3N-6 = (3 X 3) -6 = 3 Symmetrical stretching s OH (3652 cm-1) Asymmetrical stretching As OH (3756 cm-1) Bending scissoring s HOH (1596 cm-1) Benzoic acid Acetone 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 positions of these bands is summarized in correlation tables as shown below. Wavenumbers listed in cm−1. IR spectrometer Source of IR radiation 1. Nernest glower 2. Globar 3. Incandescent wire 4. High pressure mercury arc lamp السلك المتوهج لمبة نرنست المتوهجة القضيب المتوهج لمبة الزئبق القوسية ذات الضغط العالي MONOCHROMATORS: Grating: The most modern equipment use GRATING to separate different wavelengths of infrared rays after passing through the sample. One disadvantage in GRATING is the increase of the amount of radiation scattered. To overcome this problem PRISM or FILTER is used with GRATING at the same time. PRISM is made from flint glass to be IR transparent. Sample cell Samples can be used as liquid or solid or gaseous. The thickness of the sample should be too small, therefore minute metal cells fitted with two windows are used to permit the passage of radiation through the sample. The material used of windows should not absorb IR in the measurement area. Windows material The wavelength of the rays that pass without absorption NaCl 40,000 - 625 cm-1 KBr 40,000 - 400 cm-1 AgCl 25,000 - 435 cm-1 Cesium bromide 10,000 - 270 cm-1 Cesium iodide 10,000 - 200 cm-1 Germanium 20,000 - 600 cm-1 Polyethylene 625 - 33 cm-1 Windows must be cleaned by organic solvents only and keep them away from water because they melt in water. In case of aqueous samples, AgCl windows could be used. Gas samples: Cylindrical cells made of Pyrex glass with length 10 cm are used. The windows of cells are made of sodium chloride or calcium fluoride or potassium bromide Liquid samples: Placed a thin film of pure neat sample of about 0.01 mm (1-10 mg sample) between two disks of material salts of sodium chloride, or calcium fluoride, or potassium bromide. In case of very small samples, ultra micro cavity cells fitted with beam condenser are used. Solid samples: Sample (2 – 5 mg) is crushed in carborundum container, attached in a liquid high molecular weight (mulling oil named Nujol), and then placed them thin film called mulls. Solid sample can also prepared in a form of a compact disc. 1 mg sample is mixed carefully and homogeneously with approximately with 100 mg of potassium bromide by dry ball mill, and then compressing the mixture under a pressure of up to 20,000 -50,000 lb / in2 Stuff used in the processing of samples for IR spectral analysis: Metal blocks KBr Die sets for KBr Discs Laboratory hydrolyic press product Detector 1. Thermocouple detector 2. Bolometer detector 3. Golay cell detector IR gas sampling supplies cells Thermocouple detector Bolometer Golay Cell Recorder Recording unit is used to record the absorption either at different wavelengths wavelength (nm), or different wave numbers (cm-1), and thus can be recorded absorption spectrum in the long-desired In general, IR spectrum of organic compounds can be divided into two parts: High frequency portion: absorption of function groups appeared in this region 1300-3600 cm-1 Low frequency portion: absorption of aromatic groups appeared in this region 650-909 cm-1 More distinct division into four areas, namely: 1- Region between 2700 – 3600 cm-1 A special area for stretching between hydrogen atom and other high atomic weight atoms such as oxygen or nitrogen or carbon, O-H, N-H, C-H 2- Region between 1850 – 2700 cm-1 A special area for triple bonds CC , CN 3- Region between 1500 – 1850 cm-1 A special area for double bonds C=N, C=O, C=C 4- Region between 700 – 1500 cm-1 A special area for finger print. This region for single bonds between carbon atoms and atoms other than hydrogen, such as C-C, C-O, C-Cl and others, any links that are the basic structure of the molecule. Compound Group IR Absorption νmax cm-1 C=C Alkene 1690 – 1600 CC Alkyne 3300 C=O Ketone 1700 – 1750 -COH Aldehyde 1700 – 1750 -COOH Carboxyl 3520 -NH2 Amido 3400 – 3180 -NO2 Nitro 1850 – 1555 -CN Nitrile 2250 – 2225 -S=O Sulfoxide 2600 – 2550 O=S=O Sulfone 2600 – 2550 R-OH Alcohol –OH 3650 – 3584 Ar-OH Phenol –OH 3650 – 3584 Phenyl Aromatic structure 909 – 650 INTERPRETING AN INFRA-RED SPECTRUM This explains how to use an infra-red spectrum to identify the presence of a few simple bonds in organic compounds. Ethanoic acid Ethanoic acid has the structure: You have seen that 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 The carbon-carbon bond has absorptions which occur over a wide range of wavenumbers in the fingerprint region that makes it very difficult to pick out on an infra-red spectrum. The carbon-oxygen single bond also has an absorbtion in the fingerprint region, varying between 1000 and 1300cm-1 depending on the molecule it is in. You have to be very wary about picking out a particular trough as being due to a C-O bond. The other bonds in ethanoic acid have easily recognised absorptions outside the fingerprint region. The C-H bond (where the hydrogen is attached to a carbon which is singly-bonded to everything else) absorbs somewhere in the range from 2853 - 2962 cm-1. Because that bond is present in most organic compounds, that's not terribly useful! What it means is that you can ignore a trough just under 3000 cm-1, because that is probably just due to C-H bonds. The carbon-oxygen double bond, C=O, is one of the really useful absorptions, found in the range 1680 - 1750 cm-1. Its position varies slightly depending on what sort of compound it is in. The other really useful bond is the O-H bond. This absorbs differently depending on its environment. It is easily recognised in an acid because it produces a very broad trough in the range 2500 - 3300 cm-1. The infra-red spectrum for ethanoic acid looks like this: The possible absorption due to the C-O single bond is queried because it lies in the fingerprint region. You couldn't be sure that this trough wasn't caused by something else. Ethanol The O-H bond in an alcohol absorbs at a higher wavenumber than it does in an acid - somewhere between 3230 - 3550 cm-1. In fact this absorption would be at a higher number still if the alcohol isn't hydrogen bonded for example, in the gas state. All the infra-red spectra on this page are from liquids - so that possibility will never apply. Notice the absorption due to the C-H bonds just under 3000 cm-1, and also the troughs between 1000 and 1100 cm-1 one of which will be due to the C-O bond. Ethyl ethanoate This time the O-H absorption is missing completely. Don't confuse it with the C-H trough fractionally less than 3000 cm-1. The presence of the C=O double bond is seen at about 1740 cm-1. The C-O single bond is the absorption at about 1240 cm-1. Whether or not you could pick that out would depend on the detail given by the table of data which you get in your exam, because C-O single bonds vary anywhere between 1000 and 1300 cm-1 depending on what sort of compound they are in. Some tables of data fine it down, so that they will tell you that an absorption from 1230 - 1250 is the C-O bond in an ethanoate. The infra-red spectrum for a ketone Propanone You will find that this is very similar to the infra-red spectrum for ethyl ethanoate, an ester. Again, there is no trough due to the O-H bond, and again there is a marked absorption at about 1700 cm-1 due to the C=O. Confusingly, there are also absorptions which look as if they might be due to C-O single bonds - which, of course, aren't present in propanone. This reinforces the care you have to take in trying to identify any absorptions in the fingerprint region. Aldehydes will have similar infra-red spectra to ketones. The infra-red spectrum for a hydroxy-acid 2-hydroxypropanoic acid (lactic acid) This is interesting because it contains two different sorts of O-H bond - the one in the acid and the simple "alcohol" type in the chain attached to the -COOH group. The O-H bond in the acid group absorbs between 2500 and 3300, the one in the chain between 3230 and 3550 cm-1. Taken together, that gives this immense trough covering the whole range from 2500 to 3550 cm-1. Lost in that trough as well will be absorptions due to the C-H bonds. Notice also the presence of the strong C=O absorption at about 1730 cm-1. The infra-red spectrum for a primary amine 1-aminobutane Primary amines contain the -NH2 group, and so have N-H bonds. These absorb somewhere between 3100 and 3500 cm-1. That double trough (typical of primary amines) can be seen clearly on the spectrum to the left of the C-H absorptions.