What is a active gas?

Answer 1

Absorption of energy from IR radiation can only occur when the wavelength of radiation and the wavelength of the bond vibration match. If a molecule has symmetry...for example no dipole, we say it is IR inactive. CO2 has no dipole....this would be an IR inactive gas...Cl2 as well. A gas that has a dipole would be IR active. Propene would be a nice example of an IR active gas.It possesses a dipole. One would see a signal around 1650.

A:CO2 has no permanent dipole. However, when CO2 undergoes a bending vibration, its dipole moment changes from zero to some non-zero value. This vibration produces a change in dipole moment and is therefore IR active.