Interpretation of Spectral Information

1-Interpretation of Spectral Information

Infrared spectroscopy did not provide complete information about compound spectra. The information obtained from it, nevertheless often correct, is always indefinite and is used together with confirmations by chemical, physical and other means of spectroscopy. Absorption bands of complex compounds cannot interpret precisely.

1.1-Regions of spectroscopy: Infrared spectrum can be divides into two regions.

• Functional-group region

• Finger-print region

The infrared region ranges from 4050 to 600cm-1, is known as functional-group region. While the other region ranges from 1600 to 625cm-1, known as finger-print region. While observing infrared spectra, we firstly approach the functional group region. This region contains few strong bands that appear due to the specific characteristics stretching vibrations of prominent functional groups. Specific bands are assigned to each functional group at specific frequency range. For instance, functional group X-H type compounds like C-H type, O-H type and N-H type shows absorption peaks near the range of 3700 to 2500cm-1. Triple bonds like C ≡C and C ≡N shows absorption bands in the region of 2300 to 2100cm-1. While double bonds like C=O and C=C shows absorption peaks in the region of 1900 to 1600cm-1. However the presence of a particular absorption at a specific position in the functional group region does not gives us any guarantee about the presence of that particular functional group in the analyte.

Fingerprint region that is ranges from 1600 to 625cm-1 is called so because it gives unique characteristics absorption bands for each type of compounds and serves as fingerprint for each compound. Sometimes compounds almost show same types of absorption bands in the functional group region so, the fingerprint region shows some noticeable difference for each compounds. No two compounds having same molecular formula show identical spectra except enantiomers. Fingerprint region usually shows a large number of complex patterns of bands and these bands originate due to the interacting vibrational modes. Bending and stretching vibrations of functional groups like C-C, C-N and C-O are the main reasons of appearing complex bands in this region.

2-Amines:

Amines can be derived from ammonia. In amines hydrogen atoms in ammonia can be replaced by hydrocarbon group like alkyls.

2.1-Types of Amines:

Amines can be categorized into three kinds on the basis of how many hydrogen atoms of ammonia are displaced by alkyl functional groups. These are;

• Primary amines

• Secondary amines

• Tertiary amines

• Aromatic amines

2.2-Primary amines:

When one hydrogen atom of ammonia compound is displaced by an alkyl functional group, is called primary amines. The formula of primary amines is RNH2 in which “R” stands for alkyl group. It gives us two stretching bands at the frequency range of 3300 to 3000cm-1.

Examples include: 1-Amino propyl, 2-Amino propyl, Butyl Amine.

2.3-Secondary amines:

When more than one hydrogen atoms of ammonia compound are displaced by two alkyl substituents, secondary amines are formed. Secondary amines give us only one absorption band at the frequency range of 3300 to 3000cm-1.

Examples include: Dimethyl Amine and Diethyl Amine.

2.4-Tertiary amines:

When more than one hydrogen atoms of ammonia compound are displaced by three alkyl substituents, tertiary amines are formed. For tertiary amines no absorption band appears in the range of 3300 to 3000cm-1.

Examples includes: Trimethyl Amines

2.5-Aromatic amines:

In aromatic amines a benzene ring is attached by replacing one of ammonia hydrogen atom.

Example includes: N-methyl aniline

2.6-Differentiate between Primary, Secondary and Tertiary Amines:

These amines can be distinguished on the basis of their spectra in dilute solutions in infrared region. Primary amines depict two bands in the region of 3300 to 3000cm-1 due to their fundamental N-H stretching vibrations. Secondary amines show only single band in this region of 3300 to 3000cm-1 while, tertiary amines do not show any band at all.