At this point we are ready to leave diatomic molecules and start talking about complex organic molecules. Before doing so, it should be pointed out that the discussion that follows is an oversimplification of the true vibrational behavior of molecules. Many vibrational motions of molecules are motions that involve the entire molecule. Analysis of such motions can be very difficult if you are dealing with substances of unknown structure. Fortunately, the infrared spectrum can be divided into two regions, one called the functional group region and the other the fingerprint region. The functional group region is generally considered to range from 4000 to approximately 1500 cm-1 and all frequencies below 1500 cm-1 are considered characteristic of the fingerprint region. The fingerprint region involves molecular vibrations, usually bending motions, that are characteristic of the entire molecule or large fragments of the molecule. Hence the origin of the term. Used together, both regions are very useful for confirming the identity of a chemical substance. This is generally accomplished by a comparison of the spectrum of an authentic sample. As you become more proficient in analyzing infrared spectra, you may begin to assign bands in this region. However, if you are just beginning to interpret spectra of organic molecules, it is best to focus on identifying the characteristic features in the functional group region. The functional group region tends to include motions, generally stretching vibrations, that are more localized and characteristic of the typical functional groups found in organic molecules. While these bands are not very useful in confirming identity, they do provide some very useful information about the nature of the components that make up the molecule. Perhaps most importantly, the frequency of these bands are reliable and their presence or absence can be used confidently by both the novice and expert interpreter of infrared spectra. The discussion which follows focuses primarily on the functional group region of the spectrum. Some functional groups are discussed in more detail than others. You will find that all this information is summarized in Table 1 which should prove useful to you when you try to interpret an unknown spectrum. Finally, you should bear in mind that although we have developed a model that can help us understand the fundamental processes taking place in infrared spectroscopy, interpretation of spectra is to a large extent an empirical science. Information about the nature of a compound can be extracted not only from the frequencies that are present but also by peak shape and intensity. It is very difficult to convey this information in Table form. Only by examining real spectra will you develop the expertise to accurately interpret the information contained within. Be sure to examine the spectra contained in this handout carefully. Whenever you interpret a spectrum and extract structural information, check your assignments by examining the spectrum of a known substance that has similar structural features.
Factors Influencing the Location and Number of Peaks
Before beginning a detailed analysis of the various peaks observed in the functional group region, it might be useful to mentioned some of the factors that can influence the location and number of peaks we observe in infrared spectroscopy. Theoretically, the number of fundamental vibrations or normal modes available to a polyatomic molecule made up of N atoms is given by 3N-5 for a totally linear molecule and 3N-6 for all others. By a normal mode or fundamental vibration, we mean the simple independent bending or stretching motions of two or more atoms, which when combined with all of normal modes associated with the remainder of the molecule will reproduce the complex vibrational dynamics associated with the real molecules. Normal modes are determined by a normal coordinate analysis (which will not be discussed in this presentation). If each of these fundamental vibrations were to be observed, we would expect either 3N-5 or 3N-6 infrared bands. There are some factors which decrease the number of bands observed and others that cause an increase in this number. Let's discuss the latter first.
We have already mentioned overtones, which are absorption of energy caused by a change of 2 rather than 1 in the vibrational quantum number. While overtones are usually forbidden transitions and therefore are weakly absorbing, they do give rise to more bands than expected. Overtones are easily identified by the presence of a strongly absorbing fundamental transition at slightly more than half the frequency of the overtone. On occasion, combination bands are also observed in the infrared. These bands, as their name implies, are absorption bands observed at frequencies such as 1 + 2 or 1 - 2, where 1 and 2 refer to fundamental frequencies. Other combinations of frequencies are possible. The symmetry properties of the fundamentals play a role in determining which combinations are observed. Fortunately, combination bands are seldom observed in the functional group region of most polyatomic molecules and the presence of these bands seldom cause a problem in identification. Another cause of splitting of bands in infrared is due to a phenomena called Fermi Resonance. While a discussion of Fermi Resonance is beyond the scope of this presentation, this splitting can be observed whenever two fundamental motions or a fundamental and combination band have nearly the same energy (i.e. 1 and 22 or 1 and 2 + 3). In this case, the two levels split each other. One level increases while the other decreases in energy. In order to observe Fermi Resonance, in addition to the requirement that a near coincidence of energy levels occurs, other symmetry properties of these vibrations must also be satisfied. As a consequence, Fermi Resonance bands are not frequently encountered.
There are also several factors which decrease the number of infrared bands observed. Symmetry is one of the factors that can significantly reduce the number of bands observed in the infrared. If stretching a bond does not cause a change in the dipole moment, the vibration will not be able to interact with the infrared radiation and the vibration will be infrared inactive. Other factors include the near coincidence of peaks that are not resolved by the spectrometer and the fact that only a portion of the infrared spectrum is usually accessed by most commercial infrared spectrometers.
This concludes the general discussion of infrared spectroscopy. At this point we are ready to start discussing some real spectra.
Carbon-Hydrogen Stretching Frequencies
Let's take one more look at equation 7 and consider the carbon-hydrogen stretching frequencies. Since k and mH are the only two variables in this equation, if we assume that all C-H
stretching force constants are similar in magnitude, we would expect the stretching frequencies of all C-H bonds to be similar. This expectation is based on the fact that the mass of a carbon atom and whatever else is attached to the carbon is much larger the mass of a hydrogen. The reduced mass for vibration of a hydrogen atom would be approximately the mass of the hydrogen atom which is independent of structure. All C-H stretching frequencies are observed at approximately 3000 cm-1, exactly as expected. Fortunately, force constants do vary some with structure in a fairly predictable manner and therefor it is possible to differentiate between different types of C-H bonds. You may recall in your study of organic chemistry, that the C-H bond strength increased as the s character of the C-H bond increased. Some typical values are given below in Table 2 for various hydridization states of carbon. Bond strength and bond stiffness measure different properties. Bond strength measures the depth of the potential energy well associated with a C-H. Bond stiffness is a measure of how much energy it takes to compress or stretch a bond. While these are different properties, the stiffer bond is usually associated with a deeper potential energy surface. You will note in Table 2 that increasing the bond strength also increases the C-H bond stretching frequency.
Methyl groups, methylene groups and methine hydrogens on sp3 carbon atoms all absorb between 2850 and 3000 cm-1. While it is sometimes possible to differentiate between these types of hydrogen, the beginning student should probably avoid this type of interpretation. It should be
pointed out however, that molecules that have local symmetry, will usually show symmetric and asymmetric stretching frequencies. Take, for example, a CH2 group. It is not possible to isolate an individual frequency for each hydrogen. These two hydrogens will couple and will show two stretching frequencies, a symmetric stretching frequency in which stretching and compression of both hydrogens occurs simultaneously, and an asymmetric stretching frequency in which stretching of one hydrogen is accompanied by compression of the other. While these two motions will occur at different frequencies, both will be found between the 2850-3000 cm-1 envelope. Similarly for a CH3 group, symmetric and asymmetric vibrations are observed. This behavior is found whenever this type of local symmetry is present. We will find other similar examples in the functional groups we will be discussing. Some examples of spectra containing only sp3 hybridization can be found in Figures 5-6, and located at the end of this discussion. These peaks are usually sharp and of medium intensity. Considerable overlap of several of these bands usually results in absorption that is fairly intense and broad in this region.
Hydrogens attached to sp2 carbons absorb at 3000-3250 cm-1. Both aromatic and vinylic carbon hydrogen bonds are found in this region. An example of a molecule that contains only sp2 hybridization can be found in Figure 7. Other examples of molecules that contain sp2 C-H bonds along with other functional groups include Figures 13, 25 and 37. Examples of hydrocarbons that contain both sp2and sp3 hybridization can be found in Figures 8-12. These peaks are usually sharp and of low to medium intensity.
Hydrogens attached to sp carbons absorb at 3300 cm-1. An example of a spectrum that contains sp hybridization can be found in Figure 13. These peaks are usually sharp and of medium to strong intensity.
Before concluding the discussion of the carbon hydrogen bond, one additional type of C-H stretch can be distinguished, the C-H bond of an aldehyde. The C-H stretching frequency appears as a doublet, at 2750 and 2850 cm-1. Examples of spectra that contain a C-H stretch of an aldehyde can be found in Figures 14 and 15. You may (should) question why the stretching of a single C-H bond in an aldehyde leads to the two bands just described. The splitting of C-H stretching frequency into a doublet in aldehydes is due to the phenomema we called "Fermi Resonance". It is believed that the aldehyde C-H stretch is in Fermi resonance with the first overtone of the C-H bending motion of the aldehyde. The normal frequency of the C-H bending motion of an aldehyde is at 1390 cm-1. As a result of this interaction, one energy level drops to ca. 2750 and the other increases to ca. 2850 cm-1. Only one C-H stretch is observed for aldehydes that have the C-H bending motion of an aldehyde significantly shifted from 1390 cm-1.
In summary, it is possible to identify the type of hydrogen based on hybridization by examining the infrared spectra in the 3300 to 2750 cm-1 region. Before concluding, we should also mention some exceptions to the rules we just outlined. Cyclopropyl hydrogens which are formally classified as sp3 hybridized actually have more s character than 25 %. Carbon-hydrogen frequencies greater than 3000 cm-1 are observed for these stretching vibrations. Halogen substitution can also affect the C-H stretching frequency. The C-H stretching frequencies of hydrogens attached to a carbon also bearing halogen substitution can also be shifted above 3000 cm-1. This is illustrated in Figure 16. The last exception we will mention is an interesting case in which the force constant is increased because of steric interactions. The infrared spectrum of tri-t-butylcarbinol is given in Figure 17. In this case, the hydrogens are sp3 hybridized but stretching the C-H bonds leads to increased crowding and bumping, and this is manifested by a steeper potential energy surface and an increase in k, the force constant in equation 6.
Nitrogen Hydrogen Stretching Frequencies
Much of what we have discussed regarding C-H stretching frequencies is also applicable here. There are three major differences between the C-H and N-H stretching frequencies. First, the force constant for N-H stretching is stronger, there is a larger dipole moment associated with the N-H bond, and finally, the N-H bond is usually involved in hydrogen bonding. The stronger force constant leads to a higher frequency for absorption. The N-H stretching frequency is usually observed from 3500-3200 cm-1. The larger dipole moment leads to a stronger absorption and the presence of hydrogen bonding has a definite influence on the band shape and frequency position. The presence of hydrogen bonding has two major influences on spectra. First, its presence causes a shift toward lower frequency of all functional groups that are involved in hydrogen bonding and second, the peaks are generally broadened. Keep these two factors in mind as you examine the following spectra, regardless of what atoms and functional groups are involved in the hydrogen bonding.
The N-H stretching frequency is most frequently encountered in amines and amides. The following examples will illustrate the behavior of this functional group in a variety of circumstances.
Primary amines and amides derived from ammonia
The N-H stretching frequency in primary amines and in amides derived from ammonia have the same local symmetry as observed in CH2. Two bands, a symmetric and an asymmetric stretch are observed. It is not possible to assign the symmetric and asymmetric stretches by inspection but their presence at approximately 3300 and 3340 cm-1 are suggestive of a primary amine or amide. These bands are generally broad and a third peak at frequencies lower than 3300 cm-1, presumably due to hydrogen bonding, is also observed. This is illustrated by the spectra in Figures 18 and 19 for n-butyl amine and benzamide.
Secondary amines and amides
Secondary amines and amides show only one peak in the infrared. This peak is generally in the vicinity of 3300 cm-1. This is illustrated in Figures 20 and 21. Again notice the effect of hydrogen bonding on the broadness of the N-H peak.
Tertiary amines and amides
Tertiary amines and amides from secondary amines have no observable N-H stretching band as is illustrated in Figures 22 and 23.
You may recall that we will be ignoring most bending motions because these occur in the fingerprint region of the spectrum. One exception is the N-H bend which occurs at about 1600 cm-1. This band is generally very broad and relatively weak. Since many other important bands occur in this region it is important to note the occurrence of this absorption lest it be mistakenly interpreted as another functional group. Figure 18 illustrates the shape and general intensity of the bending motion. Most other functional groups absorbing in this region are either sharper or more intense.
The hydroxyl stretch is similar to the N-H stretch in that it hydrogen bonds but does so more strongly. As a result it is often broader than the N-H group. In those rare instances when it is not possible to hydrogen bond, the stretch is found as a relative weak to moderate absorption at 3600-3650 cm-1. In tri-t-butylmethanol where steric hindrance prevents hydrogen bonding, a peak at 3600 cm-1 is observed as shown in Figure 17. Similarly for hexanol, phenol, and hexanoic acid, Figures 24, 25, and 26, gas phase and liquid phase spectra illustrate the effect of hydrogen bonding on both the O-H stretch and on the rest of the spectrum. In should be pointed out that, in general, while gas phase spectra are usually very similar, frequencies are generally shifted to slightly higher values in comparison to condensed phase spectra. Gas phase spectra that differ significantly from condensed phase spectra are usually taken as evidence for the presence of some sort of molecular association in the condensed phase.
The hydroxyl group in phenols and alcohols usually is found as a broad peak centered at about 3300 cm-1 in the condensed phase as noted above and in the additional examples of Figures 24, 28, and 29. The O-H of a carboxylic acid, so strongly associated that the O-H absorption in these materials, is often extended to approximately 2500 cm-1. This extended absorption is clearly observed in Figures 26, 27, and 29 and serves to differentiate the O-H stretch of a carboxylic acid from that of an alcohol or phenol. In fact, carboxylic acids associate to form intermolecular hydrogen bonded dimers both in the solid and liquid phases.
The nitrile group is another reliable functional group that generally is easy to identify. There is a significant dipole moment associated with the CN bond which leads to a significant change when it interacts with infrared radiation usually leading to an intense sharp peak at 2200-2280 cm-1. Very few other groups absorb at this region with this intensity. The spectrum in Figure 31 illustrates the typical behavior of this functional group. If another electronegative atom such as a halogen is attached to the same carbon as the nitrile group, the intensity of this is markedly reduced.
The carbon-carbon triple bond
The CC bond is not considered to be a very reliable functional group. This stems in part by considering that the reduced mass in equation 7 is likely to vary. However it is characterized by a strong force constant and because this stretching frequency falls in a region where very little else absorbs, 2100-2260 cm-1, it can provide useful information. The terminal carbon triple bond (CC-H) is the most reliable and easiest to identify. We have previously discussed the C-H stretching frequency; coupled with a band at 3300 cm-1, the presence of a band at approximately 2100 cm-1 is a strong indication of the -CC-H group. The spectrum in Figure 13 illustrates the presence of this group.
An internal -CC- is more difficult to identify and is often missed. Unless an electronegative atom such as nitrogen or oxygen is directly attached to the sp hybridized carbon, the dipole moment associated with this bond is small; stretching this bond also leads to a very small change. In cases where symmetry is involved, such as in diethyl acetylenedicarboxylate, Figure 32, there is no change in dipole moment and this absorption peak is completely absent. In cases where this peak is observed, it is often weak and difficult to identify with a high degree of certainty.
The carbonyl group is probably the most ubiquitous group in organic chemistry. It comes in various disguises. The carbonyl is a polar functional group that frequently is the most intense peak in the spectrum. We will begin by discussing some of the typical acyclic aliphatic molecules that contain a carbonyl group. We will then consider the effect of including a carbonyl as part of a ring and finally we will make some comments of the effect of conjugation on the carbonyl frequency.
Acyclic aliphatic carbonyl groups
Esters, aldehydes, and ketones
Esters, aldehydes, and ketones are frequently encountered examples of molecules exhibiting a C=O stretching frequency. The frequencies, 1735, 1725, 1715 cm-1 respectively, are too close to allow a clear distinction between them. However, aldehydes can be distinguished by examining both the presence of the C-H of an aldehyde (2750, 2850 cm-1) and the presence of a carbonyl group. Examples of an aliphatic aldehyde, ester, and ketone are given in Figures 14, 34, 36, and 35, respectively.
Carboxylic acids, amides and carboxylic acid anhydrides
Carboxylic acids, amides and carboxylic acid anhydrides round out the remaining carbonyl groups frequently found in aliphatic molecules. The carbonyl frequencies of these molecules, 1700-1730 (carboxylic acid), 1640-1670 (amide) and 1800-1830, 1740-1775 cm-1 (anhydride), allow for an easy differentiation when the following factors are also taken into consideration.
A carboxylic acid can easily be distinguished from all the carbonyl containing functional groups by noting that the carbonyl at 1700-1730 cm-1 is strongly hydrogen bonded and broadened as a result. In addition it contains an O-H stretch which shows similar hydrogen bonding as noted above. Spectra which illustrate the effect of hydrogen bonding include Figures 27, and 29.
Amides are distinguished by their characteristic frequency which is the lowest carbonyl frequency observed for an uncharged molecule, 1640-1670 cm-1(Amide I). In addition, amides from ammonia and primary amines exhibit a weaker second band (Amide II) at 1620-1650 cm-1 and 1550 cm-1 respectively, when the spectra are run on the solids. Amides from secondary amines do not have a hydrogen attached at nitrogen and do not show an Amide II band. The Amide I band is mainly attributed to the carbonyl stretch. The Amide II involves several atoms including the N-H bond. We will return to the frequency of the amide carbonyl when we discuss the importance of conjugation and the effect of resonance on carbonyl frequencies. The spectra of benzamide, a conjugated amide (Figure 19), and N-methyl acetamide (Figure 21) clearly identify the Amide I and II bands. The spectrum of N,N dimethyl acetamide (Figure 23) illustrates an example of an amide from a secondary amine.
Anhydrides can be distinguished from other simple carbonyl containing compounds in that they contain and exhibit two carbonyl frequencies. However, these frequencies are not characteristic of each carbonyl. Rather they are another example of the effects of local symmetry similar to what we have seen for the CH2 and NH2 groups. The motions involved here encompass the entire anhydride (-(C=O)-O-(O=C-) in a symmetric and asymmetric stretching motion of the two carbonyls. The two carbonyl frequencies often differ in intensity. It is not possible to assign the peaks to the symmetric or asymmetric stretching motion by inspection nor to predict the more intense peak. However, the presence of two carbonyl frequencies and the magnitude of the higher frequency (1800 cm-1) are a good indication of an anhydride. Figure 36 contains a spectrum of an aliphatic anhydride.
Cyclic aliphatic carbonyl containing compounds
The effect on the carbonyl frequency as a result of including a carbonyl group as part of a ring is usually attributed to ring strain. Generally ring strain is believed to be relieved in large rings because of the variety of conformations available. However as the size of the ring gets smaller, this option is not available and a noticeable effect is observed. The effect of increasing ring stain is to increase the carbonyl frequency, independent of whether the carbonyl is a ketone, part of a lactone(cyclic ester), anhydride or lactam (cyclic amide). The carbonyl frequencies for a series of cyclic compounds is summarized in Table 3.
Table 3. The Effect of Ring Strain on the Carbonyl Frequencies of Some Cyclic Molecules
Ring Sizeketone: cm-1lactones: cm-1lactams: cm-1
4cyclobutanone: 1775b-propiolactone: 1840
5cyclopentanone: 1751g-butyrolactone: 1750g-butyrolactam: 1690
6cyclohexanone: 1715d-valerolactone: 1740d-valerolactam: 1668
7cycloheptanone: 1702e-caprolactone: 1730e-caprolactam: 1658
Carbon carbon double bond
Like the CC bond, the C=C bond stretch is not a very reliable functional group. However, it is also characterized by a strong force constant and because of this and because the effects of conjugation which we will see can enhance the intensity of this stretching frequency, this absorption can provide useful and reliable information.
In simple systems, the terminal carbon carbon double bond (C=CH2) is the most reliable and easiest to identify since the absorption is of moderate intensity at 1600-1675 cm-1. We have previously discussed the C-H stretching frequency of an sp2hybridized C-H. The spectrum in Figure 9 illustrates the presence of this group. In addition the terminal C=CH2 is also characterized by a strong band at approximately 900 cm-1. Since this band falls in the fingerprint region, some caution should be exercised in its identification.
An internal non-conjugated C=C is difficult to identify and can be missed. The dipole moment associated with this bond is small; stretching this bond also leads to a very small change. In cases where symmetry is involved, such as in 4-octene, Figure 10, there is no change in dipole moment and this absorption peak is completely absent. In cases where this peak is observed, it is often weak. In 2,5-dihydrofuran, Figure 39, it is difficult to assign the C=C stretch because of the presence of other weak peaks in the vicinity. The band at approximately 1670 cm-1 may be the C=C stretch. In 2,5-dimethoxy-2,5-dihydrofuran, Figure 40, the assignment at 1630 cm-1 is easier but the band is weak.
There is one circumstance that can have a significant effect on the intensity of both internal and terminal olefins and acetylenes. Substitution of a heteroatom directly on the unsaturated carbon to produce, for example, a vinyl or acetylenic ether, or amine leads to a significant change in the polarity of the C=C or CC bond and a substantial increase in intensity is observed. The C=C in 2,3-dihydrofuran is observed at 1617.5 cm-1 and is one of the most intense bands in the spectrum (Figure 41). Moving the C=C bond over one carbon gives 2,5-dihydrofuran attenuates the effect and results in a weak absorption (Figure 39).
Aromatic ring breathing motions
Benzene rings are encountered frequently in organic chemistry. Although we may write benzene as a six membered ring with three double bonds, most are aware that this is not a good representation of the structure of the molecule. The vibrational motions of a benzene ring are not isolated but involve the entire molecule. To describe one of the fundamental motions of benzene, consider imaginary lines passing through the center of the molecule and extending out through each carbon atom and beyond. A symmetric stretching and compression of all the carbon atoms of benzene along each line is one example of what we might describe as a ring breathing motion. Simultaneous expansions and compressions of these six carbon atoms lead to other ring breathing motions. These vibrations are usually observed between 1450 and 1600 cm-1 and often lead to four observable absorptions of variable intensity. As a result of symmetry, benzene, Figure 7, does not exhibit these bands. However most benzene derivatives do and usually 2 or 3 of these bands are sufficiently separate from other absorptions that they can be identified with a reasonable degree of confidence. The least reliable of these bands are those observed at approximately 1450 cm-1 where C-H bending motions are observed. Since all organic molecules that contain hydrogen are likely to have a C-H bond, absorptions observed at 1450 cm-1 are not very meaningful and should usually be ignored. Two of the four bands around 1600 cm-1 are observed in ortho and meta xylene, identified by the greek letter and a third band at about 1500 cm-1 is assigned (Figure 11 and 12). We will return to a discussion of these bands when we discuss the effects of conjugation on the intensities of these motions.
The final functional group we will include in this discussion is the nitro group. In addition to being an important functional group in organic chemistry, it will also begin our discussion of the importance of using resonance to predict effects in infrared spectroscopy. Let's begin by drawing a Kekule or Lewis structure for the nitro group. You will find that no matter what you do, it will be necessary to involve all 5 valence
electrons of nitrogen and use them to form the requisite number of bonds to oxygen. This will lead to a positive charge on nitrogen and a negative charge on one oxygen. As a result of resonance, we will delocalize the negative charge on both oxygens and as shown, this leads to an identical structure. Since the structures are identical, we would expect the correct structure to be a resonance hybrid of the two. In terms of geometry, we would expect the structure to be a static average of the two geometric structures both in terms of bond distances and bond angles. Based on what we observed for the CH2 and NH2 stretch, we would expect a symmetric and an asymmetric stretch for the N-O bond in the nitro group halfway between the N=O and N-O stretches. Since both of those functional groups are not covered in this discussion, we will need to assume for the present that this is correct. Two strong bands are observed, one at 1500-1600 cm-1 and a second between 1300-1390 cm-1, Figure 28.
Effect of resonance and conjugation on infrared frequencies
Let's continue our discussion of the importance of resonance but shift from the nitro group to the carboxylate anion. The carboxylate anion is represented as a resonance hybrid by the following figure:
Unlike the nitro group which contained functional groups we will not be discussing, the carboxyl group is made up of a resonance hybrid between a carbon oxygen single bond and a carbon oxygen double bond. According to resonance, we would expect the C-O bond to be an average between a single and double bond or approximately equal to a bond and a half. We can use the carbonyl frequency of an ester of 1735 cm-1 to describe the force constant of the double bond. We have not discussed the stretching frequency of a C-O single bond for the simple reason that it is quite variable and because it falls in the fingerprint region. However the band is known to vary from 1000 to 1400 cm-1. For purposes of this discussion, we will use an average value of 1200 cm-1. The carbonyl frequency for a bond and a half would be expected to fall halfway between 1735 and 1200 or at approximately 1465 cm-1. The carboxyl group has the same symmetry as the nitro and CH2 groups. Both a symmetric and asymmetric stretch should be observed. The infrared spectrum of sodium benzoate is given in Figure 42. An asymmetric and symmetric stretch at 1410 and 1560 cm-1 is observed that averages to 1480 cm-1, in good agreement with the average frequency predicted for a carbon oxygen bond with a bond order of 1.5. While this is a qualitative argument, it is important to realize that the carboxylate anion does not show the normal carbonyl and normal C-O single bond stretches (at approximately 1700 and 1200 cm-1) suggested by each of the static structures above.
In the cases of the nitro group and the carboxylate anion, both resonance forms contribute equally to describing the ground state of the molecule. We will now look at instances where two or more resonance forms contribute unequally to describing the ground state and how these resonance forms can effect the various stretching frequencies.
Most carbonyl stretching frequencies are found at approximately 1700 cm-1. A notable exception is the amide carbonyl which is observed at approximately 1600 cm-1. This suggests that the following resonance form makes a significant contribution to describing the ground state of amides:
You may recall that resonance forms that lead to charge separation are not considered to be very important. However the following information support the importance of resonance in amides. X-ray crystal structures of amides show that in the solid state the amide functional group is planar. This suggests sp2 hybridization at nitrogen rather than sp3. In addition the barrier to rotation about the carbon nitrogen bond has been measured. Unlike the barrier of rotation of most aliphatic C-N bonds which are of the order of a few kcal/mol, the barrier to rotation about the carbon nitrogen bond in dimethyl formamide is approximately 18 kcal/mol. This suggests an important contribution of the dipolar structure to the ground state of the molecule and the observed frequency of 1600 cm-1, according to the arguments given above for the carboxylate anion, is consistent with more C-O single bond character than would be expected otherwise.
Conjugation of a carbonyl with a C=C bond is thought to lead to an increase in resonance interaction. Again the resonance forms lead to charge separation which clearly de-emphasizes their importance.
However this conjugative interaction is useful in interpreting several features of the spectrum. First it predicts the small but consistent shift of approximately 10 cm-1 to lower frequency, observed when carbonyls are conjugated to double bonds or aromatic rings. This feature is summarized in Table 4 for a variety of carbonyl groups. Next, the dipolar resonance form suggests a more polar C=C than that predicted for an unconjugated C=C. In terms of the change in dipole moment, contributions from this structure suggest that the intensity of infrared absorption of a C=C double bond would increase relative to an unconjugated system. Comparison of Figures 9, 10 and 35 with Figures 43, and 44-47 shows this to be the case. Conjugation is associated with an increase in intensity of the C=C stretching frequency. Finally, examination of Figures 43-46 reveals an intricacy not previously observed with simple non-conjugated carbonyls. The carbonyls of Figures 43-46 which are all conjugated appear as multiplets while those unconjugated carbonyls such as those in Figures 14 and 35 appear as single frequencies. Note however that not all conjugated carbonyls appear as multiplets (Figures 15 and 47. Resolution of this additional complicating feature can be achieved if we consider that conjugation requires a fixed conformation. For most conjugated carbonyls, two or more conformations are possible. The s-cis form is shown above and the s-trans form is shown below.
If the resonance interaction in these two forms differ, the effect of resonance on the carbonyl will differ leading to similar but different frequencies. The presence of multiple carbonyl frequencies is a good indication of a conjugated carbonyl. In some conjugated systems such as benzaldehyde
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