Regarding dipolar changes by vibration and rotation, most of electron transitions requires the energy of visible rays and infrared rays spectrum and infrared rays occur primarily only in the molecules which have significant difference in the energy among several vibrations and rotations.

So, to absorb or emit infrared radiant rays, the relevant molecule should change dipolar moment by vibration and rotation. If radiating infrared rays with the same wave length to the molecule, the energy of infrared rays is absorbed into the molecule, and by resonance, the molecule or its molecule group are becomes excited. And then, the infrared rays of the wavelengths with the inconsistent number of molecular vibrations transmit or reflect the molecule

In these conditions, the electric field of radiant rays is interactive with molecules and can cause changes in the molecular vibration and rotation. If the number of radiant ray vibrations agrees with that of natural molecular vibrations, energy transfer occurs and the amplitude of molecular vibration changes. As a result, radiant rays are absorbed or emitted. Similarly, if asymmetric molecules rotate around the center of mass, cyclic dipolar changes appear followed by interaction with radiant rays.