A two force-constant model for complexes B•••M-X (B is a Lewis base and MX is any diatomic molecule): Intermolecular stretching force constants from centrifugal distortion constants DJ or ΔJ

A two force-constant model is proposed for complexes of the type B center dot center dot center dot MX, in which B is a simple Lewis base of at least C-2v symmetry and MX is any diatomic molecule lying along a C-n axis (n >= 2) of B. The model assumes a rigid subunit B and that force constants beyond quadratic are negligible. It leads to expressions that allow, in principle, the determination of three quadratic force constants F-11, F-12, and F-22 associated with the r(B center dot center dot center dot M) = r(2) and r(M-X) = r(1) internal coordinates from the equilibrium centrifugal distortion constants D-J(e) or Delta(e)(J), the equilibrium principal axis coordinates a(1) and a(2), and equilibrium principal moments of inertia. The model can be applied generally to complexes containing different types of intermolecular bond. For example, the intermolecular bond of B center dot center dot center dot MX can be a hydrogen bond if MX is a hydrogen halide, a halogen-bond if MX is a dihalogen molecule, or a stronger, coinage-metal bond if MX is a coinage metal halide. The equations were tested for BrCN, for which accurate equilibrium spectroscopic constants and a complete force field are available. In practice, equilibrium values of D-J(e) or Delta(e)(J) for B center dot center dot center dot MX are not available and zero-point quantities must be used instead. The effect of doing so has been tested for BrCN. The zero-point centrifugal distortion constants D-J(0) or Delta(0)(J) for all B center dot center dot center dot MX investigated so far are of insufficient accuracy to allow F-11 and F-22 to be determined simultaneously, even under the assumption F-12 = 0 which is shown to be reasonable for BrCN. The calculation of F-22 at a series of fixed values of F-11 reveals, however, that in cases for which F-11 is sufficiently larger than F-22, a good approximation to F-22 is obtained. Plots of F-22 versus F-11 have been provided for Kr center dot center dot center dot CuCl, Xe center dot center dot center dot CuCl, OC center dot center dot center dot CuCl, and C2H2 center dot center dot center dot AgCl as examples. Even in cases where F-22 similar to F-11 (e.g., OC center dot center dot center dot CuCl), such plots will yield either F-22 or F-11 if the other becomes available. (C) 2016 AIP Publishing LLC.