Answer to Question #274401 in Chemistry for Bobby

Question

Spectroscopic methods are based on the interaction of electromagnetism ration with matter.

a. What type of electrons in a molecule are generally involve in UV or visible radiation?

b. What is the necessary criterion for absorption to occur in the infrared region?

c. What types of molecular vibration are associated with infrared absorption?

Answer 1

Question:

Spectroscopic methods are based on the interaction of electromagnetism ration with matter.

a. What type of electrons in a molecule are generally involve in UV or visible radiation?

b. What is the necessary criterion for absorption to occur in the infrared region?

c. What types of molecular vibration are associated with infrared absorption?

Answer:

a. The electrons situated on the highest occupied molecular orbitals (HOMO) and the neighbor ones (HOMO-1, HOMO-2, etc) are generally involved in UV or visible radiation absorption. In organic molecules, it is often the electrons belonging to the extended π-systems.

b. In order to absorb a light in infrared region, the wavelength must be in resonance with a vibrational mode present in the molecule.

c. Vibrational motions, associated with infrared absorption, can be divided in stretching and bending modes. In case of the former one, the distance between the atoms is changing continuously. On the other hand, bending modes (wagging, twisting, rocking and scissoring) are linked to the change of the angle between the bonds involved.