23.13: Spectroscopic Properties

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IR

The infrared spectrum of aniline is shown beneath the following table. Some of the characteristic absorptions for C-H stretching and aromatic ring substitution are also marked, but not colored.

Amine Class	Stretching Vibrations	Bending Vibrations
Primary (1°)	The N-H stretching absorption is less sensitive to hydrogen bonding than are O-H absorptions. In the gas phase and in dilute CCl₄ solution free N-H absorption is observed in the 3400 to 3500 cm^-1 region. Primary aliphatic amines display two well-defined peaks due to asymmetric (higher frequency) and symmetric N-H stretching, separated by 80 to 100 cm^-1. In aromatic amines these absorptions are usually 40 to 70 cm^-1 higher in frequency. A smaller absorption near 3200 cm^-1 (shaded orange in the spectra) is considered to be the result of interaction between an overtone of the 1600 cm^-1 band with the symmetric N-H stretching band. C-N stretching absorptions are found at 1200 to 1350 cm^-1 for aromatic amines, and at 1000 to 1250 cm^-1 for aliphatic amines.	Strong in-plane NH₂ scissoring absorptions at 1550 to 1650 cm^-1, and out-of-plane wagging at 650 to 900 cm^-1 (usually broad) are characteristic of 1°-amines.
Secondary (2°)	Secondary amines exhibit only one absorption near 3420 cm^-1. Hydrogen bonding in concentrated liquids shifts these absorptions to lower frequencies by about 100 cm^-1. Again, this absorption appears at slightly higher frequency when the nitrogen atom is bonded to an aromatic ring. The C-N absorptions are found in the same range, 1200 to 1350 cm^-1(aromatic) and 1000 to 1250 cm^-1 (aliphatic) as for 1°-amines.	A weak N-H bending absorption is sometimes visible at 1500 to 1600 cm^-1. A broad wagging absorption at 650 to 900 cm^-1 may be discerned in liquid film samples.
Tertiary (3°)	No N-H absorptions. The C-N absorptions are found in the same range, 1200 to 1350 cm^-1 (aromatic) and 1000 to 1250 cm^-1 (aliphatic) as for 1°-amines.	Aside from the C-N stretch noted on the left, these compounds have spectra characteristic of their alkyl and aryl substituents.

NMR

The hydrogens attached to an amine show up ~ 0.5-5.0 ppm. The location is dependent on the amount of hydrogen bonding and the sample's concentration.

The hydrogens on carbons directly bonded to an amine typically appear ~2.3-3.0 ppm.

Contributors

William Reusch, Professor Emeritus (Michigan State U.), Virtual Textbook of Organic Chemistry

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