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Infrared Spectroscopy<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

1. Introduction

The light our eyes see is but a small part of a broad spectrum of electromagnetic radiation. On the immediate high energy side of the visible spectrum lies the UV, and on the low energy side is the infrared. The portion of the IR region most useful for analysis of organic compounds is not immediately adjacent to the visible spectrum, but is that having a wavelength range from 2,500 to 16,000 nm, with a corresponding frequency range from 1.9*10¹³ to 1.2*10¹⁴ Hz.

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Photon energies associated with this part of the IR (from 1 to 15 kcal/mole) are not large enough to excite electrons, but may induce vibrational excitation of covalently bonded atoms and groups. The covalent bonds in molecules are not rigid sticks or rods, such as found in molecular model kits, but are more like stiff springs that can be stretched and bent. The mobile nature of organic molecules was noted in the conformational isomers. We must now recognize that, in addition to the facile rotation of groups about single bonds, molecules experience a wide variety of vibrational motions, characteristic of their component atoms. Consequently, virtually all organic compounds will absorb IR radiation that corresponds in energy to these vibrations. IR spectrometers, similar in principle to the UV-Visible spectrometer, permit chemists to obtain absorption spectra of compounds that are a unique reflection of their molecular structure. An example of such a spectrum is that of the flavoring agent vanillin, shown below.

The complexity of this spectrum is typical of most IR spectra, and illustrates their use in identifying substances. The gap in the spectrum between 700 & 800 cm^-1 is due to solvent (CCl₄) absorption. Further analysis (below) will show that this spectrum also indicates the presence of an aldehyde function, a phenolic hydroxyl and a substituted benzene ring. The inverted display of absorption, compared with UV-Visible spectra, is characteristic. Thus a sample that did not absorb at all would record a horizontal line at 100% transmittance (top of the chart).

The frequency scale at the bottom of the chart is given in units of reciprocal centimeters (cm^-1) rather than Hz, because the numbers are more manageable. The reciprocal cm is the number of wave cycles in one cm; whereas, frequency in cycles per second or Hz is equal to the number of wave cycles in 3*10¹⁰ cm (the distance covered by light in one second). Wavelength units are in microns (μ), instead of nm for the same reason.

Most IR spectra are displayed on a linear frequency scale, as shown here, but in some older texts a linear wavelength scale is used. A calculator for interconverting these frequency and wavelength values is provided on the right.

IR spectra may be obtained from samples in all phases (liquid, solid and gas). Liquids are usually examined as a thin film sandwiched between two polished salt plates (note that glass absorbs IR radiation, whereas NaCl is transparent). If solvents are used to dissolve solids, care must be taken to avoid obscuring important spectral regions by solvent absorption. Perchlorinated solvents such as CCl4, CHCl3 and tetrachloroethene are commonly used. Alternatively, solids may either be incorporated in a thin KBr disk, prepared under high pressure, or mixed with a little non-volatile liquid and ground to a paste (or mull) that is smeared between salt plates.

2. Vibrational Spectroscopy

A molecule composed of n-atoms has 3n degrees of freedom, six of which are translations and rotations of the molecule itself. This leaves 3n-6 degrees of vibrational freedom (3n-5 if the molecule is linear). Vibrational modes are often given descriptive names, such as stretching, bending, scissoring, rocking and twisting. The four-atom molecule of formaldehyde, the gas phase spectrum of which is shown below, provides an example of these terms. We expect six fundamental vibrations (12 minus 6), and these have been assigned to the spectrum absorptions.

The exact frequency at which a given vibration occurs is determined by the strengths of the bonds involved and the mass of the component atoms. In practice, IR spectra do not normally display separate absorption signals for each of the 3n-6 fundamental vibrational modes of a molecule. The number of observed absorptions may be increased by additive and subtractive interactions leading to combination tones and overtones of the fundamental vibrations, in much the same way that sound vibrations from a musical instrument interact. Furthermore, the number of observed absorptions may be decreased by molecular symmetry, spectrometer limitations, and spectroscopic selection rules. One selection rule that influences the intensity of IR absorption, is that a change in dipole moment should occur for a vibration to absorb IR energy. Absorption bands associated with C=O bond stretching are usually very strong because a large change in the dipole takes place in that mode.
Some General Trends:

i) Stretching frequencies are higher than corresponding bending frequencies. (It is easier to bend a bond than to stretch or compress it.)
ii) Bonds to hydrogen have higher stretching frequencies than those to heavier atoms.
iii)Triple bonds have higher stretching frequencies than corresponding double bonds, which in turn have higher frequencies than single bonds.(Except for bonds to hydrogen).

The general regions of the IR spectrum in which various kinds of vibrational bands are observed are outlined in the following chart. Note that the blue colored sections above the dashed line refer to stretching vibrations, and the green colored band below the line encompasses bending vibrations. The complexity of IR spectra in the 1450 to 600 cm^-1 region makes it difficult to assign all the absorption bands, and because of the unique patterns found there, it is often called the fingerprint region. Absorption bands in the 4000 to 1450 cm^-1 region are usually due to stretching vibrations of diatomic units, and this is sometimes called the group frequency region.

3. Group Frequencies

Detailed information about the infrared absorptions observed for various bonded atoms and groups is usually given here. The following table provides a collection of such data for the most common functional groups. Following the color scheme of the chart, stretching absorptions are listed in the blue-shaded section and bending absorptions in the green shaded part.Most organic compounds have C-H bonds, a useful rule is that absorption in the 2850 to 3000 cm^-1 is due to sp³ C-H stretching; whereas, absorption above 3000 cm^-1 is from sp² C-H stretching or sp C-H stretching if it is near 3300 cm^-1.

Typical Infrared Absorption Frequencies
	Stretching Vibrations			Bending Vibrations
Functional Class	Range (cm-1)	Intensity	Assignment	Range (cm-1)	Intensity	Assignment
Alkanes	2850-3000	str	CH3, CH2 & CH 2 or 3 bands	1350-1470 1370-1390 720-725	med med wk	CH2 & CH3 deformation CH3 deformation CH2 rocking
Alkenes	3020-3100 1630-1680 1900-2000	med var str	=C-H & =CH2 (usually sharp) C=C (symmetry reduces intensity) C=C asymmetric stretch	880-995 780-850 675-730	str med med	=C-H & =CH2 (out-of-plane bending) cis-RCH=CHR
Alkynes	3300 2100-2250	str var	C-H (usually sharp) C≡C (symmetry reduces intensity)	600-700	str	C-H deformation
Arenes	3030 1600 & 1500	var med-wk	C-H (may be several bands) C=C (in ring) (2 bands) (3 if conjugated)	690-900	str-med	C-H bending & ring puckering
Alcohols & Phenols	3580-3650 3200-3550 970-1250	var str str	O-H (free), usually sharp O-H (H-bonded), usually broad C-O	1330-1430 650-770	med var-wk	O-H bending (in-plane) O-H bend (out-of-plane)
Amines	3400-3500 (dil. soln.) 3300-3400 (dil. soln.) 1000-1250	wk wk med	N-H (1°-amines), 2 bands N-H (2°-amines) C-N	1550-1650 660-900	med-str var	NH2 scissoring (1°-amines) NH2 & N-H wagging (shifts on H-bonding)
Aldehydes & Ketones	2690-2840(2 bands) 1720-1740 1710-1720 1690 1675 1745 1780	med str str str str str str	C-H (aldehyde C-H) C=O (saturated aldehyde) C=O (saturated ketone) aryl ketone α, β-unsaturation cyclopentanone cyclobutanone	1350-1360 1400-1450 1100	str str med	α-CH3 bending α-CH2 bending C-C-C bending
Carboxylic Acids & Derivatives	2500-3300 (acids) overlap C-H 1705-1720 (acids) 1210-1320 (acids) 1785-1815 ( acyl halides) 1750 & 1820 (anhydrides) 1040-1100 1735-1750 (esters) 1000-1300 1630-1695(amides)	str str med-str str str str str str str	O-H (very broad) C=O (H-bonded) O-C (sometimes 2-peaks) C=O C=O (2-bands) O-C C=O O-C (2-bands) C=O (amide I band)	1395-1440 1590-1650 1500-1560	med med med	C-O-H bending N-H (1¡-amide) II band N-H (2¡-amide) II band
Nitriles Isocyanates,Isothiocyanates, Diimides, Azides & Ketenes	2240-2260 2100-2270	med med	C≡N (sharp) -N=C=O, -N=C=S -N=C=N-, -N3, C=C=O

To illustrate the usefulness of IR absorption spectra, eg. five C₄H₈O isomers are presented below their corresponding structural formulas. Try to associate each spectrum (A - E) with one of the isomers in the row above it. When you have made assignments check your answers by clicking on the structure or name of each isomer.

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4. Other Functional Groups

IR absorption data for some functional groups not listed in the preceding table are given below. Most of the absorptions cited are associated with stretching vibrations. Standard abbreviations (str = strong, wk = weak, brd = broad & shp = sharp) are used to describe the absorption bands.