Vibration-rotation interaction ($B_v=B_e-{\alpha }_e(v+1/2)$)

The dependence of rotational energy on the vibrational state. For a diatomic molecule a common leading approximation is $B_v=B_e-{\alpha }_e(v+1/2)$, so vibrational satellites can have different rotational line spacings.

The first-order dependence of a polyatomic rotational constant on vibrational occupations $v_i$, where $d_i$ is the degeneracy of normal mode $i$. The factor $d_i/2$ accounts for all of that mode's zero-point energy. A single band's change in rotational constant generally determines one ${\alpha }_i$, rather than every zero-point correction needed for $B_e$.