NIST: Diatomic Molecules - Equilibrium Internuclear Distances

7. Equilibrium Internuclear Distances

When precise values for the Dunham coefficients, Y_lj, are obtained from experiment, the equilibrium internuclear distance can be determined from the equilibrium rotational constant B_e. Dunham [3] derived the following relationship between Y₀₁ and B_e:

(eq27)

where the a_i are the anharmonic potential constants that can be evaluated from the higher Y_ij coefficients. The equilibrium internuclear distance is then expressed as:

eq28 (eq28)

Since Dunham's treatment utilizes the Born-Oppeniheimer approximation, the value of B_e must be modified for breakdown of this approximation. Several methods have been developed for handling these corrections; the most important are those of Rosenblum et al. [58008], Bunker [17], and Watson [73023].