← Spectroscopic data

4 Infrared (IR) spectroscopy

Infrared spectroscopy studies the vibrational properties of a molecule. The bonds and atoms present in a molecule vibrate in different ways and at different frequencies, and these vibrations are generally quite distinct for each molecule. Each vibration can be measured by passing IR radiation through a sample and measuring which frequencies are absorbed: measured in wavenumbers, cm⁻¹. Watch: Infrared (IR) spectroscopy Opens a YouTube search: pick a video that clicks for you

Two ways to analyse a molecule with IR

• Compare the spectrum to a library of spectra. If there is a match you can usually have great confidence in the molecule’s identity. This has been used historically in forensics, especially to identify illicit drugs.
• Examine the distinct absorptions produced by particular functional groups. This is the approach used in this course.

The two absorptions to focus on

• Broad absorption(s) around 3000–3500 cm⁻¹ ⟹ the presence of either N–H or O–H bonds.
• A sharp, intense absorption around 1600–1800 cm⁻¹ (depending on functionality) ⟹ the presence of a C=O bond.
• You should also note a peak or series of peaks around 3000 cm⁻¹ for C–H, but this is present in virtually all samples analysed, so on its own it tells you almost nothing.

The six 4-carbon compounds: varying combinations of C=O, N–H and O–H

Compound	O–H / N–H	C=O
Butane (reference)	none	none: only C–H at 2950
Butan-1-ol	O–H at 3350	none
1-aminobutane	N–H at 3350	none
Butanone	none	clear C=O at 1700
Butanoic acid	O–H at 3100	C=O at 1700
Ethyl ethanoate	none	C=O at 1700
Butanamide	N–H at 3200 AND 3400	C=O at 1650

✓

How to use that table

These seven compounds are the whole decision tree in miniature. Ask two questions in order:

1. Is there a broad absorption at 3000–3500? No → butane, butanone or ethyl ethanoate. Yes → alcohol, amine, acid or amide.
2. Is there a C=O at 1600–1800? Combine the two answers and you have already narrowed seven compounds to at most two.

Then use the finer detail: butanamide shows TWO N–H bands (3200 and 3400) where the amine shows one, and its C=O sits lower at 1650. ℹ

Why some absorptions are weak

IR absorption involves a change in dipole, so molecules which have no overall dipole (totally symmetrical, e.g. methane) have extremely weak IR absorption. This is also why the data sheet lists the C=C stretch as variable intensity: symmetry reduces its intensity.

AS91388 data sheet: IR stretching vibrations

Functional group	Range (cm⁻¹)	Intensity	Assignment
Alkanes	2850–3000	med–str	C–H, 2 or 3 peaks
Haloalkanes	600–800	str	C–Cl
Haloalkanes	500–600	str	C–Br
Alkenes	3020–3100	med	=C–H & =CH₂ (usually sharp)
Alkenes	1630–1680	var	C=C (symmetry reduces intensity)
Alcohols	3200–3550	str	O–H (H-bonded), usually broad
Alcohols	970–1250	str	C–O
Amines & amides	3300–3500	wk–med	N–H (1° amines); N–H (amides) – 2 peaks; N–H (2° amines)
Amines & amides	1000–1250	med	C–N
Aldehydes & ketones	2690–2840	med	C–H, 2 peaks (aldehyde C–H)
Aldehydes & ketones	1720–1740	str	C=O (saturated aldehyde)
Aldehydes & ketones	1710–1720	str	C=O (saturated ketone)
Carboxylic acids & derivatives	2500–3300 (c.acids), overlaps C–H	str	O–H (very broad)
Carboxylic acids & derivatives	1705–1720 (c.acids)	str	C=O (H-bonded)
Carboxylic acids & derivatives	1210–1320 (c.acids)	med	O–C (sometimes 2 peaks)
Carboxylic acids & derivatives	1785–1815 (acid chlorides)	str	C=O
Carboxylic acids & derivatives	1735–1750 (esters)	str	C=O
Carboxylic acids & derivatives	1000–1300	str	O–C (2 peaks)
Carboxylic acids & derivatives	1630–1695 (amides)	str	C=O

AS91388 data sheet: IR bending vibrations

Functional group	Range (cm⁻¹)	Intensity	Assignment
Amines & amides	2850–3000	med	N–H (1° amine)
Amines & amides	1550–1640	str	N–H (1° amine)
Amines & amides	1590–1650	med	N–H (amide)
Alkane	1450–1470	med	C–H
Carboxylic acids	910–950	med	O–H
Alkene	650–1000	med–str	=C–H

Key: str = strong, med = medium, wk = weak, var = variable.

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