rotational constants

Coefficients of quantum numbers in the rotational term expression and inversely proportional to the principal moments of inertia. Symbols: $A$ , $B$ , $C$ . $A ˜ = h 8 π 2 c I A$ (dimension wavenumber), $A = h 8 π 2 I A$ (dimension frequency) where $h$ is the Planck constant and $c$ the speed of light in vacuum.

Source:

Green Book, 2nd ed., p. 23