Generalized 1/Z expansion for heteronuclear molecules

We have first studied empirical regularities in various series of heteronuclear diatomic molecules between the energy E , the total number of electrons N , the equilibrium distance R e and Z̄ = (Z 1 Z 2 ) 1/2 where Z 1 e and Z 2 e are the nuclear charges in the diatom. In particular, for various alkali halide series, R e 2 | E |/ N 5/3 is shown to correlate rather simply with Z̄ R e 3 . Some theoretical basis is afforded by generalizing the 1/ Z expansion used early by the writers in work on homonuclear diatomics. Finally, when Z 2 /Z 1 → ∞, a model is presented which predicts a finite asymptotic bond length and this prediction is confronted with available experimental data for both heteronuclear diatoms and for the polyatomic series CH 4 to SnH 4 .