Generally, rotation-vibration molecular spectra are described by the Dunham expression in the form E(v, I)=sum from n=i,j Y_(i,j)(v+1/2)~i[I(I+1)]~j , (1)where I is the angular momentum and v is the vibrational quantum number. The numerical coefficients Y_(i,j) of the first few terms of the Dunham expression are obtained by the solution of the Schrdinger equation for the Morse potential, while the rest is fixed by experimental data. For rotational spectra, the Dunham expansion can be written as