= Rotational constant
{title2=$B=h/(8\pi^2I)$}
In this article the <rotational constant> is a <frequency>,
$$
B=\frac{h}{8\pi^2I}.
$$
Its dimensions are $\mathrm{s^{-1}}$, since the <Planck constant> has dimensions $\mathrm{kg\,m^2\,s^{-1}}$ and a <moment of inertia> has dimensions $\mathrm{kg\,m^2}$. Some authors instead call $hB=\hbar^2/(2I)$ the energy-valued <rotational constant>; this is the convention in the earlier synthetic <molecular beam electric resonance> diagram. A third convention uses the <spectroscopic wavenumber> $\widetilde B=B/c$, with $c$ the <speed of light>. Always check units before using $\nu=2B(J+1)$.
If $B$ is known and the two <masses> are specified, the rigid two-atom model gives the <bond length> $r=\sqrt{h/(8\pi^2\mu B)}$. This is why <rotational spectroscopy> can constrain <molecular geometry>. A real fitted constant can depend on <molecular vibration>, so this inference alone does not establish an equilibrium <bond length>.
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