Why are N2 and O2 not greenhouse gasses?

Answer 1

The O2 and N2 are homonuclear molecules. So the rotational wave function of the diatomic homonuclear molecules should be symmetric or antisymmetric under the exchange of the two identical nuclei, depending on whether they are bosons or fermions. This would imply that the l quantum number is either even or odd. In either case, in rotational transitions the change in l should be at least 2. This violates the selection rule for electric dipole transitions, namely the change in l should be one. So the electric dipole transitions are forbidden in the rotational transitions of these molecules. The electric dipole transitions are the most prominent transitions whenever the wavelength is much larger than the size of the system, which is evidently true for the rotational transition wavelengths of the diatomic molecules.

The vibrational transitions of these molecules can only absorb one wavelength because in electric dipole transitions, the n quantum number of the vibrational spectrum can only change by one unit and the vibrational spectrum is equally spaced, just like the energy spectrum of a one dimensional harmonic oscillator.

Answer 2

N2 and O2 are not greenhouse gases because they do not have the molecular structure to effectively absorb and re-emit infrared radiation, which is necessary for trapping heat in the atmosphere. This makes them ineffective in contributing to the greenhouse effect that warms the Earth's surface.