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Infrared Spectral Interpretation

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Contents Page

1 Infrared Spectral Quality 2-3 2 Orbital Hybridization – sp, sp2, and sp3 Carbon 4-6

2.1. sp3 Hybridization 4-5

2.2. sp2 Hybridization 5-6

2.3. sp Hybridization 6

3 General Infrared Interpretation Concepts 7-8

4 IR Frequencies 9 5 Alkanes 10-11 6 Alkenes 12-13 7 Alkynes 14 8 Aromatic Compounds 15-17 9 Alcohols 18-19 10 The –OH Group and Hydrogen Bonding 20 11 Ethers 21-22 12 Amines 23-24 13 Carbonyl Frequencies 25-26 14 Aldehydes 27 15 Ketones 28-29 16 Carboxylic Acids 30-31 17 Esters 32-33 18 Amides 34-35 19 Anhydrides 36 20 Acid Chlorides 37 21 Fermi Resonance 38-39 22 Stretching Frequencies 40-44 22.1. Carbonyls 40-41 22.2. Nitrogen 42 22.3. Sulfur 43 22.4. Phosphorus, Boron, and Halogen 44 22.5. Inorganic 44 References 45

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1. Infrared Spectral Quality

Infrared interpretation must be performed on a high quality spectrum; otherwise, misleading results can be obtained. More often than not, bad sample preparation is the main cause of poor quality IR spectra.

A high quality infrared spectrum must possess a flat (level) baseline (positioned near or around 100% transmittance and with a low level of noise). Shifting of the baseline position would indicate at least one of the following conditions:

The background spectrum does not correspond to the sample spectrum (i.e. different solvent). The aperture size (where appropriate) for the background and sample spectra do not

correspond. The sample strongly absorbs. The sample solvent absorbs strongly when obtaining spectra in the solution phase. The NaCl or KBr discs are cloudy.

A sloping baseline usually indicates the electromagnetic radiation has been diffracted or scattered as it interacts with the sample. This may happen when the particles in the KBr pellet are not ground properly or if the sample surface of a thin film is not homogeneous (air bubbles etc.).

Additional bands should be avoided, in particular, carbon dioxide and water which can absorb infrared radiation, therefore, their presence should be minimized (usually below 2% transmittance, Figure 1). This can be achieved by ensuring samples are made with dry solvents (solution IR), dry KBr (solid state IR), and by taking a background spectrum to account for atmospheric water and carbon dioxide within the instrument.

Figure 1: IR background spectrum.

Dispersive IR instruments are dual beam instruments with equivalent beams passing independently through both the sample and reference chambers. During analysis the sample and reference beams are alternately focused on the detector by an optical chopper, allowing for comparison and subtraction of the sample and reference spectra. Therefore, the reference chamber should be filled with the same matrix as the sample to account for any IR active components (solvent, water, CO2).

FTIR instruments are single beam instruments, therefore, before a sample spectrum is obtained a spectrum of the sample matrix is acquired which can be subtracted from the sample spectrum to

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account for any IR active solvents or dissolved gases within the sample matrix which may obscure analytically relevant peaks in the sample spectrum.

Remember:

Water produces small, sharp absorption bands in the regions from 4000-3000 and 1800-1600 cm-1.

Carbon dioxide produces a strong doublet near 2340 cm-1.

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2. Orbital Hybridization – sp, sp2, and sp3 Carbon

Hybridization is used to explain molecular structures and describes the various orbital types which

are involved in the bonding between atoms. In infra-red analysis, the nature of the bonding can

make a big difference to the region of the spectrum in which the signal appears, and as such, this

brief refresher is intended to remind you of the concept of Orbital Hybridization which can be used

to help interpret infrared spectra.

Using carbon as an example it is known that for a tetrahedrally coordinated carbon (e.g. methane

CH4) the carbon atom will have four orbitals with the correct symmetry to bond to the four hydrogen

atoms. With respect to IR spectroscopy, the energy of the infrared light absorbed by a C-H bond is

dependent on the type of hybridization of the bonding orbitals. The C-H bond strengths are in the

order sp3>sp2>sp, due to the increased s character of the hybrid orbital which results in better

overlap with the hydrogen s-orbital. This results in the different IR stretching frequencies that are

observed for sp3, sp2, and sp carbons.

The orbitals that are used by carbon to form hybrid bonding orbitals are s- and p-orbitals (Figure 2).

Figure 2: s- and p-orbitals.

2.1. sp3 Hybridization

The ground state configuration of carbon is 1s2 2s2 2px1 2py1. The p orbitals are equal in energy and

said to be degenerate. The two singly occupied p orbitals can be utilized for bonding to give

methylene CH2, an unstable free radical (Figure 3). Excitation of an electron from the doubly

occupied 2s orbital to the empty 2p orbital results in four singly occupied orbitals. Excitation of an

electron from the 2s to the 2p orbital requires an input of energy; this is offset by the release of

energy that is obtained by the formation of the two additional bonds, making this an energetically

favored process. Quantum mechanics states that the lowest energy will be obtained if the four

bonds are equivalent which requires that they are formed from equivalent orbitals on the carbon.

Therefore, a set of four equivalent orbitals can be obtained via hybridization of the valence-shell

(core orbitals that are not involved in bonding) s- and p-orbitals to form four sp3 orbitals, each

consisting of 25% s character and 75% p character (Figure 3). The hybrid orbitals are orientated at a

bond angle of 109.5° from each other, giving tetrahedral geometry as seen in methane (Figure 4). In

methane the four sp3 hybrid orbitals will overlap with the hydrogen 1s orbital resulting in four

covalent bonds (σ bonds) which will be of equal length and have equal bond strengths.

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Figure 3: Formation of sp3 hybrid orbitals.

Figure 4: Molecular geometry arising from sp3 hybridization, for example in methane.

2.2. sp2 Hybridization

sp2 hybridization results in the formation of molecules with trigonal planar structures (e.g.

aluminium trihydride). The three sp2 hybridized orbitals are formed as follows (Figure 5):

Figure 5: Formation of sp2 hybrid orbitals.

Each of the three sp2 orbitals has 33% s character and 67% p character. The orbitals are orientated

to minimize electron repulsion giving bond angles of 120°. The p-orbital that is not used to form the

hybrid orbitals remains unchanged and sits perpendicular to the plane of the three sp2 orbitals

(Figure 6).

In compounds such as alkenes there is a double bond between the carbon atoms, i.e. ethene

C2H2. In the case of ethene, two of the sp2 hybridized orbitals (on each of the individual carbon

atoms) are used to form σ bonds with the 1s orbitals on hydrogen. The remaining sp2 orbitals on

each of the carbon atoms overlap to form a C-C σ-bond. The remaining two p-orbitals contain a

single electron; overlap of these orbitals forms a π-bond, creating a double bond between the two

carbon atoms. The double bond results in ethene having linear geometry with the carbon atoms

being trigonal planar (Figure 6).

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Figure 6: Geometry and bonding resulting from sp2 hybridization.

2.3. sp Hybridization

The linear geometry of molecules such as alkynes can be explained by sp hybridization. The 2s

orbital and one 2p orbital hybridize to form two sp hybrid orbitals, which will each have 50% s and

50% p character (Figure 7). These orbitals are aligned to minimize electron repulsion and give a

bond angle of 180°, as seen in linear molecules (Figure 8). There are two remaining p-orbitals that

contain a single electron that can be utilized by the molecule. As in sp2 hybridization these p-orbitals

are orientated perpendicular to the two sp orbitals.

Figure 7: Formation of sp hybrid orbitals.

The singly occupied p-orbitals can be used in molecules such as ethyne to form two additional π-

bonds resulting in a triple bond. This can only occur when two atoms, such as carbon, both have two

p-orbitals that each contains a single electron (Figure 8).

Figure 8: Geometry and bonding resulting from sp hybridization.

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3. General Infrared Interpretation Concepts

In order to infer any useful information from an infrared spectrum, it is necessary, not only to identify the region in which signals appear, but also their shape and intensity. In general terms:

Band frequency: indicates the presence of functional groups. Band shape: provides information on functional groups as well as purity. Single absorptions

are symmetric, so deviations in symmetry (such as shoulders or tailing) would indicate overlapping.

Band intensity: indicates the type and amount of functional groups within the molecule.

The following rules will help you to interpret any mid-infrared spectrum:

Band frequency assignment is usually straightforward in the region (4000-1500 cm-1) and more difficult in the fingerprint region (1500-400 cm-1) (Figure 9).

Figure 9: IR spectrum with analytically relevant regions indicated.

For each band, make a list of possible functional groups. Since most organic compounds have C-H bonds, a useful rule is that absorption between 2850

and 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.

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Figure 10: Position of unsaturated and aliphatic C-H stretching frequencies.

“Cross-checking” whenever possible. For example, an organic alcohol should exhibit O-H and C-O stretching bands.

Do not expect to be able to assign every single band in the spectrum. For a given functional group, band intensities may vary considerably.

Infrared spectroscopists have classified the intensity of signals in four groups (Figure 11):

Strong signals (s): when signal intensity is approximately 5% T. Medium signals (m): when signal intensity is between 30 and 60% T. Weak signals (w): when signal intensity does not exceed 70% T. Variable signals (v): signal strength may vary depending on the particular molecular structure,

presence of hydrogen bonding, state of the sample (solid, liquid etc.).

Figure 11: Classification of IR signals (oversimplified).

The following pages will detail the major IR spectroscopic features that can be used in structural elucidation.

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4. IR Frequencies

The interpretation of infrared spectra can be aided by an ‘overview’ of the major signals of interest as is presented below (Figure 12). Whilst the diagram is by no means comprehensive, it is intended to be a guide for beginners to recognize the major features of common infrared spectra. The topics which follow will highlight more specific spectral features generated by some common functional groups.

Figure 12: Infrared frequencies of selected functional groups.1-2

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5. Alkanes

The spectra of alkanes are usually simple (few peaks, Figure 13). The sp3 C-H stretch occurs at frequencies not exceeding 3000 cm-1.

Group Frequency (cm-1) Explanation

2930-2850(s)

Two or three bands C-H stretching

2890-2880(w)

1470-1430(m) C-H deformations

1390-1370(m) CH3 symmetrical deformation

~720(w) -CH2 rocking

Table 1: Important frequencies in alkanes.

REMEMBER: (s) – strong signal (~5% T), (m) – medium signal (~ 40% T), (w) – weak signal (~70% T), (v) – variable.

Figure 13: The infrared spectrum of octane.

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Wavenumber (cm-1) Explanation

2850 CH3 symmetric and asymmetric stretch

1450 Aliphatic CH2 bending (scissoring)

1380 Aliphatic CH3 bending (symmetrical)

720 Aliphatic CH2 rocking

Table 2: Octane IR frequencies.

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6. Alkenes

The spectra of alkenes are usually more complex (more peaks) than the spectra of alkanes (Figure 14).

The sp2 C-H stretch occurs at frequencies above 3000 cm-1. Symmetrically substituted double bonds do not absorb in the infrared region.


3095-3075(m) C-H stretching (sp2 absorption). Often obscured by the

stronger bands of saturated C-H groups 3040-3010(m)

970-960(s) C-H out of plane deformation

1416 C-H in plane bending

1680-1620(v) Non-conjugated C=C stretching

May be very weak if symmetrically substituted

Table 3: Important frequencies in alkenes.

Figure 14: The infrared spectrum of cis-2-pentene.


2950 C-H stretching (sp2 absorption)


1600 C=C stretch

700 =C-H out of plane bending

Table 4: Cis-2-pentene IR frequencies.

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970-960(s)

995-985(s) 940-900(s)

895-885(s)

840-790(m)

730-675(m)

Table 5: Alkene C-H out of plane deformations.


C=C 1680-1620(v) Non-conjugated C=C stretching

May be very weak if symmetrically substituted

Conjugated with aromatic ring 1625(m)

Dienes, trienes etc. 1650(s)

and 1600(s)

Overlap of bands is common with the low frequency band being more

intense

α,β-Unsaturated carbonyl 1640-1590(s) Weaker than C=O

Enol esters, enol ethers, and enamines 1690-1650(s)

Table 6: Alkene C=C vibrations.

Figure 14b: Alkene containing compounds.

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7. Alkynes

Terminal alkynes exhibit strong ≡C-H and C≡C signals near 3300 and 2150 cm-1 respectively (non-terminal alkynes tend to lack such signals).

The alkyl chain will exhibit similar IR frequencies to alkanes. Symmetrically substituted triple bonds do not absorb in the infrared region.

In the case of octyne (Figure 16) the alkyne bond is asymmetrically substituted, therefore, it will be IR active.


3300(m) C-H stretching (sp absorption)

900-600 C-H out of plane bending

2140-2100(w) C≡C stretch

2260-2150(v) C≡C stretch

2150-2110(s) Isonitriles

2305-2280 Nitrile oxides

~2260 Diazonium salts

2175-214(s) Thiocyanates Aryl substitutes at the upper end of the range

and alkyl at the lower end

Table 7: Important frequencies in alkynes.

Figure 16: The infrared spectrum of 1-octyne.


3300 C-H stretching (sp absorption)


2150 Asymmetric C=C stretch

600 =C-H out of plane bending

Table 8: 1-Octyne IR frequencies.

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8. Aromatic Compounds

The aromatic sp2 =C-H stretch occurs at frequencies beyond 3000 cm-1 (Figure 18). The out of plane =C-H stretch is usually intense and occurs at 900-690 cm-1. Overtone/combination bands at 2000-1600 cm-1.


3040-3010(m) C-H stretching (sp2 absorption)

1280-1000 C-H in-plane bending

970-960(s) C-H out of plane bending

~1600(m) ~1580(m) ~1500(m)

Aromatic C=C Stretch

Table 9: Important frequencies in aromatic compounds.

The IR stretching frequencies found in the region 900-675 cm-1, produced by the C-H out-of-plane bending modes, are highly characteristic of the aromatic substitution pattern (Figure 17).

Figure 17: Aromatic substitution (oversimplified).

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Figure 18: The infrared spectrum of toluene.

Note that the aromatic C-H stretches are to the left of 3000 cm-1 and the alkyl C-H stretches are to the right.


3010 Aromatic C-H stretching (sp2 absorption)


2000-1600 Aromatic overtone/combination bands

1600-1475 Aromatic C=C stretch

900-790 =C-H out of plane bending

760-690 Mono substitution pattern

Table 10: Toluene IR frequencies.


Aromatic rings

~1600(m)

~1580(m) Stronger when the ring is further conjugated

~1500(m) Usually the strongest of the 2 or 3 bands

Table 11: Aromatic compounds.

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R-C6H5 770-730(s)

and 720-680(s)

Monosubstituted

R2-C6H4

770-735(s)

810-750(s), 720-685(m) 860-800(s)

Disubstituted Ortho Meta Para

R3-C6H3

810-750(s), 720-685(m) 900-820(m), 860-800(s) 900-820(s), 720-685(m)

Trisubstituted 1,2,3- 1,2,4- 1,3,5-

R4-C6H2

840-800(s) 880-840(s) 880-840(s)

Tetrasubstituted 1,2,3,4- 1,2,3,5- 1,2,4,5-

R5-C6H 900-860(m-s) Pentasubstituted

R6-C6 415-385(m-s) Hexasubstituted

Table 12: Benzene ring substitution patterns.

Figure 19: Benzene substitution patterns.

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9. Alcohols

The free ‒OH band appears as a sharp signal of medium to low intensity near 3620 cm-1. This band usually appears when the alcohol is dissolved in a solvent (Figure 20).

The hydrogen bonded ‒OH band appears as a broad peak between 3400 and 3300 cm-1. This band usually appears when the alcohol is not dissolved in a solvent.

The hydrogen bonded and the free ‒OH bands are present together with the relatively weak free ‒OH signal appearing to the left or completely overlapped by the hydrogen bonded signal.


3650-3590(v) Free –OH stretch (sharp peak)

3600-3200(s) H-bonded –OH (solid, liquid, and dilute solution, broad band)

3200-2500(v) Intramolecular H-bonded –OH in chelate form (broad)

3600-3100(w) Water of crystallization (solid state spectra)

1410-1260(s) –OH bending

1150 - 1040 C-O stretching

Table 13: Important frequencies in aromatic compounds.

Figure 20: The infrared spectrum of 1-hexanol.

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3650 Free –OH stretch

3400-3300 Hydrogen bonded –OH stretch




1100 C-O stretching


Table 14: 1-Hexanol IR frequencies.

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10. The –OH Group and Hydrogen Bonding

The stretching frequency of –OH groups can be used to measure the strength of hydrogen bonds.

The stronger the hydrogen bond the longer the ‒OH bond will be, resulting in a lower vibrational frequency, and a broader and more intense absorption band.

Sharp free ‘monomeric’ –OH bands (3650-3590(v) cm-1) will often be seen when there is little possibility for hydrogen bonding i.e. in the vapor phase, dilute solutions, and when steric hindrance precludes H bonding.

Pure liquids, solids, and solutions will often only exhibit a broad ‘polymeric’ H bonded band for the –OH group (3600-3200(s) cm-1).

However, liquids will sometimes show both monomeric and polymeric bands.

A good diagnostic test of whether an inter- or intramolecular H bond is present is to dilute the solution; intramolecular H bonds will be unaffected, and therefore, the absorption band will remain unchanged, whereas, intermolecular bonds will be broken resulting in a decrease in the H-bonded –OH absorption and an increase in, or appearance of, a free –OH band.

Figure 21: The O-H stretch region. (a) Hydrogen-bonded O-H only (neat liquid). (b) Free and

hydrogen-bonded O-H (dilute solution). (c) Free and hydrogen-bonded O-H (very dilute solution).

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11. Ethers

The spectra of ethers are usually simple. The main difference between the IR spectra of ethers and alcohols is that ethers lack O-H

signals. The C-O stretching allows for the differentiation of ethers from alkanes (Figure 22).


1150-1070(s) C-O stretching

1275-1200(s) 1075-1020(s)

C-O stretching alkyl aryl ether

2850-2810(m) C-H stretching aryl ethers at high end of the range

1225-1200(s)

1270-1030(s)

1260-1240(m-s)

880-805(m) Monosubstituted

950-860(v) Trans form

865-785(m) Cis form

770-750(s) Trisubstituted

Table 15: Important frequencies in ethers.

Figure 22: The infrared spectrum of dibutyl ether.

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1100 C-O stretching


Table 16: Dibutyl ether IR frequencies.

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12. Amines

Primary amines have two N-H bands in the range 3500–3300 cm-1 corresponding to symmetrical and unsymmetrical stretching (Figure 23 - 24).

Figure 23: N-H stretching frequencies in primary (left) and secondary (right) amines.

Secondary amines absorb weakly and have one N-H band in the range 3500–3300 cm-1. Tertiary amines lack N-H signals.


3500-3300(m) Amine and imine N-H stretching

1650-1560(m) N-H bend in primary amines result in a broad band

1580-1490(w) N-H bend in secondary amines

1600(s) 1500(s)

Secondary amine salts exhibit a 1600 cm-1 band

~800 N-H out of plane bending

1350-1000 C-N stretch

Table 17: Important frequencies in amines.

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Figure 24: The infrared spectrum of n-hexylamine.


3500 NH2 symmetric and asymmetric stretch


1600 N-H bending



1100 C-N stretching

800 N-H out of plane bending


Table 18: N-hexylamine IR frequencies.

N-H bands can often be confused with OH bands, however, due to their lower propensity for forming hydrogen bonds the bands will be sharper which can aid in identification.

N-H absorption is of weaker intensity.


Aliphatic

1650-1560(m)

1090-1020(w-m) α-carbon branching at 795 cm-1

850-810(w-m) Strong

495-445(m-s) Broad

Aromatic 1350-1260(s) Also for secondary aryl amines

445-345

Amino acids

3100-3030(m) Solid state

2800-2400(m) 1625-1560(m)

1550(m) Sharp bands; dilute solution

Table 19: Primary, secondary, and tertiary amines.

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13. Carbonyl Frequencies

Composed of a carbon atom double bonded to an oxygen atom, carbonyl groups absorb energy in the infrared region between approximately 1600 to 1900 cm-1 (Figure 25). Weak bands, overtones of the strong C=O absorption (1800-1600 cm-1) appear in the 3600-3200 cm-1 region.

Figure 25: IR frequency values for selected carbonyl compounds.

Electron-Withdrawing Effect: electron withdrawing centers next to the carbonyl group will shift its frequency to higher values.

Figure 26: The electron withdrawing effect will shift the carbonyl band to higher frequencies.

Further conjugation has relatively little effect.

When one or more of the structural influences is affecting a carbonyl group the effect is close to additive.

Carbonyl intensities: Acids absorb more strongly than esters.

Esters absorb more strongly than ketones or aldehydes.

Hydrogen Bonding Effect: Intramolecular hydrogen bonding will shift the carbonyl signal to lower frequencies. In the diagram below, the molecule methyl salicylate (an ester) exhibits an unusually low carbonyl frequency of 1680 cm-1 instead of being between 1735 and 1750 cm-1. This phenomenon is due to the formation of an intra-molecular hydrogen bond.

Figure 27: The hydrogen bonding effect will shift the carbonyl band to lower frequencies.

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Resonance Effect: when the unpaired electrons on a nitrogen atom conjugate with the carbonyl group, then the carbonyl band is shifted to a lower frequency.

Figure 28: The resonance effect will shift the carbonyl band to lower frequencies.

Ring Size: six-membered rings with carbonyl groups will exhibit similar frequencies to their non-cyclic counterparts; however, decreasing the ring size (rings with less than six members) will increase the frequency of the carbonyl absorption.

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14. Aldehydes

The spectra of aldehydes usually contain strong C-O stretching bands between 1740 and 1680 cm-1 (Figure 29).

Aldehydes exhibit a pair of C-H stretching bands; however, while the lower frequency (2760–2700 cm-1) signal is usually found in most spectra, the one at high frequency values (2860–2800 cm-1) is usually obscured by alkyl C-H signals.


1740-1720(s) Aliphatic C=O stretching

1715-1695(s) Aromatic C=O stretching

2900-2700(w) C-H aldehyde stretch consists of a pair of weak signals (one of

them is usually obscured by alkyl signals)

Table 20: Important frequencies in aldehydes.

Figure 29: The infrared spectrum of nonanal.



2800 and 2700 Aldehyde C-H stretch consists of a pair of signals

1720 Aliphatic C=O stretching




Table 21: Nonanal IR frequencies.

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15. Ketones

Ketones show strong C=O carbonyl stretching signals between 1780 and 1615 cm-1 (Figure 31). Aromatic and conjugated ketone systems will exhibit lower carbonyl frequencies than their

aliphatic counterparts. The infrared spectra of 1,2-diketones will show one strong carbonyl signal near 1716 cm-1. The infrared spectra of 1,3-diketones are usually complex; this is because, 1,3-diketones

typically undergo keto-enol tautomerism (Figure 30).

Figure 30: Keto-enol tautomerism in 1,3-diketones. Note that for these compounds not only

carbonyl but alcohol and alkene frequencies can be detected in the infrared spectrum.



1700-1680(s) Aromatic C=O stretching

Table 22: Important frequencies in ketones.

In liquid or solid state spectra C=O stretching frequencies will be lowered by ~10-20 cm-1, while in vapor phase spectra they will be raised by ~20 cm-1.

Figure 31: The infrared spectrum of acetophenone.

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1650 Aromatic C=O stretching

1600 Aromatic C=C stretching

1280 Aromatic =C-H in-plane bending

800-680 Mono substituted aromatic ring

Table 23: Acetophenone IR frequencies.


α,β-Unsaturated 1685-1665 Often two bands

α,β-, α’,β’-Unsaturated and diaryl 1670-1660

Cyclopropyl 1705-1685

Six-ring ketones and larger Similar to the corresponding open chain

ketone

Five-ring ketones 1750-1740

Four-ring ketones ~1780

α-Chloro or α-bromo 1745-1725 Affected by conformation; when halogen is in the same plane as C=O higher

frequencies are observed α,α’-Dichloro or α,α’-Dibromo 1765-1745

1,2-Diketones s-trans (i.e. open chains)

1730-1710 Asymmetrical stretching of both C=O

groups

1,2-Diketones s-cis, six-ring 1760 and 1730

1,2-Diketones s-cis, five-ring 1775 and 1760

Enolised 1,3-diketones 1650 and 1615 C=O lowered by H bonding and C=C

Table 24: Ketone carbonyl absorption bands C=O (all bands are strong).

Figure 31b: Ketone compounds.

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16. Carboxylic Acid

Carboxylic acids exhibit a very strong carbonyl signal from 1760-1690 cm-1. The exact position of this band depends on whether the carboxylic acid is saturated, unsaturated, dimerized, or has internal H-bonding (Figure 32).

Centered near 3000 cm-1, the O-H stretch of carboxylic acids appears as a broad band between 3400 and 2400 cm-1. The reason the O-H band is broad is that carboxylic acids usually exist as H-bonded dimers.

The broad O-H stretch often obscures C-H vibrations occurring in the same region The presence of a broad O-H band plus a strong carbonyl frequency, almost certainly

confirms the analyte is a carboxylic acid.


3000-2500(s) Carboxylic acid O-H stretch (broad peak)

1725-1700(s) Saturated

1715-1690(s) α,β-Unsaturated

1700-1680(s) Aryl

1740-1720(s) α-halo

1760-1690 C=O stretching

1440-1395(s) and 995-915(s) C-O-H bending

1320-1210 C-O stretching

Table 25: Important frequencies in carboxylic acids.

Figure 32: The infrared spectrum of benzoic acid.

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3000 O-H stretch from carboxylic acid


1690 C=O stretch from carboxylic acid

1600 Aromatic C=C stretching

1420 C-O-H bending

1300 C-O stretching


Table 26: Benzoic acid IR frequencies.

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17. Esters

In esters, the C-O stretch appears as two or more bands (usually one stronger than the others) in the range 1300-1000 cm-1 (Figure 33).

Cyclic esters, known as lactones, will exhibit lower carbonyl frequencies than their non-cyclic counterparts.

The ring size effect in esters is important whenever the size of the ring is equal to or lower than six membered (the smaller the ring size, the higher the carbonyl frequency).



1730-1715(s) Aromatic and α,β-unsaturated C=O stretching

1310-1050(s) Usually two strong bands

Table 27: Important frequencies in esters.

Figure 33: The infrared spectrum of hexyl butyrate.


3500 Carbonyl overtone


1740 C=O stretch from ester

1360 Aliphatic CH3 bending (symmetric)

1250-1100 Aliphatic C-O stretching


Table 28: Hexyl butyrate IR frequencies.

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1800-1750(s) Aryl and vinyl esters

1770-1745(s) Esters with electronegative α-substituents

(e.g. Cl)

α-Keto esters 1755-1740(s)

Six-ring and larger lactones Similar values to the corresponding open

chain esters

Five-ring lactone 1780-1760(s)

α,β-Unsaturated five-ring lactone 1770-1740(s) Two bands seen in the presence of αC-H

β,γ-Unsaturated five-ring lactone ~1800(s)

Four-ring lactone ~1820(s)

β-Keto ester in H-bonding enol form

~1650(s)

Table 29: Ester stretching frequencies.

Figure 33b: Ester compounds.

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18. Amides

Amides exhibit a strong carbonyl stretching frequency in the region 1670-1630 cm-1 (Figure 34).

Amides give two bands which can be attributed to forms 1 and 2.

The carbonyl region of many amide compounds will also exhibit two bands. Hydrogen bonding lowers and broadens N-H stretching frequencies, but to a lesser extent

than is seen for O-H functional groups.


3500(m) 3400(m)

N-H stretch primary amide (two signals)

3460-3400(m) N-H stretch secondary amide (two bands. Only one band with

lactams)

1650-1560(m) N-H bending primary amide

1580-1490(w) N-H bending secondary amide

750-650 N-H wagging

1670-1630(s) C=O stretching

1560 - 1530 C-N stretch

Table 30: Important frequencies in amides.

Figure 34: The infrared spectrum of N-methylacetamide.

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3280 N-H stretch secondary amine

3100 Overtone signal located in the region 3100-3060 cm-1


1680 C=O amide stretch

1560 N-H bending secondary amide

1530 C-N stretch

750-650 N-H wagging

Table 31: N-methylacetamide IR frequencies.

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19. Anhydrides

Anhydrides exhibit two strong carbonyl signals in the 1850-1740 cm-1 region. These correspond to the symmetric and asymmetric stretch (Figure 35).

The two carbonyl bands are usually separated by ~60 cm-1, with the higher frequency band being more intense in acyclic anhydrides and the lower frequency band is more intense in cyclic anhydrides.

One or two strong bands may be observed due to the C-O stretching frequency in the 1300-1050 cm-1 region.


1850-1800(s) 1790-1740(s)

C=O stretch Two bands usually

separated by ~60 cm-1

1830-1780(s) 1770-1710(s)

Aryl and α,β-unsaturated

1870-1820(s) 1800-1750(s)

Saturated five-ring

1300–1050(s) C-O stretch

One or two strong bands

Table 32: Important frequencies in anhydrides.

Figure 35: The infrared spectrum of propionic anhydride.



1819 Anhydride C=O symmetric stretch

1750 Anhydride C=O asymmetric stretch

1100 Anhydride C-O stretch

Table 33: Propionic anhydride IR frequencies.

i

37


20. Acid Chlorides

A weak overtone signal located in the region 3100 - 3060 cm-1 is sometimes detectable. In some aromatic acid chlorides, a strong band on the lower side of the carbonyl C=O

stretching will be present, resulting in the carbonyl appearing as a doublet. This phenomenon is known as a Fermi resonance (named after the great Italian physicist Enrico Fermi, who first explained the phenomenon).


1815-1790(s)

C=O stretching saturated (Acid fluorides higher, bromides and

iodides lower)

1790-1750(s) C=O stretching aryl and α,β-unsaturated

800-600(s) C-Cl stretch

Table 34: Important frequencies in acid chlorides.

Figure 36: The infrared spectrum of benzoyl chloride.



1780 C=O stretching (conjugated chlorides)

1700 The second carbonyl band exhibited by benzoyl chloride is attributable to Fermi Resonance

(named after the Italian physicist Enrico Fermi who first explained the phenomenon)

1600 Aromatic C=C stretch


730-550 C-Cl stretch

Table 35: Benzoyl chloride IR frequencies.

i

38


21. Fermi Resonance

A Fermi resonance is the shifting of the energies and intensities of absorption bands in an infrared or Raman spectrum, and is a consequence of quantum mechanical mixing.

Two criteria must be fulfilled for Fermi Resonance to occur:

1. The two vibrations must have the same symmetries 2. The transitions have almost the same energy

Fermi resonance most often occurs between normal and overtone modes if they are approximately coincident in energy.

Fermi resonance results in two effects:

1. The high energy mode is shifted to higher energy, while the low energy mode is shifted to lower energy

2. An increase in intensity will be seen for the weaker band, and the more intense band will decrease in energy

The two transitions can be thought of as a linear combination of the parent modes.

Figure 37: Example of intensity and frequency shifts due to Fermi resonance. The top bands represent two fundamental vibrations without Fermi resonance, and the bottom bands show the change in bands as a result (left). The two energy levels are split such that one increases and the other decreases in energy, known as a “Fermi doublet” and they move away from each other.

39


In the case of saturated acid chlorides, i.e. acetyl chloride, strong carbonyl stretching frequencies are present at 1815-1790 cm-1 (Figure 38). When the carbonyl group is conjugated to an aromatic system, as in benzoyl chloride, a second (usually less intense) carbonyl stretching frequency will appear at ~1730 cm-1 (Figure 38). The occurrence of a second stretching frequency is attributable to Fermi resonance involving the fundamental C=O stretching frequency (~1780 cm-1) and the first overtone of an intense aryl C-H bending mode (~900 cm-1).

Figure 38: IR spectrum of acetyl chloride (top) and benzoyl chloride (bottom).

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22. Stretching Frequencies

22.1. Carbonyl


Imides

+15 cm-1 shift with α,β-unsaturation Cyclic six-ring

~1710 and 1700(s)

Cyclic five-ring ~1770 and 1700(s)

Ureas

RNHCONHR ~1660(s)

Six-ring ~1640(s)

Five-ring ~1720(s)

Urethanes

1740-1690(s) Non- or mono-substitution on

N will give amide II band 1700-1670(s)

Thioesters & Acids

~-25 cm-1 shift with α,β-

unsaturation RCOSH ~1720(s)

RCOS-alkyl ~1690(s)

RCOS-aryl ~1710(s)

Table 36: IR frequencies of carbonyl containing compounds. Carbonyl absorption frequencies are always strong.

Figure 39: Carbonyl containing compounds.

41



Carbonates

~1780(s)

~1740(s)

~1785(s)

Cyclic five-ring

~1820(s)

~1645(s)

~1715(s)

Table 37: IR frequencies of carbonyl containing compounds. Carbonyl absorption frequencies are always strong.

42


22.2. Nitrogen


3400-3300(m) N-H stretching

(lower when H-bonded)

1690-1640(v) Large variations in intensity and

close proximity to C=C stretching which makes identification difficult α,β-unsaturated 1660-1630(v)

Conjugated cyclic systems 1660-1480(v)

Table 38: IR frequencies of imines, oximes etc.


1500-1400(v)(m) Very weak or inactive

in IR

1480-1450 1335-1315

Symmetric and asymmetric stretch

Table 39: IR frequencies of azo compounds.


1570-1540(s) 1390-1340(s)

~30 cm-1 lower when conjugated. Two bands due to symmetrical and asymmetrical

NO stretch

Nitrates

1650-1600(s) 1270-1250(s)

Nitramines

1630-1550(s) 1300-1250(2)

1600-1500(s) 1585-1540 1510-1490

Saturated aryl

1680-1610(s) Two bands

1500-1430(s)

1300-1200(s) 970-950(s)

Aromatic Aliphatic (very strong)

1410-1340

860-800

Table 40: IR frequencies of nitro, nitroso etc.

Figure 40: Imine compounds.

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22.3. Sulfur


2600-2550(w) SH stretch

1200-1050(s) C=S stretch

~3400

1550-1450(s)

1300-1100(s)

N-H stretch

Amide II

Amide I

~1225

~1170

1340-1130

~1050

~1070

1060-1040(s)

1350-1310(s)

1160-1120(s)

1370-1330(s)

1180-1160(s)

‒SO2‒O‒ 1420-1330(s)

1200-1145(s)

1410-1375

1205-1170

‒S‒F 815-755

Table 41: IR frequencies of sulfur containing compounds.

44


22.4. Phosphorus, Boron, and Halogen


P-H 2440-2350(s) Sharp

P-Ph 1440(s) Sharp

P-O-alkyl 1050-1030(s)

P-O-aryl 1240-1190(s)

P=O 1300-1250(s)

P=S 750-580

P-O-P 970-910 Broad

2700-2560 H-bonded O-H

1240-1180(s) P=O stretch

p-F 1110-760

Table 42: IR frequencies of phosphorus containing compounds.


B-H 2640-2200(s)

B-O 1380-1310(vs)

B-N 1550-1330(vs)

B-C 1240-620(s)

Table 43: Boron containing compounds.

Group Frequency (cm-1) Explanation C-F 1400-1000(s) Sharp

780-680 Weaker

C-Cl 800-600(s)

C-Br 750-500(s)

C-I ~500(s)

Table 44: Halogen containing compounds.

22.5. Inorganic

Group Frequency (cm-1) Explanation Ammonium 3300-3030(s) NH3

+

Cyanide, thiocyanate, cyanate 2200-2000(s) CN-, SCN

-, OCN

-

Carbonate 1450-1410(s) CO32-

Sulfate 1130-1080(s) SO42-

Nitrate 1380-1350(s) NO3-

Nitrite 1250-1230(s) NO2-

Phosphates 1100-1000(s) PO43-

Table 45: IR frequencies of inorganic ions.

45


23. References

1. John A. Dean. “Lange’s Handbook of Chemistry” Section 7.5. Fifteenth Edition. United States of America. McGraw Hill. 1999.

2. Dudley H. Williams and Ian Fleming “Spectroscopic Methods in Organic Chemistry” Chapter 2. McGraw Hill. 1995.

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