Publication Date:
2016-02-19
Description:
A two force-constant model is proposed for complexes of the type B⋯MX, in which B is a simple Lewis base of at least C 2 v 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⋯M) = r 2 and r (M–X) = r 1 internal coordinates from the equilibrium centrifugal distortion constants D J e or Δ J e , 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⋯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 Δ J e for B⋯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 Δ J 0 for all B⋯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⋯CuCl, Xe⋯CuCl, OC⋯CuCl, and C 2 H 2 ⋯AgCl as examples. Even in cases where F 22 ∼ F 11 (e.g., OC⋯CuCl), such plots will yield either F 22 or F 11 if the other becomes available.
Print ISSN:
0021-9606
Electronic ISSN:
1089-7690
Topics:
Chemistry and Pharmacology
,
Physics
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