CHM427 IR Scans

INTRODUCTION TO INFRARED (IR) SPECTROSCOPYVirtually all organic compounds absorb IR radiation. The frequency absorbed varies with the functional groups present, e.g., OH, NH, C=O, C=C, etc. Thus IR spectroscopy is a powerful method of classifying unknown organic compounds by identifying the functional groups present in the compounds.

The IR portion of the electromagnetic spectrum lies between visible light and microwaves. It is divided into three regions; the near IR, mid IR and far IR as shown

Wavelength Boundaries of Important EM Radiation Bands

(m) (m) (m) (cm-1 )Vacuum UV 10 - 200 .01 - .20

Near UV 200 - 400 .20 - .40Visible 400 - 800 .40 - .80Near IR .80 - 2.5 13000 - 4000Mid IR 2.5 - 25 4000 - 400Far IR 25 - 1000 400 - 10

Microwave 103 - 106 .001 - 1Radio & TV 1 - 104

The last column labeled (cm-1) is a frequency unit called ‘wavenumbers’ or ‘reciprocal centimeters’. It is calculated as the mathematical inverse of the wavelength in cm. Thus wavenumber ( ) has units of cm-1 and is directly proportional to the frequency of the radiation.

An example of the conversion between wavenumbers and wavelength is shown below.

Note that wavenumber is directly proportional to frequency. Mathematically, it differs from frequency by a constant multiplier, i.e., ‘c’, the speed of light in a vacuum...

compare

1

and =c

(by rearranging c = )

Most organic functional group absorptions occur in the mid IR range, between 4000 and 400 cm-1 (25 and 1000 m, respectively).

IR Spectroscopy Introduction 1 of 14

When IR radiation strikes a molecule it is absorbed and causing the molecule to vibrate. Picture a molecule and its bonds as an assembly of spheres (atoms) and springs (bonds).

There are 2 kinds of molecular vibrations; i.e., stretching and bending.

1. Stretching vibrations involve bond length change, i.e., the distance between atoms increase or decreases but the bond angle remains unchanged.

2. Bending vibrations involve a change in bond angle, i.e., atoms change positions while maintaining a constant distance between atoms. Bending vibrations are also called ‘deformations’.

Terminology and Types of Fundamental Vibrations:Consider a methylene group (CH2)

1. symmetrical stretching (s CH2 @ 2850 cm-1)

(same symbol as used for frequency)

2. asymmetrical stretching (as CH2 @ 2930 cm-1)(Note that asymmetric stretching occurs at slightly higher frequencies than symmetric stretching)

3. in-plane bending (scissoring) (s CH2 @ 1465 cm-1)

(Greek symbol 'delta')

4. in-plane bending (rocking) ( CH2 @ 720 cm-1)

(Greek symbol 'rho')

5. out-of-plane bending (wagging) ( CH2 @ 1350-1150 cm-1)

(Greek symbol 'omega')

6. out-of-plane bending (twisting) ( CH2 @ 1350-1150 cm-1)

(Greek symbol 'tau')


VIBRATIONS

STRETCHING BENDING

++

+_

The mid IR region is subdivided as shown below and the approximate frequency (cm-1) at which various functional groups absorb is shown.

In the group frequency region, absorption bands are characteristic of specific functional groups (OH, NH2, C=O, C-H, etc.). These appear at fairly constant positions, rather independent of the rest of the molecule.

In the fingerprint region, vibrational frequencies are greatly affected by the whole molecular structure and spectra are considered specific for a particular molecule. Some functional group absorption can be identified in the fingerprint region, especially below 1000 cm-1.

The region between 1400-1000 cm-1 is often congested with deformation (bending) bands and is difficult to interpret and many (but not all) bands in this region are sometimes ignored.

In general, more polar bonds absorb more intensely than less polar (and non polar) bonds.

IR analysis can be carried out on gas, liquid, or solid samples.

Obtaining and Interpreting and IR SpectrumIR spectrometers are scanning instruments. A sample of the substance to be tested is irradiated with the entire range of mid IR radiation; various functional groups absorbing at different frequencies. An IR spectrum is produced. It is a plot of transmittance vs. frequency (cm-1) and wavelength (m). Peaks (absorption bands) appear as dips in the baseline; the baseline running at the top of the spectrum, near 100% transmittance.

The theory of IR spectroscopy will be covered in greater detail in CHM 526, Organic Chemistry 3. The purpose of this introductory unit is to give the student a head start at training your eye to recognize the main functional groups of organic compounds.

Credits: The following IR scans are from SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/ (May, 2004). This is an excellent Spectroscopy database web site that contains IR, MS, NMR and CNMR scans for many organic compounds.


4000 3000 2000 1400 1250 1000 800 (cm-1)

fingerprint regiongroup frequency region

CC C=CN-HC=NCNC-HO-H C=O

congested region

Alkanes: CH3 @ 2960 & 2850 cm-1 and @ 1460 & 1380 cm-1

CH2 @ 2930 & 2850 cm-1 and @ 1460 & 720 cm-1

multiple C-C bands appear over a broad range of 1200 to 800 cm-1 Note: all CH bands are below 3000 cm-1

Alkenes: have all the same absorptions as alkanes plus: terminal =CH2 @ 3080 or internal =CHR @ 3020 cm-1 (both above 3000 cm-1) C=C @ 1650 cm-1 and =CH2 between 1000 to 720 cm-1

Alkynes: have all the same absorptions as alkanes plus: terminal C-H @ 3300 cm-1 and C-H between 700 to 600 cm-1 CC @ 2120 cm-1


CH3 CH2 CH34

hexane CH2 & CH3

< 3000 cm-1

CH2 & CH3

1460 cm-1

CH3

‘umbrella’ 1380 cm-1

CH2

720 cm-1

1-hexene =CH2

3080 cm-1

C=C 1650 cm-1

CH2 CH(CH2)3CH3

=CH2 910 & 990 cm-1

C-H 3300 cm-1

1-hexyneCH C(CH2)3CH3

CC 2120 cm-1

C-H 2120 cm-1

Nitriles: have the same absorptions as alkanes plus CC @ 2240 cm-1 is stronger than an alkyne otherwise it looks like a terminal alkyne

Aromatics: have some unique absorption bands several =C-H bands @ 3030 cm-1, ring stretches @ 1500 &/or 1600 cm-1

group of 2 to 6 small CH bands (‘overtones’) between 2000 & 1700 cm-1

characteristic overtones and oop patterns are appended group of 1 to 3 large CH ‘out of plane bends’(‘oop’) between 900 & 700 cm-1

Monosubstituted aromatics: have oop bands @ ca. 700 & 750 cm-1

Meta disubstituted aromatics: have oop bands @ ca. 700, 770 & 870 cm-1


hexanenitrilee

CC 2240 cm-1

CH3 CH2 C N4

toluene

=C-H 3030 cm-1

CH3

phenyl ring 1600 & 1500 cm-1

overtones 2000 – 1700 cm-1

mono oop 700 & 750 cm-1

=C-H 3030 cm-1

m-xylene

CH3

H3C

meta oop 700, 770 & 870 cm-1




Ortho disubstituted aromatics: have a single oop band below 800 cm-1

Para disubstituted aromatics: have a single oop band at or above 800 cm-1

Ethers: have the same absorptions as alkanes plusaliphatic ethers: one broad C-O-C band between 1150 & 1085 cm-1


=C-H 3030 cm-1

o-xylene overtones 2000 – 1700 cm-1

phenyl ring 1600 & 1500 cm-1 ortho oop

750 cm-1

p-xylene

=C-H 3030 cm-1



para oop 800 cm-1

CH3H3C

C-O-C 1125 cm-1

diethyl ether

Ethers (Continued): aryl-alkyl, diaryl, and vinyl ethers: two broad C-O-C bands between 1275 & 1200 cm-1 and between 1075 & 1020

Also note the aromatic bands in methoxybenzene

Alcohols: are easily identified. Characteristic bands are: strong, broad O-H band @ ca. 3300 cm-1 broad C-O band between 1260 & 1000 cm-1 (like the ether C-O-C stretch) broad, weak O-H band between 770 & 650 cm-1

Phenols: show the OH and CO stretches as well as the aromatic bands


methoxybenzene(anisole)

C-O-C 1250 & 1030 cm-1

O CH3

C-O 1050 cm-1

1-butanolCH3CH2CH2CH2 OH

O-H 3300 cm-1

O-H650 cm-1

phenol

O-H 3300 cm-1

OH

C-O 1240 cm-1

Carbonyl Compounds: all contain the C=O group which absorbs strongly between 1870 & 1540 cm-1. Electron withdrawing groups (-NO2, -Cl, etc.) increase its frequency while electron withdrawing groups (-NH2, -OH, phenyl, etc.) lower its frequency.

Ketones: without donating/withdrawing groups show a strong C=O band @ 1715 cm-1

The aryl group in acetophenone lowers the C=O frequency to 1690 cm-1 and shows the characteristic aromatic bands

Aldehydes: The normal (aliphatic) aldehyde C=O band occurs at 1740 to 1720 cm-1. Aldehydes have a unique C-H stretch @ 2820 – 2695 cm-1 (otherwise they are

indistinguishable from ketones)

In Aromatic Aldehydes the frequency of the C=O stretch is lowered to 1705 cm-1 and the characteristic aromatic absorption bands are evident


C=O 1715 cm-1

2-hexanoneCH3 C

O

CH2CH2CH2CH3

C=O 1690 cm-1

acetophenone

C

O

CH3

C=O 1725 cm-1

cyclohexanecarbaldehyde

C

O

H

C-H2700 cm-1

Carboxylic Acids: The C=O band occurs at 1710 cm-1 in aliphatic carboxylic acids. A very broad O-H band occurs from 3300 all the way to 2500 cm-1 C-O and in-plane O-H occur in the 1400-1200 cm-1 region broad O-H oop occurs near 920 cm-1-

In Benzoic Acid the C=O frequency is lower (1690 cm-1) due to resonance with the phenyl ring and the monosubstituted aromatic bands are evident

Acid Anhydrides: have two C=O groups joined by an O atom. Their IR spectra are unique as their carbonyl peak is split into two or more peaks.

C=O peak stretch occurs between 1860 to 1720 cm-1


C=O 1710 cm-1

hexanoic acid

benzaldehyde

C=O 1700 cm-1

C

O

H C-H

2700 cm-1 C-H

2700 cm-1

O-H 3300 - 2500 cm-1

O-H 1420 & 940 cm-1

C-O 1260 cm-1

benzoic acid

C=O 1690 cm-1

O-H 3300 - 2500 cm-1

C-O 1300 cm-1

O-H 1430 & 940 cm-1

C

O

OH

a broad C-O-C stretch occurs between 1300 to 1050 cm-1

In Phthalic Anhydride, the C=O band is split into a multiplet.

Acid Chlorides: have an IR spectra much like ketones except that the C=O band occurs at a higher frequency (1800 to 1700 cm-1)

Esters: show characteristic absorption bands for C=O and C-O stretches. Esters might be mistaken for ketones except for the broad C-O ester bands

C=O at 1750 to 1735 cm-1 for aliphatic esters and at 1730 to 1715 cm-1 for aromatic esters

all esters have two broad C-O bands at ca. 1300 and at ca. 1050 cm-1


acetic anhydride

split C=O 1800 cm-1

C-O-C 1100 cm-1

split C=O 1800 cm-1

phthalic anhydride

OC

C

O

O

acetyl chloride

C=O 1800 cm-1

C

O

ClH3C

C-Cl 590 cm-1

Phenyl Benzoate: has a C=O band @ 1725 cm-1, C-O bands at 1050 and 1275 cm-1

as well as characteristic absorption bands of a monosubstituted aromatic.

Halogenated Alkanes: The C-H stretch may be raised above 3000 cm-1 with several electronegative halogens. Halogens are difficult to identify by IR. Mass Spec is much better.

CH2-X occurs between 1300 and 1150 cm-1

C-Cl occurs between 850 and 550 cm-1

C-Br occurs between 690 and 515 cm-1

Amides: Amides:

slightly below 1700 cm-1, all amides show a C=O band (‘amide 1’ band) (reduced by resonance with unpaired electrons on N)

between 1650 and 1515 cm-1, 1° and 2° amides and some lactams show and N-H bend (‘amide 2’ band). This band is usually partially or completely overlapped by the amide 1 band and may not be distinguishable. 3° have no N-H bonds.


methyl acetate C=O

1740 cm-1 C-O

1250 & 1050 cm-1

C

O

OH3C CH3

phenyl benzoate C=O 1725 cm-1

C-O 1270 & 1050 cm-1

methyl acetate

C=O 1725 cm-1

C-Cl 850 - 550 cm-1

chlorocyclohexane

Cl

between 800 and 630 cm-1, 1° and 2° amides (and amines) show medium to weak N-H oop bands (similar to the O-H bend of alcohols)

between 3500 and 3300 cm-1, 1° amides show two N-H bands while 1° amides show one N-H bands. 3° amides have none. They are narrower and less intense than O-H bands of alcohols and phenols.

between 1400 and 1050 cm-1, all amides show a medium intensity C-N band

Amines: The and N-H bands and C-N bands in amines are basically the same as in amides.

in the 3500-3300 cm-1region, 1º amines (R-NH2) show two N-H bands, 2º amines (R2-NH) show one N-H band and 3º amines (R3-N) have no N-H bonds

between 1650-1500 cm-1, in-plane N-H band (scissoring) occurs. (equivalent to the Amide 2 band of 1 and 2 amides). The band is strongest in 1º amines, small and difficult to locate in 2º amines, and does not occur in 3º amines


benzamide(1° amide)

N-H 3200 & 3370 cm-1

C

O

NH

H

amide 1 1670 cm-1

N-H 630 cm-1amide 2

1580 cm-1

C-N1400 cm-1

N-methylbenzamide(2° amide)

N-H 3300 cm-1

C

O

NCH3

H

amide 1 1640 cm-1

C-N1320 cm-1

N-H 650 cm-1

amide 2 1550 cm-1

N,N-dimethylbenzamide(3° amide)

C-N & CH3

1380 cm-1

amide 1 1630 cm-1

C

O

NCH3

CH3

has no N-H or N-H

between 910-670 cm-1, out-of-plane N-H occurs. The band is strongest in 1º amines, and does not occur in 3º amines

between 1250-1020 cm-1, medium to weak C-N occurs in aliphatic amines. Oddly, the frequencies are increased to 1340-1265 cm-1 in aromatic amines


N-H3300 cm-1

diethylamine (2° amine)

CH3CH2 NH CH2CH3

N-H730 cm-1

N-H, CH2 & CH3

1465 -1480 cm-1 C-N

1130 cm-1

hexanamine(1° amine)

N-H3300 & 3450 cm-1

CH3 CH2 CH2 NH24

‘ amide 2’ N-H

1600 cm-1 N-H

800 cm-1

triethylamine (3° amine)

N(CH2CH3)3

C-N1100 cm-1

has no N-H or N-H

CH2 & CH3

1480 cm-1


Documents

CHM427 IR Scans