Example of Amines and their Infrared Spectra

1-Examples of Amines

1.1-Infrared spectra of the diethyl amine:

Similarly dimethyl amine is an example of the secondary amines. As we discussed above secondary amines shows only single absorption band because they contain only one C-H group. At 3280cm-1 frequency range it shows N-H stretching absorption band and at the range of 1140cm-1 C-H stretching bands are appeared.

1.2-infrared spectra of Aniline:

Aniline is an example of the primary aromatic amines. The infrared spectra of aniline show absorption peaks at different frequency ranges. It shows two N-H stretching bands that appear 3445cm-1 to 3360cm-1. Beside N-H bands C-H stretching absorptions bands are also appeared in the range of 1280cm-1.

1.3-Infrared spectra of Trimethyl amine:

Trimethyl amine is an example of the tertiary amine. It does not show any absorption peak because there was no C-H bond directly attached with nitrogen. However, it shows C-H stretching vibrations in the range of 1215cm-1.

Deforming Vibrations of Amines (N-H):

Primary amines shows absorption bands of medium to strong intensity in the infrared region of 1650 to 1580cm-1. While secondary amines show weaker bands in the region range from 1580 to 1490cm-1. Range for aromatic amines is 1615 to 1580cm-1. Hydrogen bonding moves the N-H bands to higher frequency range. This shift totally depends upon the physical strength of hydrogen bond [8]. Primary amines show weak to medium intensity of broad absorption bands at the range of 895 to 650cm-1. The shapes and position of these absorption bands highly depend upon the hydrogen bonding. It is noteworthy, that all the bonds present in the molecule have not enough capable of absorbing infrared radiations, even though these radiations exactly match to vibrational frequencies of the bonds.

Factors Affecting N-H absorption frequencies:

There are different factors that influence the IR spectra of NH and shift the vibrational frequency of absorption bands [13].

• Hydrogen Bonding

• Electronic effect

• Fermi resonance

• Coupled vibrations

2.1-Hydrogen Bonding

Hydrogen bonding is the important phenomenon because it plays important role to shift the frequencies on away from normal ranges. In case of NH the electronegative difference is very important [14]. Nitrogen is electronegative and due to electronegative difference between the nitrogen and hydrogen leads the intensity towards maximum intensity absorption. In case of hydroxyl group the electronegative difference is very high as compared to NH so its peak is more prominent then the NH peak. And the intensity of CH is less than that of NH.

Two types of hydrogen bonding appears in the molecule that That are;

1-INTRAMOLECULE HYDROGEN BONDING

2-INTERMOLECULAR HYDROGEN BONDING

In case of intermolecular hydrogen bonding shows the broad peaks while in case of intramoleculer hydrogen bonding sharp peaks is observed.

Conclusion:

In this article we discussed about the infrared spectra of Amines. Amines can be derived from the ammonia in which the hydrogen atoms are displaced by alkyl functional substituents. Amines can be further divided into many types, like Primary, Secondary, Tertiary and Aromatic amines. We differentiate between these kinds of amines by observing its spectral information. All amines gives different types of absorption bands which helps to distinguish them. Primary amines give us two stretching bands at the frequency range of 3300 to 3000cm-1. Secondary amines give us only one absorption band at the frequency range of 3300 to 3000cm-1. For tertiary amines no absorption band appears in the range of 3300 to 3000cm-1. Similarly, aromatic amines show different absorption bands at different frequency range. Beside this, there are many factors that influence the vibrational frequency ranges and shift the absorption band position from lower to higher range. These factors are, Hydrogen bonding, Electronic effect, Fermi resonance, and coupled vibrations.