Spectroscopy study

IR spectroscopy

Start with the functional groups suggested by the formula and structure, then use broad regions to confirm or reject them.

Region	Common signal	Interpretation note
3600-3200 cm⁻¹	O-H and N-H stretches	Broad shape often matters as much as position.
3100-2850 cm⁻¹	C-H stretches	sp2 C-H appears slightly above most sp3 C-H stretches.
2260-2100 cm⁻¹	Triple-bond region	Alkynes and nitriles can be distinctive here.
1800-1650 cm⁻¹	Carbonyl region	Ketones, aldehydes, esters, acids, and amides separate by environment.
1600-1450 cm⁻¹	Aromatic and alkene C=C	Useful with out-of-plane C-H bands and molecular formula.
1300-1000 cm⁻¹	C-O and fingerprint bands	Best interpreted alongside the whole spectrum.

Search functional groupsIntensityShape

High wavenumberLow wavenumber

40003000200015001000400 cm⁻¹

alcohol or phenol

O-H stretch

3200-3650 cm⁻¹

strongbroad

Hydrogen bonding broadens O-H bands; phenols and alcohols overlap.

carboxylic acid

very broad acid O-H stretch

2500-3300 cm⁻¹

strongbroad

Often overlaps C-H stretches and is most convincing with a carbonyl band.

amine or amide

N-H stretch

3300-3500 cm⁻¹

mediumsharp

Primary amines can show two bands; secondary amines often show one.

alkene or aromatic

sp2 C-H stretch

3000-3100 cm⁻¹

mediumsharp

Usually interpreted with C=C or aromatic ring-region evidence.

alkane or alkyl group

sp3 C-H stretch

2850-3000 cm⁻¹

strongsharp

Common in many organic molecules, so it is rarely diagnostic alone.

terminal alkyne

terminal alkyne C-H stretch

3260-3330 cm⁻¹

mediumsharp

Look for a C≡C stretch around 2100-2260 cm⁻¹ as support.

nitrile

C≡N stretch

2210-2260 cm⁻¹

mediumsharp

A sharp band in this region is strongly suggestive of nitrile.

alkyne

C≡C stretch

2100-2260 cm⁻¹

variablesharp

Internal symmetric alkynes can be weak or absent.

acid chloride

acid chloride C=O stretch

1780-1815 cm⁻¹

strongsharp

Higher-frequency carbonyl due to the electron-withdrawing chloride.

anhydride

anhydride paired C=O stretches

1740-1850 cm⁻¹

strongsharp

Two carbonyl bands are more meaningful than one isolated peak.

ester

ester C=O stretch

1735-1750 cm⁻¹

strongsharp

Often paired with C-O absorptions in the fingerprint region.

aldehyde

aldehyde C=O stretch

1720-1740 cm⁻¹

strongsharp

Aldehyde C-H bands near 2720 and 2820 cm⁻¹ help distinguish it.

ketone

ketone C=O stretch

1705-1725 cm⁻¹

strongsharp

Conjugation and ring strain shift the exact position.

carboxylic acid

carboxylic acid C=O stretch

1680-1725 cm⁻¹

strongsharp

Most convincing with the broad acid O-H envelope.

amide

amide C=O stretch

1630-1690 cm⁻¹

strongsharp

Amide I bands overlap alkene/aromatic regions.

alkene

C=C stretch

1620-1680 cm⁻¹

mediumsharp

May be weak in symmetric alkenes.

aromatic ring

aromatic C=C ring stretch

1450-1600 cm⁻¹

mediumsharp

Best read with aromatic C-H and substitution-region evidence.

nitro compound

NO2 asymmetric/symmetric stretches

1340-1550 cm⁻¹

strongsharp

Nitro groups often show two strong absorptions.

ether, alcohol, or ester

C-O stretch

1000-1300 cm⁻¹

strongsharp

Fingerprint-region C-O bands need supporting evidence.

Fingerprint region

The fingerprint region contains many coupled bending and stretching motions. It is powerful for comparing a known reference spectrum with an unknown, but it is weaker for naming a functional group from one isolated peak. Treat C-O, aromatic substitution, and out-of-plane C-H bands as supporting evidence unless the rest of the spectrum agrees.

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