1459227637CHE_P1sdsdsdsdsds2_M8_e-Text.pdf

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CHEMISTRY PAPER No. 12: ORGANIC SPECTROSCOPY
Module 8: Factors affecting vibrational frequencies and IR
Spectroscopy of Hydrocarbons
Subject Chemistry
Paper No and Title Paper 12: Organic Spectroscopy
Module No and Title Module 8: Factors affecting vibrational frequencies and IR
Module Tag CHE_P12_M8_e-Text

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TABLE OF CONTENTS
1. Learning Outcomes
2. The position of an IR-Absorption Band
3. The Intensity of Absorption Bands
4. The Shape of Absorption Bands
5. Factors Influencing Vibrational Frequencies
5.1 Effect of Bond Order
5.2 Resonance and Inductive Electronic Effect
5.3 Hydrogen Bonding
5.4 Fermi Resonance
5.5 Bond Angles
6. Characteristic absorption bands for hydrocarbons
7. Aliphatic hydrocarbons
7.1 Alkanes
7.2 Cycloalkanes
7.3 Alkenes
7.4 Cycloalkanes
7.5 Alkynes
8. Aromatic hydrocarbons
9. Summary

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1. Learning Outcomes
After studying this module, you shall be able to
• To understand the position, intensity and shape of an absorption band
• To understand the effect of bond strength on absorption frequency
• To understand how H-bonding affect the absorption band
• To know about the possible modes of vibration in aliphatic and aromatic hydrocarbons and
their frequency values
2. The position of an IR-Absorption Band
The amount of energy required to stretch a bond depends on the strength of the bond and the
masses of the bonded atoms. The stronger the bond, the greater the energy required to stretch it.
The frequency of the vibration is inversely proportional to the mass of the atoms, so heavier
atoms vibrate at lower frequencies.
The approximate vibration frequency of a bond can be calculated from the following equation
derived from Hooke’s law.
Where µ is the reduced mass
And µ=m1×m2/m1+m2
m1 and m2 are the masses of the two atoms
C is the velocity of light (2.998×1010
cms-1
)
K = force constant of the bond
[For a single C-C bond, it is approximately 5×105
gs-2
and it doubles and triples for double and
triple bond respectively.]
The force constant is a measure of the strength of a bond. Thus the value of stretching frequency
of a bond increases with increasing the bond strength and decreasing the reduced mass of the
bonded atoms.
The triple bonds are stronger than the double bonds followed by single bonds between the same
two atoms and hence vibrational frequency is in the order of triple>double>single.

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As the atom attached to carbon a particular atom say carbon increases in mass, the reduced mass
increases and hence the frequency of vibration decreases.
3. The Intensity of Absorption Bands
The intensity of an absorption band depends on the change in the dipole moment of the bond
associated with the vibration. The greater the change in dipole moment, the more intense is the
absorption. The dipole moment of a bond is equal to the magnitude of the charge on one of the
bonded atoms, multiplied by the distance between the two charges. If we have two different
atoms, there will be an electro-negativity difference and larger the electro-negativity difference,
the more intense the absorption will be. The stretching vibration of an O-H bond will be
associated with a greater change in dipole moment than that of an N-H bond because O-H bond is
more polar. Consequently, the stretching vibration of the O-H bond will be more intense.
Likewise, the stretching vibration of an N-H bond is more intense than that of a C-H bond
because the N-H bond is more polar.
The intensity of an absorption band also depends on the number of bonds responsible for the
absorption. For example, if you compare the IR spectra of methane to that of octane, the octane
molecule will have a much more intense C-H peak because it has many more C-H bonds than
methane. The concentration of the sample used to obtain an IR spectrum also affects the intensity
of the absorption bands. Concentrated samples have greater numbers of absorbing molecules and,
therefore, more intense absorption bands.
4. The Shape of Absorption Bands
The shape of an absorption band can be helpful in identifying the compound responsible for an IR
spectrum. For example, both O-H and N-H bonds stretch at wave numbers above 3100 cm-1
but
the shapes of their bands are distinct. An N-H absorption band (3300 cm-1
) is comparatively
narrower and less intense than an O-H absorption band in an alcohol and the O-H absorption band
of a carboxylic acid (3300-2500 cm-1
) is broader than the O-H absorption band of an alcohol.
5. Factors Influencing Vibrational Frequencies
The value of vibrational frequency of a bond calculated by Hooke’s Law is not always equal
to their observed value. The force constant is changed with the electronic and steric effects

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caused by other groups present in the surroundings. Following are some important factors
affecting the vibrational frequency of a bond.
5.1 Effect of Bond Order
Bond order affects the position of absorption bands. Higher the bond order larger is the band
frequency. A C-C triple bond is stronger than a C=C bond, so a C-C triple bond has higher
stretching frequency than does a C=C bond. The C-C bonds show stretching vibrations in the
region from 1200-800 cm-1
but these vibrations are weak and of little value in identifying
compounds. Similarly, a C=O bond stretches at a higher frequency than does a C-O bond and a
C-N triple bond stretches at a higher frequency than does a C=N bond which in turn stretches at a
higher frequency than does a C-N bond.
5.2 Resonance and Inductive Electronic Effects
Whenever we discuss about the IR band, a range of frequency for each stretch is usually assigned
as the exact position of the absorption band depends on other structural features of the molecule,
such as electron delocalization, the electronic effect of neighbouring substituents, and hydrogen
bonding. Important details about the structure of a compound can be revealed by the exact
position of the absorption band. For example, the IR bands for the carbonyl group in 2-pentanone,
2-cyclohexenone appears at different frequencies. The 2-Cyclohexenone absorbs at a lower
frequency because the carbonyl group has less double-bond character due to electron
delocalization.
If you put an atom other than hydrogen (aldehyde) or carbon (ketone) next to the carbonyl group,
it also causes the shifting of the position of the carbonyl absorption band. Whether it shifts to a
lower or to a higher frequency depends on the electronic effect (inductive and mesomeric effect)
of the atom.
Different substituents on carbonyl carbon changes the electronegativity of carbonyl group due to
the inductive effect arises due to the different electronegativities of the carbonyl carbon and of the
substituent in compounds of the type RCOZ. It involves the electrons in the sigma bonds.
The mesomeric effect involves the electrons in the pie and nonbonding orbitals and it operates in
general opposite to the inductive effect. These effects cannot be isolated from one another and at
a time only one of them can be applied to determine approximate results.
Electron releasing groups attached to the carbonyl group tend to favour the polar contribution by
mesomeric effect and thus lower the bond order of the C=O bond (less double bond character)
and hence resulting in a decrease of the carbonyl stretching frequency.
Electron withdrawing groups suppress the polar contribution with an effective increase in the
double bond character and hence resulting in the increase of the frequency of absorption.

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The predominant effect of the nitrogen of an amide is electron donation by resonance. Therefore,
the carbonyl group of an amide has less double bond character than the carbonyl group of a
ketone and hence amide has low C=O bond IR frequency than the ketone. In contrast, the
predominant effect of the oxygen of an ester is inductive electron withdrawal, therefore the
carbonyl group of an ester has more double bond character than the carbonyl group of a ketone,
so the former appears at larger frequency value.
If we talk about the C-O stretching frequency it appears in the range of 1250-1050 cm-1
. If C-O
bond is in an alcohol or in ether, the stretching frequency will be toward the lower end of the
range. If this C-O bond is a part of a carboxylic group, the stretching frequency will occur at the
higher end of the range. The position of the absorption varies because the bond in an alcohol is a
pure single bond, whereas the bond in a carboxylic acid has partial double bond character that is
due to resonance electron donation by OH group in acids. Esters show stretches at both ends of
the range because esters have two single bonds, one that is a pure single bond and one that has
partial double-bond character.
In a specific example of benzamide and phenyl acetate, as the nitrogen atom is less
electronegative than the oxygen atom, in amide and is more labile and participates to a greater
extent in conjugation. The presence of greater degree of conjugation, C=O absorption frequency
is relatively lesser than in esters.
In most of the cases, mesomeric effect works along with inductive effect and cannot be ignored. It
is important to note that only inductive effect is considered when the compound in meta
substituted. In para substitution, both inductive and mesomeric effects become important and the
domination of one over the other will decide the wavenumber of the absorption.

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In the above examples, due to the low electronegativity of nitrogen atom, the lone pair of
electrons participates more in conjugation in compound I as compared to that compound III.
Thus, in compound I, ν(C=O) absorption occurs at lower wave number compared to that in
compound III. In compounds II and IV, inductive effect dominates over mesomeric effect and
hence absorption takes place at comparatively higher frequencies.
In ortho substitution, inductive effect, mesomeric effect along with steric effect is considered. In
ortho substituted compounds, the lone pairs of electrons on two atoms influence each other
through space interactions and change the vibrational frequencies of both the groups. This effect
is called field effect.
5.3 Hydrogen Bonding
The presence of hydrogen bonding changes the position and shape of an infrared absorption band.
Frequencies of both stretching as well as bending vibrations are changed because of hydrogen
bonding. The X-H stretching bands move to lower frequency usually with increased intensity and
band widening. The X-H bending vibration usually shifts to higher frequencies. Stronger the
hydrogen bonding, greater is the absorption shift from the normal values. The two types of
hydrogen bonding (intramolecular and intermolecular) can be differentiated by the use of infrared
spectroscopy.
Inter-molecular hydrogen bonding involves association of two or more molecules of the same or
different compound, and it may result in dimer molecules as in carboxylic acids. Intra-molecular
hydrogen bonds are formed when the proton donor and acceptor are present in a single molecule
under special conditions that allow the required overlap of orbitals, e.g. formation of a 5 or 6
member rings.
The extent of inter-molecular hydrogen bonding depends upon the concentration of the solution
and hence the position and the shape of an absorption band also depend on the concentration of
the solution. The more concentrated the solution, the more likely it is for the OH-containing
molecules to form intermolecular hydrogen bonds. It is easier to stretch an O-H bond if it is
hydrogen bonded, because the hydrogen is attracted to the oxygen of neighbouring molecule.
Therefore, the O-H stretching of a concentrated (hydrogen bonded) solution of an alcohol occurs

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at about 3550 cm-1
, whereas the O-H stretching band of a dilute solution (with little or no
hydrogen bonding) appears at 3650 cm-1
. Additionally, hydrogen-bonded OH groups also have
broader absorption bands whereas the absorption bands of non-hydrogen–bonded OH groups are
sharper.
Hydrogen bonding interactions can also occur between functional groups of solvent and solute.
Three classes of solvent exist which may lead to trouble when used for hydrogen-bonding studies
is solution. First, compounds which contain hydrogen donor groups, e.g. halogenated compounds
which contain a sufficient number of halogens to activate the hydrogens present, such as
chloroform. Secondly, compounds which contain non-bonded electron pairs, such as ethers,
aldehydes and tertiary amines.
In addition, there are compounds which contain both types of group, e.g. water and alcohols.
There are only a few solvents that do not have the above characteristics, such as carbon
tetrachloride (CCl4) and carbon disulfide (CS2). These still contain lone electron pairs, but being
on S and Cl are less available, and any interactions will be extremely weak.
Apart from the concentration and solvent effects, temperature also affects the degree of hydrogen
bonding in a compound. High temperature decreases the extent of hydrogen bonding.
5.4 Fermi Resonance
Fermi resonance is a phenomenon which was first explained by the Italian
physicist Enrico Fermi to account for shifting of the energies and intensities of absorption
bands in an infrared spectroscopy. Sometimes, it happens that two different vibrational
levels have nearly the same energy. If such vibrations belong to the same species of
molecules, then a mutual perturbation of energy may occur, resulting in the shift of one
towards lower frequency and the other towards a higher frequency. It is also accompanied
by a substantial increase in the intensity of the respective bands.
Fermi resonance was first discovered in carbon dioxide by Fermi and it is also found in the
vibrational spectra of aldehydes, where the C-H bond in the CHO group interacts with the second
harmonic level, derived from the fundamental frequency of the deformation vibration of the CHO
group (2*1400 cm-1
). The result is a Fermi doublet with branches around 2830 cm-1
and 2730 cm-
1
. It is important for Fermi resonance that the vibrations connected with the two interacting levels
be localized in the same part of the molecule.

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5.4 Bond angles
Smaller ring requires the use of more p-character to make the internal C-C bonds for the requisite
small angles. This gives more s character to the C=O sigma bond which causes the strengthening
and stiffening of the exocyclic double bond. The force constant K is then increased and the
absorption frequency increases.
6. Characteristic absorption bands for hydrocarbons
These compounds have carbon and hydrogens only and may be saturated, unsaturated, aliphatics,
cyclics and aromatics. The characteristic vibrational modes of aliphatic and aromatic
hydrocarbons and their absorption frequencies are described in subsequent sections.
7. Aliphatic hydrocarbons
The stretching and bending vibrations frequencies of alkanes, alkenes and alkynes are described
in table 1
Table 1: Characteristic frequencies of absorption bands for aliphatic hydrocarbons

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7.1 Alkanes
In alkanes only two types of bonds are present that is C–H and C–C bonds. The most useful
bands in alkanes are those arising from C–H stretching and C–H bending. A methyl group usually
shows two strong and distinct bands, one at 2960 cm-1
due to asymmetric stretching and the other
at 2870 cm-1
due to symmetric stretching. While the methylene group show asymmetric stretching
at 2930 cm−1
and symmetric stretching at 2850 cm−1
. The C-H bonds do not participate in H-
bonding and hence the frequencies are not affected by chemical environment. The presence of
several methyl groups in a molecule results in strong absorptions at these positions.
The C-H in-plane bending vibrations of a methyl group give two bands, i.e. a symmetrical band at
1380 cm−1 and an asymmetrical band at 1460 cm−1
. Methylene groups give rise to four bending
vibrations: scissoring (1465 cm−1
), rocking (720 cm−1
), wagging (1305 cm−1
) and twisting (1300
cm−1
). The band at 1460 cm−1
indicates the C-H asymmetric bending of both methyl and
methylene groups whereas the band at 1380 cm−1
represents symmetric bending of methyl group
only. Thus the peak at 1380 cm−1
is extremely useful in detecting the presence of methyl group
because this band is hardly overlapped to the absorptions due to the methylene and methane
absorption bands.
The presence of two or more methyl groups, attached to the same carbon atom results in splitting
of the C-H symmetric bending vibrations. For example the isopropyl group gives rise to two
bands 1397 cm−1
and 1370 cm−1
and in t-butyl group the two bands appear at 1385 cm−1
and 1370
cm−1
(Figure 1).
The methine C-H shows C-H stretching and bending vibrations at 2890 cm−1
and 1340 cm−1
respectively and are very weak with no practical significance (Figure 1).
Figure 1: IR-spectrum of decane
7.2 Cycloalkanes
In case of unstrained cycloalkanes, the C-H stretching frequencies are the same (2800 – 3000
cm−1
) as in the case of acyclic compounds. However, methylene scissoring bands shift slightly to
smaller wavenumber (1470 cm−1
in hexane and 1448 cm−1
in cyclohexane). In sterically strained
cyclic compounds, the C-H stretching normally occurs at slightly higher frequencies. For example
the ring CH2 and CH groups in a monoalkyl cylcopropane ring absorb in the range of 3100-2990
cm−1
.

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Figure 2: IR-spectrum of cyclohexane
Methylene C-H in-plane bending vibrations in cyclic alkenes shift slightly to smaller frequency as
compared to their acyclic counterparts. For example, cyclohexane absorb at 1452 cm−1
whereas n-
hexane absorb at 1468 cm−1
. The C-C bond vibrations appear as weak bands in 1200-800 cm−1
region and are seldom used for structural study. Whereas the C-C bending absorptions occur at <
500 cm−1
and are usually below the range of IR instrument (Figure 2)
7.3 Alkenes
There are four major bands associated with alkene molecular framework. These are the out-of-
plane and in-plane =C–H bending, C=C stretching and =C–H stretching. In completely
symmetrical alkenes, such as ethylene, tetrachloroethylene etc., C=C stretching band is absent,
due to lack of change in dipole moment in completely symmetrical molecule. On the other hand,
non-symmetrically substituted double bonds exhibit strong absorption bands.
Olefinic C-H stretching occurs near 3100-3000 cm−1
. C=C stretching band occurs near 1680-1600
cm−1
. The C=C absorption bands are more intense for cis isomers than for trans isomers. Also,
terminal olefins show stronger C=C double bond stretching vibrations than internal double bonds.
Conjugation moves C=C stretching bands towards lower frequency with increased intensity than
in comparison to non-conjugated ones. In case of olefins, conjugated with an aromatic ring, the
C=C stretching appears at 1625 cm-1
(s) and an additional band at ~1600 cm-1
is observed due to
aromatic double bond.

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Figure 3: IR-spectrum of Cis-2-pentene
Figure 4: IR-spectrum of trans-2-pentene
The =C-H in-plane bending vibrations occur near 1400 cm−1
whereas the =C-H out-of-plane
bending band occur in the range from 1000-680 cm−1
(Figure 3,4).
Note: In compounds containing both olefinic and alkyl C-H bonds, the bands above 3000 cm-1
are generally attributed to aromatic or aliphatic C-H stretching, whereas between 3000-2840 cm-1
are generally assigned to the alkyl C-H stretching.
7.4 Cycloalkenes
The absorption frequency of a double bond in a cyclic ring is very sensitive to ring size. The
absorption frequency decreases as the internal bond angle decreases and is becomes lowest in
cyclobutene (90o
angle) but after that the frequency increases again for cyclopropene (Figure 5).
Figure 5: Vibrational frequencies of endocyclic C=C bonds
The exocyclic double bonds exhibit an increase in frequency with decrease in ring size. The
exocyclic double bond on six-membered ring absorbs at 1651 cm−1
and it is shifted to 1780 cm−1
in case of exocyclic double bond on cyclopropane. The allenes show the highest double bond
absorptions at 1940 cm−1
.

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Figure 6: Vibrational frequencies of exocyclic C=C bonds
Six-membered rings with carbonyl group e.g. cyclohexanone absorb at normal value i.e. 1715 cm-
1
. Decrease in ring size increases the C=O stretching frequency. Smaller rings require the use of
more p- character to make C-C bonds for the requisite small angles (Figure 6).
7.5 Alkynes
Alkynes contain the C≡C group and three characteristic bands can be present, i.e. ≡C–H
stretching, ≡C–H bending and C≡C stretching. As the force constant for a triple bond is greater
than that for a double bond. Consequently, whereas a C-C stretching vibrations occur between
1300-800 cm−1
and the C=C stretching vibration occur in the region 1700-1500 cm−1
, the C triple
C vibrations are observed at significantly higher frequencies in the region of 2300 to 2050 cm−1
.
The terminal alkynes show C-C triple bond stretching vibration at 2140-2050 cm−1
, whereas the
symmetrical or unsymmetrical internal alkynes show either no band or show a very weak C-C
triple bond absorption at 2260-2190 cm−1
.
The terminal alkynes show a strong and sharp C-H stretching vibration bands in the region 3310-
3200 cm-1
. The sharpness of this band prevents form the confusion with other absorptions in this
region (NH, OH etc).
The C-H bending vibrations in alkynes occur as a strong broad band in the region 700-600 cm-1
.
The first overtone of this band appears as a weak broad band near 1375-1225 cm-1
.
Therefore the frequency of the absorption of C-H bond is a function of the type of hybridization
of the carbon to which hydrogen atom is attached. While moving from sp3
to sp2
and sp
hybridized carbons, the s-character increases and so is the bond strength (force constant of C-H
bond and the frequency of absorption (Figure 7,8).

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Figure 7: IR-spectrum of 1-octyne
Figure 8: IR-spectrum of 4-octyne
8. Aromatic hydrocarbons
The C–H stretching bands of aromatic compounds appear as a multiple weak bands in the 3100–
3000 cm-1
range. It is important to note here that the aromatic C–H stretching bands occur at
slightly higher frequency (as a shoulder on the stronger alkane C-H stretching bands) than is the
C–H stretch in alkanes which appear below 3000 cm-1
. Besides the C–H stretch above 3000 cm-1
,
two other regions of the infrared spectra of aromatic compounds distinguish aromatics from
organic compounds that do not have an aromatic ring.
In the region of 2000–1700 cm-1
, a series of weak combination and overtone bands appears.
However, they are very weak and are observed only in the case of concentrated solutions of
highly symmetric benzene derivatives.
The other most prominent bands are in the region of 900-650 cm-1
due to the out-of-plane bending
of ring C-H bonds. The C–H in-plane bending vibrational bands appear in the regions 1275–1000
cm-1
. The bands of the out-of-plane bending vibrations of aromatic compounds are strong and
characteristic of the number of hydrogens in the ring, and hence can be used to give the
substitution pattern. Mono substituted benzene derivatives exhibit strong absorption bands in the
regions 690-710 cm-1
and 730-770 cm-1
. The absence of band at 690 cm-1
indicates the absence of

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mono substituted phenyl rings in the sample. 1,2-Disubstituted benzenes give one strong
absorption band in the region 735-770 cm-1
but no band in the region 690-710 cm-1
. 1,3-
Disubstituted rings give two absorption bands in the region 680-725 cm-1
and 750-810 cm-1
. 1,4-
Disubstituted rings give only one strong absorption band in the region 800-850 cm-1
. The IR-
spectrum of all the possible di-substituted benzene derivatives are shown below (Figure 9,10,
11,12).
Figure 9: IR-spectrum of toluene
Figure 10: IR-spectrum of 1,2-diethyl benzene

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The bands related to other substitution patterns are summarized in the figure given below (Figure
13).
Figure 13: Out-of-plane bending vibrations of ring C-H bonds in substituted
aromatic compounds
Aromatic hydrocarbons show absorptions in the regions 1650-1400 cm-1
due to C=C stretching
vibrations in the aromatic ring. There are normally four bands in this region at about 1600, 1585,
1500 and 1450 cm-1
and are due to C=C in-plane vibrations. The absence of these bands indicates
that the compound is not aromatic.

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9. Summary
1. The stretching frequency of a bond depends on the strength of the bond and the masses of the
bonded atoms.
2. Hooke’s derived an equation to calculate the approximate vibration frequency of a bond.
3. The vibrational frequency of a bond increases as the number of bonds increases between the
two atoms. A C-C triple bond absorbs at higher frequency than a C=C bond followed by a C-C
single bond.
4. Hydrogen bonding changes the position and shape of an infrared absorption band. Stronger is
the hydrogen bonding, larger are the effects.
5. Ring size and electronic effect such as inductive, resonance also affects the vibrational
frequency.
6. Cis-alkenes absorb at higher frequencies and give more intense spectrum than the
corresponding trans-alkene.
7. The terminal alkynes show strong C-C triple bond stretching bands while internal alkynes
show either no band or show a very weak band due to C-C triple bond.
8. The out of plane bending vibrations for benzene gives important information about the
substitution pattern on the benzene ring.

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