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.
Green Book, 2nd ed., p. 23