The ability of a gas to function as a green house gas depends on its ability to absorb infra red rays. In turn, the absorption of infrared red rays depends on whether or not the molecule is IR active.
The triatomic molecules such as methane and water are IR active. Only IR active molecules can lead to green house effect.
Note that for a molecular vibrational mode to be IR active, the dipole moment of the molecule is changed as the vibration occurs .
