= Solution
A polar diatomic molecule can absorb microwaves through its permanent <electric dipole moment>. In the <linear rigid rotor> approximation,
$$
E_J=hB_0J(J+1),\quad J=0,1,2,\ldots,
\qquad B_0=\frac{h}{8\pi^2I_0}.
$$
The electric-dipole <selection rule> is $\Delta J=+1$ in absorption. It gives
$$
\nu_{J\to J+1}=\frac{E_{J+1}-E_J}{h}=2B_0(J+1),
$$
so adjacent lines are separated by $2B_0$. Therefore
$$
\boxed{B_0=\frac{21.999}{2}=10.9995\ \mathrm{GHz}.}
$$
Assign a line near $m\times21.999\ \mathrm{GHz}$ to $J=m-1\to m$ for positive integer $m$. The first member is $0\to1$, the next $1\to2$, and so on; if the recorded window misses the lowest lines, their absolute frequencies fix the integer offset. At high temperature many rotational states are populated, so the strongest line need not be $0\to1$.
This is the leading rotational pattern, not a claim that real ZnF lacks spin or hyperfine structure. Fine splittings and <centrifugal distortion> can perturb the ideal lines; they do not explain the three systematically displaced progressions as economically as vibrational excitation.
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