N-H Stretching Vibrations of Amines

N-H stretching vibrations of the amines:

Stretching vibrations of N-H region shows the absorption bands near the range of 3550 to 3300cm-1. In clear solutions samples the N-H shows feeble and acute bands than the observe O-H band which mostly appears in the same region.

Stretching vibrations of Amines (N-H):

In primary aliphatic amines, may be solid or liquid, hydrogen bonding is present, which shows absorption in the region of 3450 to 3160cm-1. In dilute solutions of non-polar, two bands are appeared due to the both symmetric and non-symmetric vibrations of N-H bond. For aliphatic amines the absorption region ranges from 3550 to 3160cm-1 and for aromatic amines, bands of medium intensity appears, one shows at 3520 to 3420cm-1 and the other one shows at the range of 3420 to 3340cm-1.

Secondary amines shows single absorption band in the range of 3500 to 3300cm-1. This single band appears due to the presence of only one N-H stretching bands which is mostly weak. Medium intensity bands can be visualized near the range of 3450 to 3300cm-1 for the secondary aromatic amines in liquid or solid phase. In some solvents, hydrogen bonding affect the absorption and bands appears due to N-H stretching vibrations at low frequency of 300cm-1.

For tertiary amines, sometimes it is useful to convert them into hydrochlorides and then we observe the resulting spectra and indicate the presence of band due to N-H stretching vibrations. This technique proves very useful to distinguish between amines and imines. In Raman spectra of amines stretching vibrations of N-H bond always results in bands of weak to medium intensity.

C-N stretching vibrations of Amines:

Amines show weaker C-N stretching absorption for primary aliphatic amines in the region of 1090 to 1020cm-1. Two bands of medium intensity are shown for secondary aliphatic amines in the region of 1190 to 1170 cm-1 and 1145 to 1130cm-1. Tertiary amines show band of C-N in the region of 1380 to 1265cm-1. For unsaturated and aromatic amines we clear observe two bands at different positions at 1360 to 1250cm-1 and 1280 to 1180cm-1. These bands appears due to the conjugation process of nitrogen electrons with the ring that imparting a double bond character to the visible N-H bond.

N-CH3 and N-CH2- Absorption of amines:

Medium to strong intensity of bands appears in the range of 2820cm-1due to the stretching vibrations of the N-C-H bond and C-H bonds. These bands have lower frequency and higher intensity than characterized bands and therefore, it is an easy task to identify them. Aliphatic amines contains –N(CH3) group gives us two bands, one at 2820cm-1 and the other one at 2770cm-1 frequency range.

Other types of Amines Bands:

Primary aromatic amines like aniline shown weak to medium intensity bands at the range of 445 to 345cm-1 which are likely due to the in plane deformation vibrations of the aniline ring. And primary alkyl amines give us strong absorption band in the region of 200cm-1.

N-H bending vibrations of amines:

For primary amines spectra shows in plane strong N-H scissoring absorptions at the range of 1550 to 1650cm-1 along with out of plane wagging vibrations at the range of 650 to 900cm-1. These peaks are usually broad and are the characteristics of amines.

Infrared spectra of secondary amines showed weak N-H bending absorptions that are visible in the infrared region of 1500 to 1600cm-1. An intense broad wagging absorption bands may appear in the range of 650 to 900cm-1.

Tertiary amines showed broad bands in the infrared spectra aside from the N-H stretch and these compounds shown specific characteristics for their alkyl and aryl substituents.