Certain atomic movements give rise to bands that occur in approximately the same position in a large variety of compounds and are only slightly affected by the rest of the molecule. These vibrations are assigned to groups of atoms termed functional groups. Although absorption bands are characteristic of the molecule as a whole, it is a useful approximation to consider that molecular vibrations are localized in particular functional groups. This allows one to relate absorption band position with a particular functional group and to tabulate these relationships. The tables showing the positions where the functional group absorptions occur are called correlation tables. The intensity of the absorption bands is also shown on good correlation tables.
Most of the spectral features that allow us to readily identify functional groups are found in the left part of the spectrum. The right hand portion of the spectrum is more complex, and each peak is not readily identified with a particular part of the molecule. The entire spectral pattern is unique for a given compound.
The steps used by a chemist to find information about molecular structure from the IR spectrum are as follows:
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1. Obtain a spectrum of the material on an IR spectrophotometer.
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2. Using information from correlation tables and absorbances from the functional group region of the spectrum, identify the functional groups that are present or sometimes more importantly absent.
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3. Compare this spectrum with those of known compounds or obtain a known sample of a suspected material and run its spectrum for comparison.
Many of the group frequencies vary over a wide range because the bands arise from complex interacting vibrations within the molecule. Absorption bands may, however, represent predominantly a single vibrational mode. Certain absorption bands, for example, those arising from C-H, O-H, and C=O stretching modes, remain within fairly narrow regions of the spectrum.
The two important areas for a preliminary examination of a spectrum are the region 4000-1300 cm-1 and the 909-650 cm-1 region. The high frequency portion of the spectrum is called the functional group region. The characteristic stretching frequencies for important functional groups such as OH, NH, and C=O occur in this portion of the spectrum. The absence of absorption in the assigned ranges for the various functional groups can usually be used as evidence for the absence of such groups from the molecule. The absence of absorption in the 1850-1540 cm-1 region excludes a structure containing a carbonyl group. Strong skeletal bands for aromatics and heteroaromatics fall in the 1600-1300 cm-1 region of the spectrum. These skeletal bands arise from the stretching of the carbon-carbon bonds in the ring structure.
The lack of strong absorption bands in the 909-650 cm-1 region generally indicates a nonaromatic structure. Aromatic and heteroaromatic compounds display strong out-of-plane C-H bending and ring bending absorption bands in this region.
The intermediate portion of the spectrum, 1300-909 cm-1 is usually referred to as the fingerprint region. The absorption pattern in this region is complex, with bands originating in interacting vibrational modes. Absorption in this intermediate region is probably unique for every molecular species.
Conclusions reached after examination of a particular band should be confirmed by examination of other portions of the spectrum if possible. For example the assignment of a carbonyl band to the presence of an ester should be confirmed by observation of a strong band in the C-O stretching region, 1300-1100 cm-1.
Characteristic Group Frequencies of Organic Molecules
Table 1 Characteristic Infrared Group Frequencies
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Class
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Group
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Wavenumber (cm-1) Alkane
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C-H
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2850-3000
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C-C
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800-1000
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Aromatic
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C-H
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3000-3100
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C=C
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1450-1600
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Alkene
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C-H
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3080-3140
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C=C
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1630-1670
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Alkyne
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C-H
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3300-3320
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C(C
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2100-2140
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Alcohol
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O-H
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3400-3600
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C-O
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1050-1200
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Ether
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C-O
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1070-1150
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Aldehyde
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C=O
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1720-1740
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C-H
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2700 &2900
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Carboxylic Acids
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C=O
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1700-1725
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O-H
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2500-3000
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Ester
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C=O
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1735-1750
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C-O
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1000-1300 (2 bands)
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Ketone
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C=O
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1700-1780
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Hydrocarbons
Hydrocarbons are classified as saturated or unsaturated based on the absence or presence of multiple bonds. The presence of multiple bonds decreases the number of hydrogens from the number in a saturated compound of formula CnH2n+2 . The decrease in number of hydrogens alone does not confirm the presence of a multiple bond. For example, 1-octene and cyclooctane have the same molecular formula, C8H16. What are the structural features that are present in 1-octene that are absent in cyclooctane? The carbon-carbon double bond and sp2-hybridized C-H bonds distinguish 1-octene from cyclooctane. It is characteristic group absorbances of these structures that will be present in the spectrum of 1-octene and absent in the spectrum of cyclooctane.
The energy of the infrared light absorbed by a C-H bond depends on the hybridization of the hybrid orbital. The bond strengths of carbon-hydrogen bonds are in the order of sp3>sp2>sp, because the increased s character of the hybrid gives better overlap with the hydrogen s-orbital. The sp3-hybridized C-H bonds in saturated hydrocarbons like octane absorb in the 2850-3000 cm-1 region. The sp2-hybridized C-H bonds in alkenes such as 1-octene absorbs at 3080 cm-1. A sp-hybridized C-H bond in a molecule such as 1-octyne absorbs infrared at 3320 cm-1.
Hydrocarbons can also be classified based on absorptions due to the carbon-carbon bond. Carbon-carbon bond strength increases in the order of singledoubletriple. Therefore, the wavenumber position of the absorption corresponding to the stretching of these bonds increases in the same order. Saturated hydrocarbons all contain carbon-carbon single bonds that absorb in the 800-1000 cm-1 region. Unsaturated hydrocarbons also contain carbon-carbon single bonds that absorb in this same region. This is not a very diagnostic region because we already know that most organic compounds have carbon-carbon single bonds.
Alkenes are identified by the absorption of the carbon-carbon double bond, which occurs in the 1630-1670 cm-1 region. Terminal alkenes have the most intense absorptions as the absorption decreases with increased substitution. Alkyne C(C stretches occur in a region of the IR spectrum where very little else appears. The alkyne carbon-carbon stretch occurs in the range of 2100-2260 cm-1. The intensity can very dramatically since the dipole moment change depends entirely upon what is attached to each carbon. Terminal alkynes, alkynes which have an H attached to one of the alkyne carbons, generally display greater band intensities as well as characteristic signals near 3300 cm-1.
Activity Four
Provide students with unlabelled spectra for octane, 1-octene, and 1-octyne (or any other straight chain hydrocarbons with the same number of carbon atoms). Ask students to assign the hydrocarbon to its spectra and justify their selections.
Oxygen-Containing Compounds
Many functional groups contain oxygen. These functional groups have the characteristic infrared absorptions given in Table 1. The characteristic group frequencies of aldehydes and ketones are from 1700-1780 cm-1. The carbon-oxygen double bond of carbonyl compounds requires more energy to stretch than does the carbon-oxygen single bond of ethers and alcohols. Therefore, aldehydes and ketones absorb infrared at higher wavenumber positions than alcohols and ethers. Since the carbonyl is highly polar, stretching of this bond results in a relatively large change in dipole moment producing an intense band. The carbonyl region is also free of conflicting absorptions making the recognition of the carbonyl band easy. Carefully examining the precise wavenumber of the C=O stretch, as well as the presence or absence of other signals, will usually allow one to distinguish among the many possible C=O containing compounds.
The position of the carbonyl group absorption of acyl derivatives depends on the inductive and resonance effects of atoms bonded to the carbonyl carbon atom. We can represent a carbonyl group by two contributing resonance structures. (See Figure 3) Since less energy is required to stretch a single bond than a double bond, any structural feature that stabilizes the contributing polar resonance form with a carbon-oxygen single bond will cause the infrared absorption to occur at lower wavenumber position. In other words, any group that donates electrons by resonance causes a shift in the absorption to lower wavenumbers. For example, the nitrogen atom of amides is very effective in donation of electrons to the carbonyl carbon atom. (See Figure 4) Therefore, the double bond character of the carbonyl decreases. As a result, in amides the carbonyl group absorbs in the 1650-1690 cm-1 region, which is at a lower wavenumber than for aldehydes or ketones.
When carbonyls (or other multiple bonds) are in conjugation with another double or triple bond a resonance form can be drawn in which the carbonyl oxygen bears a negative charge. The contribution of this resonance form reduces the double bond character of the carbonyl shifting the absorption to a lower frequency. (See Figure 5)
The characteristic bands observed for alcohols result from O-H stretching in addition to C-O stretching. The carbon-oxygen stretching vibration of alcohols appears in a region complicated by many other absorptions, the fingerprint region. The presence of a hydroxyl is better established by the O-H stretching. The shape and frequency of an O-H band depends on hydrogen bonding. As hydrogen bonding becomes stronger, O-H stretches appear at lower frequencies. In the vapor phase or in dilute solution in nonpolar solvents “free” hydroxyl group of alcohols absorbs strongly around 3600 cm-1. As the concentration of the solution increases intermolecular hydrogen bonding increases and we see additional bands start to appear at lower frequencies, 3550-3200 cm-1 and the “free” hydroxyl band decreases. When the hydrogen of a hydroxyl group is involved in a hydrogen bond a resonance form can be drawn in which the oxygen bears a negative charge. The contribution of this resonance form reduces the single bond character of the hydroxyl bond shifting the absorption to a lower frequency. (See Figure 6)
Carboxylic acids tend to form strongly hydrogen bonded dimers which shift the O-H stretch to frequencies lower than 3000, however, carboxylic O-H stretches can occur anywhere between 2500-3300 depending upon the strength of hydrogen bonding.
A process of elimination can identify ethers. If a compound contains oxygen and the infrared spectrum lacks absorptions characteristic of a carbonyl group or a hydroxyl group, we may conclude that the compound is an ether.
Activity Five
As we talk about the characteristic bands for functional groups students will be given actual IR spectra that illustrate each of the bands being discussed. At the culmination of the activity they will be given the spectra of carefully selected unknown compounds and will be asked to determine which functional groups are present and/or absent.
Activity Six
Students will be given the IR spectrum and other data about a particular compound, such as elemental analysis and molecular mass. They will be asked to draw a structural formula for the compound consistent with the information given and be required to justify their choice of structure.
Investigation 1: What factors affect the frequency of oscillation of a swing?
Work with a partner to design and conduct an experiment to determine what factors, if any, affect the frequency of oscillation of a swing. Frequency is defined as the number of complete oscillations per unit of time. For the purposes of comparison report your frequency in oscillations/minute. As you plan keep in mind the variables you want to control and the ones you want to test. The experiment will be conducted across the street at College Woods.
After gathering your data address the following questions in the results and conclusions section of your lab report.
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1.
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From your data, which factor(s) affect the frequency of oscillation of a swing?
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2.
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For each factor that you found affected the frequency, describe the relationship between frequency and the factor. In other words, how does varying the factor change the frequency?
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Investigation 2: Masses on a spring
The bonds between atoms may be modeled using masses connected by springs. If one mass is held stationary and the other is allowed to move, the stretching vibration of a bond between the two atoms may be modeled by stretching the spring and releasing it.
Using the model above investigate the effect of mass on the frequency of vibration and the effect of bond strength on the frequency of vibration.
After gathering your data, address the following questions in the results and conclusions section of your lab report.
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1.
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Did the mass affect the frequency of vibration? If so, quantitatively describe the relationship between mass and frequency?
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2. Did the number of springs or spring tightness affect the frequency of vibration? If so, describe the relationship between spring tightness and the frequency.
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3. Relate what you discovered about the effects of mass and spring strength to atoms joined by a chemical bond. Make a statement about the relationship between atomic mass and frequency and bond strength and frequency.
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4. Compare your results from this investigation to the results for the swing in Investigation 1.
Worksheet 1: Predicting Vibrational Frequencies
Consider the following hypothetical diatomic molecules C-H, C-C, and C-I. The carbon atom is common to the molecules. If we were to hold the carbon atom in a clamp and represent the bond by a spring we could attach masses representing the H, C, and I atoms one by one and measure the frequency of vibration as we did in investigation 2. In your investigation you found that the frequency of vibration was inversely related to the mass (as mass increased the frequency decreased). In the infrared spectrum the infrared vibrations of these atoms will occur at:
C-H, 3000 cm-1 C-C, 1000 cm-1 C-I, 500 cm-1
This is consistent with our observation that as we increased the mass the frequency decreased.
If the diatomic grouping consisted of C-O, near to which of the three frequencies would you expect to find the absorption band?
Would the frequency of the absorption band be higher or lower than this frequency? Why?
Suppose now we connect two of these diatomic groupings having sufficiently different frequencies to make a hypothetical triatomic grouping. We now have two bond springs and there will be two ways to stretch each grouping.
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( ( ( (
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H - C - C
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C - C - I
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( ( ( (
( (: direction of motion of the atoms when the two springs are vibrating.
The bonds in the H-C-C grouping are found to vibrate practically independently, when the C-H bond vibrates the C-C hardly changes, and the spectrum of this compound would show two absorption bands at 3000 cm-1 and 1000 cm-1. Similarly in the C-C-I grouping the two vibrations are shown to be almost independent of each other and to occur very close to their diatomic positions.
Where would you expect to find the absorption bands for the C-C-I grouping?
Worksheet 1
Where would you expect to find the absorption bands for the grouping H-C-I ?
If we were to link two identical bonds together to make a C-C-C grouping we would still get two absorption bands but these would involve both of the bonds vibrating. The two frequencies would arise from an in-phase and an out-of-phase vibrational motion.
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(
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C- C- C
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C- C- C
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( ( ( (
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symmetric
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asymmetric
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in-phase
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out-of-phase
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The two vibrations are termed symmetric and asymmetric. The two bonds do not move independently of each other, therefore, we say there is interaction between the groups and the frequencies of vibration will be displaced from the diatomic frequency.
In which of these groupings will there be symmetric and asymmetric vibrations?
H-C-I C- C- I O- C- O
If each bond is represented by a spring then the C-N will have one spring, the C=N will have two springs and the C(N will have three. This means the strength of the bond has been increased as you go from C-N to C(N. If the strength of the spring has been increased and the atoms remain constant the frequency of the vibration will also be increased. The frequencies of these groups are approximately:
C-N, 1070 cm-1 C=N, 1650 cm-1 C(N, 2250 cm-1
Consider the following triatomic groupings, which will have symmetric and asymmetric vibrations? Why?
N-C=N N=C=N N-C(N
Worksheet 2: Deformations
Vibrations other than the stretching of a bond also occur. These vibrations are called deformations and they refer to the bond angle changing between the atoms of a molecule. These deformations occur at frequencies lower than those of the stretching vibrations. If we consider groups of atoms of the type XY2 there are some general descriptions of the deformational vibrations we can apply to these groups.
The terms used are scissors, rock, wag, and twist. These four motions may be further divided into in-plane and out-of-plane motions of the atoms.
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H(
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In-plane symmetric deformation
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C
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(scissors)
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H(
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If you think of the C as the pivot and the H as the points of a pair of scissors then there is a plane through all of the atoms and the motion is in-plane.
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C
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In-plane asymmetric deformation
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(rock)
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H H
Consider the hydrogens to be the tips of a rocker on a rocking chair and the carbon is sitting on the chair. There is a plane through all of the atoms. This is a poor group frequency because all of the atoms move.
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H +
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Out-of-plane symmetric deformation
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C
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(wag)
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H +
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Consider the C to be the body of a horse and the H to be the tail. When the horse wags its tail the motion is from side to side and out of the plane of the horse (in the diagram the plane of the paper). The “+” indicates motion perpendicular to the plane of the paper. This is also a poor group frequency.
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+ H H -
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Out-of-plane asymmetric deformation
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C
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(twist)
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Look at the diagram of the CH2ClBr molecule and consider the CH2 part only. What is the total number of vibrations that may occur?
Worksheet 2
The infrared spectrum has in its composition three types of absorption bands, fundamental, combination, and overtone. When we stretched the masses on the springs we observed that they vibrated at a certain frequency, this is the fundamental frequency for that system. The masses were vibrating with simple harmonic motion.
If a molecule with a fundamental vibration occurring at a frequency (1 is subjected to radiation at a frequency 2(1 , an absorption may also be observed at this frequency and is called an overtone absorption band. This does not mean that the molecule itself is vibrating at 2(1 , but the fundamental is being excited by the radiation at twice its frequency and such bands are generally much weaker than the fundamental. If a molecule has two different fundamental absorptions at (a and (b it is possible that an absorption may be observed at frequencies corresponding to (a + (b and (a -(b and these are called combination bands. Combination bands will usually be weaker than the fundamentals involved.
What would be the frequency of the fundamental absorption if its first overtone absorption was observed at 2000 cm-1?
