Infrared mode of Absorption and its Stretching Vibrations

1.1-The infrared mode of absorption

When molecules absorb radiations with specific energy they get excited and move from lower to higher energy state. Similarly, molecules absorb infrared radiations that are quantized. Radiations of selected frequencies of infrared radiations are absorbed by the molecules. This absorption leads to energy change on the order of 9 to 40KJ/mole. Electromagnetic radiations of that energy range correspond to the stretching and bending vibrational frequencies of different organic and inorganic molecules. In the infrared mode of absorption, frequencies of infrared radiations that are correctly matched with the natural vibrational frequencies of the covalent compounds are simply absorbed. This absorbed frequency lead to increase the vibrational amplitude of the moving bonds in the organic molecule. Only those compounds have a dipole moment that may change as a function of time has the ability to absorb infrared radiations.

Compounds like H2, Cl2 contains symmetric bonds are not capable of absorbing infrared radiations. So, it is necessary for the bond to show a changing electrical dipole that is may be same at the same frequency range like the incoming radiations for the molecular energy that is to be transmitted. This electrical dipole that changes with time of the molecular covalent bonded compound can then couple with the observing electromagnetic field of the radiations. Thus, compounds having symmetrical bond contain almost same groups attached on both end have not absorbs IR radiations.

2-Modes of stretching and bending of vibrations

Stretching and bending are the most common modes of vibrations for a molecule that are more IR active. Other complex types of molecular vibrations are also active.

2.1-Vibrational frequencies and their modes: All molecules vibrate at specific wavelength. It was observed that when a radiations of certain wavelength fall on a molecule, the molecule start vibrating. Some vibrations are associated with specific bonds or a specific functional group in a molecule. These types of vibrations are called fundamental vibrations. Other types of vibrations associated with the whole molecule. Fundamental vibrations show two types of vibrational mode. These are;

• Stretching Vibrations

• Bending Vibrations

2.2-Stretching Vibrations: In stretching vibrations the two bonded atom along a bond horizontal shows a periodic movement in which the distance between both the atoms may increase or decrease. Stretching vibrations can be sectioned into;

• One is Symmetric

• Other is Asymmetric.

2.3-Bending Vibrations: Bending vibrations is the periodic movement of the groups of atoms in which the angle of two common bonded atoms usually change. However, the bond length remains the same. In bending vibrations more than three atoms must be present in a molecule. Bending vibrations consist of four different types;

• Rocking

• Scissoring

• Twisting

• Wagging

2.4-Others types of Molecular vibrations:

These types of vibrations arise due to excitations of molecules from the lower orbital state to the highest energy orbital exited state. Complicated peaks observe in the spectrum due to the presence of vibrations like,

• Overtones

• Difference bands

• Combinations bands.

1.2-Applications of the IR spectrum:

We observe that each kind of molecular bond shows totally different ranges for natural frequency of vibrations. Similarly, two same identical covalent bonds present in two totally different molecules may show almost different environment. So, no two compounds having different structural formula show exact infrared spectrum. However, rarely in two cases some of the vibrational frequencies might be same, but no two different compounds shows their infrared absorption pattern be identical. Therefore, we use the infrared spectrum for the identification of compounds much like the fingerprint that we use for human identification. We simply compare the infrared spectra of two compounds to confirm whether they are identical or not. If their spectra match peak for peak, they must be identical compounds.