Coefficients of quantum numbers in the rotational term expression and inversely proportional to the principal moments of inertia. Symbols: \(A\), \(B\), \(C\). \(\overset{˜}{A}=\frac{h}{8\ \pi ^{2}\ c\ I_{\rm{A}}}\) (dimension wavenumber), \(A=\frac{h}{8\ \pi ^{2}\ I_{\rm{A}}}\) (dimension frequency) where \(h\) is the Planck constant and \(c\) the speed of light in vacuum.