INTRODUCTION to FOURIER TRANSFORM INFRARED(FTIR) SPECTROSCOPY and INTERPRETATION of IR SPECTRA

ATR

Attenuated total reflectance

Each peak represents

a particular bond in a compound

Each bond

has a characteristic amount of energy different from other bonds

peak

Commonly uses the wavenumber (cm-1 ) unit which increases from left to right

The stronger the peak,

the more the peak dips to the bottom

wavenumber

reciprocal of wavelength

CHARACTERISTICS OF IR SPECTRUM

• The absorption peaks go

down the spectrum

CHARACTERISTICS OF IR SPECTRUM

y-axis

measures the percent transmittance

finger print region

not considered

CHARACTERISTICS OF IR SPECTRUM

The y-axis measures the percent transmittance: 100% T

means that all the IR light that pass through the sample is transmitted, and none is absorbed.

CHARACTERISTICS OF IR SPECTRUM

The y-axis measures the percent transmittance: 0% T

means that none of the IR light pass through the sample and is transmitted, and all is absorbed resulting to the presence of peaks

2 regions of IR spectrum

• functional grp

• fingerprint region

functional grp region/ diagnostic region

• occurs at ≥ 1500 cm-1

• common functional grp gives distinct peaks in this region at a characteristic frequency

fingerprint region

• occurs at <1500 cm-1

• the region often contains complex set of peaks and is unique for every cmpd

IR Absorptions

Stronger bonds

vibrate at higher frequency thus they are absorbed at higher wavenumber

bonds absorb in

four predictable region in an IR spectrum

R1 range

4000-3000

R2 range

3000-2000

R3 range

2000-1500

R1

• bonds to hydrogen

• lighter atoms higher freq

R1 cmpds/ bond

• C-H

• O-H

• N-H

R2

• TRIPLE BONDS

• stronger bonds higher freq

R2 cmpds/ bonds

• alkynes

• nitrile

R3

double bonds

R3 cmpds/bonds

• c=c

• c=o

• c=n

REGION 1

• ROH • RCOOH • RNH2 • Saturation

REGION 2

• Alkynes • Nitriles

REGION 3

• Alkenes/ arenes

• RCHO

• RCOR

• RCOOH all and derivatives

O-H

• 3600-3200

• strong, broad

N-H

• 3500-3200

• medium

C-H

• ~3000

sp3

• 3000-2850

• strong

sp2

• 3150-3000

• medium

sp

• 3300

• medium

alkynes

• 2250

• medium

nitrile

• 2250

• medium

c=o

• 1850-1650 (often ~1700)

• strong

c=c

• 1650

• medium

benzene

• 1600-1500

• medium

ir bands can be classified depending on

relative intensities in infrared spectrum

strong band

covers most of the y axis

medium band

falls to abt half of the y axis

weak band

falls to about one third or less of the y axis

narrow bands

thin and pointed like a dagger

broad bands

wide and smoother

amine

amide

alcohol

aldehyde

carboxylic acid (o-H)

alkyne

nitrile

ester

aldehyde

ketone

carboxylic acid (c=o)

amide (c=o)

alkene

FTIR in Determining the outcome of a chemical reaction

Oxidation of hydroxyl group in Compound C to form the carbonyl group in periplanone B is accompanied by the disappearance of the OH group and appearance of carbonyl group