In the IR absorption spectrum of diatomic molecules vibrate. frequencies are observed only in heteronuclear molecules (HF, NO, CO, etc.), and the selection rules are determined by the change in their electric. dipole moment with fluctuations.
Linear rotors: 1 moment of inertia (molecular axis) equal to zero : HF, CO, NO.
Rotationally inactive molecules: non-polar, homonuclear diatomic, symmetrical linear, spherical rotors (unless distorted by rotation) H2.